Prostate-Specific Antigen as a Marker of Disease Activity in Prostate Cancer: Part 1

ABSTRACT: Despite the impact of prostate-specific antigen (PSA) testing on the detection and management of prostate cancer, controversy about its usefulness as a marker of disease activity continues. This review, based on a recent roundtable discussion, examines whether PSA measurements can be used rationally in several clinical settings. Following radical prostatectomy and radiation therapy, prediction of survival by PSA level is most reliable in high-risk patients. PSA doubling time after radiation therapy is the strongest predictor of biochemical failure. PSA measurements have been associated with inconsistent results following hormonal treatment; reduced PSA levels may result from antiandrogen treatment, which decreases expression of the PSA gene, and therefore, the level of PSA production. In the setting of primary and secondary cancer prevention, PSA is important in risk stratification when selecting patients for studies. Part 1 of this two-part article, which concludes in the September issue, focuses on the physiology of PSA, its measurement and use in clinical practice, and its predictive value following radical prostatectomy and radiation therapy. [ONCOLOGY 16:1024-1051, 2002]

Prostate-specific antigen (PSA) testing has changed early detection and management of prostate cancer dramatically since its introduction into clinical practice in the early 1980s. Nevertheless, its usefulness as a marker of disease activity and its correlation with survival remain controversial because, in some disease settings, definitive data are lacking for its rational use. This review defines the clinical settings in which PSA can be rationally used as an indicator of disease activity and therapeutic effectiveness.

Because the natural history of prostate cancer can be long-20 to 50 years in some cases-it is impractical to use survival as the only test of a therapy’s usefulness in clinical trials. This is especially true of chemopreventive agents that are active at the earliest stages of a disease that may not be detectable for as long as 20 years. Even the longest-running trial of a potential chemopreventive therapy-the Selenium and Vitamin E Cancer Prevention Trial (SELECT)-is slated for only 12 years.[1]

Using a marker of disease activity as an end point to demonstrate therapeutic efficacy is imperative for trials at various stages of the disease process. Because patients are encouraged to participate in early-detection programs for some cancers at age 40, the time from early detection to clinical diagnosis of recurrent disease may be as long as 10 years, and the time from diagnosis to death may be another 20 to 30 years after definitive therapy-timelines that would pose practical impossibilities for clinical trials.

FIGURE 1

Algorithm for Estimating the Likelihood of Remaining Free of Metastatic Disease After Initial PSA Recurrence

In the Johns Hopkins series reported by Patrick C. Walsh, MD, and colleagues, the time from surgery to biochemical recurrence as measured by PSA ranged from 2 to 16 years, and from PSA recurrence to metastases averaged 8 years; the median time from metastases to death was 5 years (Figure 1).[2] Chemoprevention aside, these intervals are too long when planning clinical trials to evaluate the effectiveness of therapies for prostate cancer. There is a need for markers as surrogate end points, and PSA has the potential to fill such a role.

TABLE 1

Role of Prostate-Specific Antigen in Prostate Cancer

The purpose of this review is to define the physiology and clinical use of PSA, and to determine whether PSA measurements can be used rationally as a marker of disease activity in the following clinical settings (Table 1):

• post-radical prostatectomy
• after radiation therapy for local disease
• during hormonal and other drug therapies
• in primary and secondary chemoprevention.

Part 1 of this two-part review, which concludes in the September issue, looks at the physiology of PSA and how its measurement can predict time to disease recurrence and, in a limited setting, survival of patients following radical prostatectomy and radiation therapy.

Physiology of PSA

PSA is a serine protease in the kallikrein family of proteases; it is also called human kallikrein 3 (hK3).[3] Produced in high concentrations by prostatic epithelium, PSA is secreted mainly into seminal fluid, where it dissolves the gel that forms after ejaculation by digesting the major gel-forming proteins, thereby resulting in increased sperm motility.[4]

PSA is not a traditional tumor marker that is produced only by malignant tissue. Normal prostate tissue as well as hyperplastic and neoplastic tissue express PSA and, in fact, often produce more PSA protein than malignant prostate tissue.[5] In prostate cancer, however, the architecture and polarization of epithelial cells are deranged, disrupting normal secretory pathways and causing PSA to "leak" or to be actively secreted into extracellular space and escape into the circulation. As a result, PSA is found in the serum in concentrations 30 times higher per unit weight of cancerous tissue than of normal tissue, and 10 times higher per unit weight of cancerous tissue than of the epithelial tissue in benign prostatic hyperplasia (BPH).[3,6,7]

This is the basis for the use of PSA levels in the detection of prostate cancer. However, there is much overlap between PSA values in BPH and prostate cancer, and various refinements have been made to improve the diagnostic value of this test.[8] Some of these modifications will be considered later.

It is possible that PSA itself plays a modulating role in prostate cancer. Recent studies have suggested that men with high plasma levels of insulin growth factor (IGF)-I may have an increased risk of developing prostate cancer. IGF-I in the circulation is complexed to IGF-binding proteins, which also influence the biological activity of IGF. The major IGF binding protein is IGFBP-3, and it has been shown that PSA digests IGFBP-3.[9] This may result in higher-concentrations of IGF-I and, consequently, stimulation of cancer growth.[10,11] On the other hand, PSA may suppress tumor growth by inducing formation of angiostatin.[12]

PSA is also expressed in far lower concentrations by tissues other than prostate epithelium, including the periurethral glands, endometrium, normal breast tissue, breast tumors, breast milk, adrenal neoplasms, parotid gland, and renal cell carcinomas.[8] Nevertheless, in the sera of women, concentrations detected by ultrasensitive assays are rarely higher than 0.5 ng/mL and usually range from 0 to 0.2 ng/mL.[13,14]

Much of the PSA released into the circulation forms complexes with protease inhibitors in plasma. Between 50% and 90% (typically 70% to 85%) of the total assayed PSA in the circulation is complexed to alpha-1-chymotrypsin.[15] Trace amounts complex with an alpha-1-protease inhibitor and alpha-2-macroglobulin[16,17]; the remaining serum PSA is unbound or free. The proportion of complexed PSA delivered by cancerous tissue is higher than that of BPH tissue, possibly because PSA reaching the bloodstream directly can easily form complexes, whereas PSA that reaches circulation through extracellular space is susceptible to extensive proteolysis (it is then said to be "nicked" or "clipped") and is less likely to be bonded by endogenous protease inhibitors.

For this reason, the percentage of free PSA has been used to help increase the sensitivity of cancer detection when PSA is in the normal range (4.0 ng/mL or less)[18] and the specificity when PSA is in the "gray zone" (4.1 to 10.0 ng/mL).[4,8] In this context, use of the ratio of "free PSA to total PSA" has increased the specificity of total PSA levels up to 20 ng/mL without undue loss of sensitivity.[19]

PSA Measurement in Clinical Practice

Prostate-specific proteins were identified by several groups in the 1970s.[3] In 1979, Wang and colleagues, who purified the antigen from prostatic tissue and demonstrated its relationship to prostate cancer, were the first to call the protein PSA.[5]

Soon after PSA assays were developed, total serum PSA was shown to detect residual disease after radical prostatectomy and recurrence of tumor on long-term follow-up. In light of such evidence, PSA assays received approval from the US Food and Drug Administration (FDA) for monitoring therapy, and by 1988, PSA became widely used as a marker for prostate cancer.[20] In the context of monitoring, the patient provides his own reference for the assay, but in the detection of prostate cancer, interassay variability became a concern.

The first widely used assay in the United States, Tandem-R (Hybritech), established a reference range for PSA-4.0 ng/mL or less-which was found in 97% of 207 apparently healthy men aged 40 years and older. This and subsequent studies confirming the validity of this reference range[21,22] provided the basis for the commonly used cutoff of 4.0 ng/mL as the normal total PSA in men age 40 years and older.

Most other tests were interpreted on the basis of this reference range, but not all assay methods measure the same PSA concentration, and each assay has its own reference range. Even when reference ranges are similar, tests differ greatly in their upper limits for BPH. The only two assays available in the United States until 1991-the monoclonal Tandem-R and the polyclonal Pros-Check (Yang Laboratories)-differed considerably, sometimes by a factor of two.[23] Since that time, most assays have been thoroughly evaluated and modified to use reference samples as a standard.

Fine Tuning the Accuracy of PSA Tests

At present, most commercial PSA tests are sensitive monoclonal immunoassays that measure PSA-alpha-1-chymotrypsin and free PSA (total PSA). The affinity of the antibodies used in these assays against the different forms of PSA varies, sometimes resulting in a nonequimolar response to free and complexed forms of PSA. Some assays may preferentially detect more free PSA forms and thus overreport percent free PSA levels in patients with BPH, which may limit interpretation.[24] Only assays that have an equimolar reaction with free PSA and PSA complexes show similar measurements. An assay that does not detect all forms of PSA on an equimolar basis is most accurate when it corresponds with a calibration standard.[4,23]

Recognizing these problems, many manufacturers of nonequimolar assays have modified their assays to produce equimolar responses to the complexed and free forms of PSA.[23] In addition, through an international effort, the Expert Committee on Biological Standardization of the World Health Organization recently established standard reference preparations to validate and calibrate PSA assays.[25,26] One reference preparation is a standard containing 1 mg of free PSA, and the other is a preparation containing 1 mg of total PSA, including 0.1 mg free PSA and 0.9 mg PSA complexed with alpha-1-chymotrypsin. This 90:10 proportion is typical of the sera in men with prostate cancer. Applying this 90:10 calibrator to nine different PSA assays reduced the coefficients of variation among them from 28.3% to 9.5%.[25] This standard has been used to calibrate the last five PSA assays approved by the FDA, and most manufacturers now use these standards.[25]

PSA as a Marker of Therapeutic Response

Especially after radical prostatectomy, but also after other definitive treatments such as radiation or hormonal therapy, PSA levels have been found to correlate with disease activity. In early series from several institutions, patients with no detectable PSA postprostatectomy had no evidence of residual disease, whereas detectable postoperative PSA concentrations correlated with subsequent local recurrence or distant metastases-sometimes occurring as long as 1 to 3 years later. Similar findings were made after radiation and hormonal therapy.[13,27]

The limits of PSA detectability have been extended by ultrasensitive PSA assays that can accurately measure serum PSA levels as low as 0.01 to 0.001 ng/mL.[8,28] Ultrasensitive PSA assays have made it more difficult to distinguish between serum PSA concentrations produced by prostate tissue (including regrowth of cancerous prostate cells) and those produced by other tissues. But long-term follow-up of radical prostatectomy patients has indicated that postsurgical PSA levels as low as 0.01 to 0.07 ng/mL may predict recurrence.[29]

In a recent study using PSA thresholds of 0.2, 0.3, and 0.4 ng/mL, the risks of a continued rise in PSA in the next 3 years were 49%, 62%, and 72%, respectively, leading the investigators to conclude that PSA levels of 0.4 ng/mL or greater best defined a recurrence prompting further therapy.[30] Other large series have used 0.2 ng/mL to indicate disease recurrence.[31] These studies have firmly established the clinical utility of PSA in the postoperative setting, and as these and other studies mature with longer follow-up, it is likely that clinical progression will be seen in virtually all patients experiencing any PSA recurrence.

Other PSA-Based Parameters

FIGURE 2

Using PSA Velocity to Distinguish Prostate Cancer From Benign Prostatic Hyperplasia

Additional measures that improve the predictive value of PSA after definitive therapy are the time to PSA recurrence and the rate at which PSA rises. Pound et al[2] found that the timing of PSA recurrence after surgery, along with preoperative PSA level, accurately predicted disease-free survival and the pattern of recurrence-ie, local vs distant disease recurrence.

The concepts of PSA velocity and PSA doubling time have been used to characterize the rate at which PSA rises. PSA velocity requires at least three measurements over a 2-year period, or at least 12 to 18 months apart, to characterize the change in PSA level per unit of time. First introduced to improve the ability of PSA to detect prostate cancer, velocity differences were significant between men with cancer and men with BPH up to 9 years before diagnosis (Figure 2).[32]

FIGURE 3

Predicting Disease Progression

• PSA Doubling Time-When PSA becomes detectable after radical prostatectomy, it tends to increase exponentially, so that a plot of log PSA values against time is linear. PSA doubling time is calculated by dividing the slope of this plot into the natural log of 2 (0.693).[2,33] A doubling time of less than 6 months was found to suggest metastatic disease.[34] Patel et al[33] found doubling time to be a better predictor of risk of recurrence and time to clinical recurrence after radical prostatectomy than preoperative PSA, specimen Gleason sum, or pathologic stage.

In a large radical prostatectomy series (1,997 men) at The Johns Hopkins Hospital, the timing and natural history of disease progression of men with an elevated PSA after therapy were studied. The time to PSA elevation, Gleason score, and PSA doubling time were significant predictors of the probability of and time to development of metastases. A doubling time of 10 months provided the most statistically significant predictor of time to distant disease progression after an isolated elevation in PSA (Figure 3).[2]

Monitoring PSA After Radical Prostatectomy

Follow-up of men after radical prostatectomy is extensive and clearly shows that PSA is an excellent marker of tumor recurrence, progression, and response. Far fewer trials, however, have adequately addressed the question of whether altering PSA levels correlates with survival.

Progression and Treatment Response

Lange et al[35] demonstrated that a measurable PSA level (> 0.4 ng/mL) in the early postoperative period correlated with disease recurrence in patients who received no additional radiation or hormonal therapy. By 1991, several large series demonstrated that rising postsurgical PSA levels correlated with advancing pathologic stage of disease.[27]

PSA doubling time was later shown to be a significant predictor of postprostatectomy risk of recurrence and time to recurrence. In the study by Patel et al[33] cited above, patients had a 41% risk of disease recurrence in 2 years when their PSA doubling time was less than 4.6 months (log slope > 0.15), compared with a 17% risk when their PSA doubling time was greater than 14 months (log slope = 0.05-0.1).

FIGURE 4

Predicting Time of Death

Today, large series with longer follow-up have more completely characterized how PSA levels mark the natural history of postprostatectomy cancer progression. The large Johns Hopkins Hospital study[2] definitively established that a rising PSA heralds the recurrence of prostate cancer. This study also clearly established that the timing of the reappearance of PSA after surgery, along with the preoperative PSA level, could accurately predict how long a man would remain disease-free and whether the metastases were local or distant. Moreover, the study was the first to document the typical period from metastasis to death (among patients who received no additional therapy) and demonstrated that the timing of the development of distant metastases after surgery could accurately predict the time from metastasis to death for an individual patient (Figure 4).[2]

Highlighting the long natural history of prostate cancer, this study showed that the likelihood of surviving metastasis-free for 15 years after prostatectomy is 82%. Although 45% of patients who had a recurrence of PSA manifested evidence of recurrence within 1 to 2 years, the time to recurrence can be very long-a small percentage (4%) of men in this study had no detectable PSA for at least 10 years after surgery.

Among the 104 men who developed distant metastases, the mean time from an elevation of PSA to clinically evident metastasis was 8 years. A PSA doubling time greater or less than 10 months was the most significant predictor of that interval (Figure 3). Men who had a PSA doubling time of less than 10 months had only a 35% chance of remaining free of metastatic disease for 5 years after a rise in PSA.

Alone or in combination with other factors, however, PSA levels did not predict the time to death after the development of metastatic disease. The best predictor of time to death was time from surgery to the development of clinically evident metastatic disease. The median time to death after the development of metastatic disease was a little less than 5 years. Men who did not develop metastases until 8 or more years after surgery had a 78% chance of surviving another 5 years, whereas those who developed metastases within 1 to 3 years had only a 50% likelihood of surviving that long. This study clearly demonstrates that changes in PSA levels can serve as a surrogate for survival (or development of metastases) following surgery.

• Ultrasensitive Assays-Ultrasensitive PSA assays have the potential to allow earlier, more accurate prediction of prostate cancer recurrence after radical prostatectomy. In 1992, it was shown that ultrasensitive assay could have detected a rise in PSA 175 and 581 days earlier than standard assays in two patients who had a clinical recurrence of disease.[36] The ultrasensitive assay detected PSA at levels of 0.1 ng/mL, compared with a 0.3 ng/mL detection limit for the standard assays used. In a subsequent study, PSA recurrence at levels from 0.01 to 0.07 ng/mL (within an average of 569 to 589 days) was correlated with later recurrence of prostate cancer in 23 cases that had apparently been cured (showing no residual prostate cells).[29]

In a study in 148 patients with postoperative PSA values less than 0.1 ng/mL on conventional PSA assays, postoperative PSA increases of 0.1 ng/mL to as little as 0.001 ng/mL detected by ultrasensitive assay correlated significantly with indicators of a poor prognosis, such as positive surgical margins, larger tumor volumes, greater preoperative PSA, and extracapsular extension.[37]

Detectable PSA limits have been lowered by new assays that improve antibody affinity or specificity to ligand or use monoclonal-monoclonal or monoclonal-polyclonal combinations that differ from standard assays. In addition, standard PSA assays have been made ultrasensitive by applying them to concentrated serum.

Using the latter method, Haese et al[38] improved the timing of the detection of recurrence by nearly 300 days, discovering 86% of relapses within 1 year of radical prostatectomy, compared with 25% found by standard assay. Of 422 patients observed from 1 month to 5 years, 88 demonstrated a PSA recurrence (an increase of at least 0.10 ng/mL) by ultrasensitive assay with concentrated serum. Of these, 28 (31%) had recurrence detected by ultrasensitive assay that was subsequently detected on standard assay of native serum. A total of 37 patients (42%) showed evidence of recurrence simultaneously on both assays, and 23 (26%) had recurrence measured by ultrasensitive assay but not on standard assay.

The investigators noted that in the simultaneous detection group, ultrasensitive detection would likely have been achieved earlier if sampling intervals (6 months during the first year and 1 year thereafter) had been shorter. They also speculated that follow-up was likely not long enough to detect recurrence by standard assay in patients with recurrence detected by ultrasensitive assay only.

• Percent Free PSA-Measurement of the percentage of free PSA has been less successful than the use of ultrasensitive assays as a means of improving prediction of the postprostatectomy course of prostate cancer. Although measuring the percentage of free PSA increases the sensitivity of cancer detection when PSA is in the normal range (£ 4.0 ng/mL),[18] and the specificity of cancer detection with PSA levels up to 10 ng/mL,[4] such measurement has proven to be of variable value after prostatectomy.

In a study of 46 men with recurrent or persistent cancer after radical prostatectomy, the median value of free PSA was significantly lower (P < .0001) than median PSA values in a previously defined population of 413 men who had either benign disease (225 men) or prostate cancer before prostatectomy (188 men).[39] The median value for men with recurrent cancer was 8.5% free PSA, compared with 11.7% for the preoperative cancer patients and 17.4% for the benign group. Nevertheless, a significant proportion of men with aggressive tumors showed high free PSA values: Four (9%) had values ranging from 15% to 19%, and another four had values of 20% or greater. All men in the latter group had evidence of seminal vesicle or lymph node involvement.

Despite its promise, percent free PSA has proven too variable to assess patients postprostatectomy. Instead, that role may eventually be filled by further refining measurement of PSA complexes or by the use of new markers, such as human glandular kallikrein 2 (hK2)[40] or benign PSA (BPSA, a specific molecular form of free PSA).[41]

• Conclusions-Although refinements of PSA measurement and new related markers are being sought to improve prediction of the postprostatectomy course of disease, that PSA is a marker of disease activity in this setting is indisputable. Without question, a detectable PSA after surgery means that cancer has returned. The timing of PSA recurrence and its rate of change accurately predict the timing and pattern of disease recurrence.

PSA and Survival

Whether PSA elevation after prostatectomy predicts survival is less clear, and fewer studies have addressed this question. Two longer-term studies have attempted to do so, but both suffer from limitations.

• Cleveland Clinic Foundation Study-Jhaveri et al[42] attempted to correlate recurrence of PSA after radical prostatectomy with overall survival in a series of 1,132 patients followed for up to 10.4 years. The median time to PSA recurrence was 22 months. Overall, 10-year survival did not differ significantly between those who had a recurrence of PSA and those who did not (88% vs 93%).

The study was limited in its ability to demonstrate the correlation. Unlike the Johns Hopkins series,[2] men who received additional therapy (hormones or radiation or both) were included in the analysis, but no significant differences in PSA and survival emerged between those two groups overall. The main limitation was that the small number of deaths during the 12-year study period-only 24-limited the statistical power of the observations.

Differences did emerge, however, based on pathologic characteristics, although they did not reach statistical significance. PSA recurrence was a better predictor of 10-year survival among patients at high risk of progression, such as those with a specimen Gleason score of 7 or greater, extracapsular extension, positive surgical margins, or seminal vesicle invasion.

• Duke University Study-Iselin et al conducted a study that examined survival after radical prostatectomy among patients with organ-confined disease and had the longest follow-up yet reported.[43] The study included 1,242 men who underwent surgery from 1972 to 1996. The longest follow-up of a patient in the study was 22 years, and the median follow-up was 4 years. Median follow-up of patients who had PSA measured preoperatively was 2.7 years. Patients did not receive additional therapy, such as hormones or radiation, until there was clinical or PSA evidence of disease recurrence.

Even with this long follow-up, the death rate was low, with an estimated 103 cancer-associated deaths, or a cancer-associated death rate of 8.3%. Cancer-associated death was defined as any death of a patient who either had a detectable PSA level (0.5 ng/mL or above) or had undergone androgen-deprivation therapy. The median time to noncancer death was 19.3 years, and the median time to cancer-associated death in patients with margin-positive disease was 12.7 years.

There were no differences among subgroups in the overall number of deaths. Differences emerged, however, in cancer-associated deaths when patients were stratified by organ-confined, specimen-confined, or margin-positive disease. Cancer-associated survival was significantly shorter among men with margin-positive disease than among men in the other two groups. Survival was not significantly different, however, between the organ- and specimen-confined groups. In contrast, time to PSA recurrence (0.5 ng/mL or greater) was significantly different among all three groups. At 5 years, the PSA recurrence rate for men with organ-confined disease was only 8%, but for men with specimen-confined disease, it was 35% and for men with margin-positive disease, 65%. For the whole population, recurrence of a detectable PSA preceded death by about 10 years.

The widest differences in time to PSA recurrence emerged when the population was stratified by Gleason score. For patients with Gleason scores of 5 and 6, the median time to recurrence had not been reached. For those with a Gleason score of 7, the median time to recurrence was 6 years, and for those with Gleason scores of 8 and higher, it was 2.2 years. PSA recurrence preceded cancer-associated death by 11 years for a Gleason score of 7, by 9.5 years for a Gleason score of 8, and by 7.5 years for Gleason scores of 9 to 10. The median time to death had not been reached for lower grades, but it is longer than the 11 years observed for patients with Gleason 7 cancers. Assuming that, on average, death occurs 10 years after PSA recurrence, cancer-associated death for patients with Gleason 7 tumors is estimated to occur about 16 years later, and for patients with lower Gleason scores, whose median time to recurrence is greater than 6 years, the interval to death is likely to be longer.

• PSA, Survival, and Risk-These estimates by Iselin et al are consistent with Pound and colleagues’ characterization of the natural history of postprostatectomy cancer,[2] and also with the data from the study by Jhaveri et al[42]; no correlation was demonstrated between PSA levels and survival at 10 years. Even with a follow-up as long as 20 years, these data suggest that such a period of time is not long enough to correlate PSA with survival for lower grades of prostate cancer. With a time from PSA recurrence to death of 15 years or longer, and times to PSA recurrence also very long for lower-risk patients, PSA data cannot be used to predict survival for the average patient who undergoes radical prostatectomy today. Patients in this era of early detection by PSA are likely to have earlier-stage, more curable disease than in years past.

Indeed, Jhaveri and colleagues[44] have found that the rate of organ-confined disease in PSA-detected cancers has increased over the past decade, while the mean PSA level at diagnosis has remained stable. Thus PSA screening produces not only migration to an earlier clinical stage of disease,[45] but also a downward pathologic stage migration, resulting in a more favorable pathologic picture for each clinical stage of disease. This has important implications in the management of patients with prostate cancer.

In the radical prostatectomy setting, PSA is a better predictor of survival in patients at high risk. As a result, trials using survival as an end point must focus on additional therapy for high-risk patients-those with high-grade tumors or metastases-in the postprostatectomy period.

PSA and Radiation Therapy

PSA values do not change in the same way after radiation therapy as they do after radical prostatectomy. Typically, PSA levels decline more slowly after radiation therapy than after radical prostatectomy and may never reach levels that are undetectable.[46] Nevertheless, with radiation therapy, posttreatment PSA measurement successfully assesses treatment efficacy and disease recurrence, predicts the time from biochemical to clinical recurrence, and identifies patients at high risk (ie, those with aggressive disease who are candidates for additional therapy). Also, as with radical prostatectomy, pretreatment PSA assessments predict the time to biochemical disease recurrence.

Defining PSA Recurrence

Before 1996, various PSA end points were used to mark biochemical disease recurrence (also termed biochemical failure) after radiation therapy: the lowest posttreatment value (PSA nadir), the time to achieve the PSA nadir, and whether PSA is continually rising after a period of stabilization.

In reviewing data on PSA and disease recurrence, a consensus panel of the American Society for Therapeutic Radiology and Oncology (ASTRO) developed guidelines that more specifically defined biochemical (PSA) failure after radiation therapy.[46] The panel found the best correlate with disease recurrence to be a consistently rising PSA level, which it defined as three consecutive increases in PSA. For clinical trials, the panel defined the date of biochemical failure as the midpoint between the postradiation nadir PSA and the first of three consecutive rises. The panel recommended measuring PSA levels at 3- or 4-month intervals in the first 2 years after treatment and every 6 months thereafter.

Three consecutive rises in PSA were recommended because in a subset of radiation therapy patients, PSA may rise on two successive measurements and then fall, a pattern that is dubbed "bouncing."[46] At Fox Chase Cancer Center in Philadelphia, where about one-third of patients who underwent three-dimensional conformal radiation therapy (3D CRT) for prostate cancer experienced a bounce,[47] this phenomenon was associated with lower radiation doses, high pretreatment PSA levels, and increased risk of failure. Nevertheless, nearly half of the patients who experienced a PSA bounce then demonstrated a sustained PSA nadir. Similarly, temporary increases in PSA were found in about one-third of patients undergoing radioactive seed implantation followed by external-beam radiation therapy,[48] but the phenomenon has not yet proven to have prognostic significance.

Predicting Postirradiation Disease Recurrence

FIGURE 5

Predicting Postirradiation Disease Recurrence

Although the absolute PSA nadir does not define the timing and site of disease recurrence, it is nevertheless an important predictor of both biochemical and clinical disease recurrence after radiation therapy (Figure 5).[49]

Lee et al[50] found that PSA nadir was an independent predictor of time to biochemical disease-free survival in men who underwent external-beam irradiation for clinically localized (T1-3) prostate cancer. In this study, the 364 patients were followed for a minimum of 24 months and a mean of 46 months. At 3 years, 93% of men with the lowest PSA nadirs-0 to 0.99 ng/mL-had no biochemical evidence of recurrence vs 49% of those with nadirs of 1.0 to 1.99 ng/mL and 16% of those with nadirs of 2.0 ng/mL or more (P = .0001). In men with less favorable pretreatment characteristics, the lower the PSA nadir, the better the outcome. In a recent study,[51] PSA nadir was found to be an independent predictor of 5-year freedom from distant metastases in 246 men who were treated mainly with 3D CRT (27% also received initial hormonal therapy).

FIGURE 6

Time to Biochemical Failure After 3D CRT in Men With Prostate Cancer

The value and robustness of the ASTRO definition of biochemical failure after radiation therapy for prostate cancer, as described above,[46] was confirmed by Hanlon and Hanks.[52] In their study of 670 men with prostate cancer treated with 3D CRT, they noted that the use of ASTRO guidelines demonstrated little risk of biochemical failure after 4 years following treatment (Figure 6).[52]

Pretreatment PSA has long been recognized as a strong predictor of biochemical and clinical disease recurrence after radiation therapy,[53] with lower PSA values predicting better outcome. In a series of 371 patients who received external-beam irradiation, the pretreatment PSA proved to be an independent predictor of clinical recurrence of disease (Figure 7),[49] and PSA nadir values were highly dependent on pretreatment PSA levels. Among patients with pretreatment PSA values less than 10 ng/mL, 85% were free of clinical evidence of disease at 5 years. Of those with pretreatment PSA levels between 10.1 and 20 ng/mL, nearly 60% were free of disease at 5 years. When pretreatment PSA levels exceeded 20 ng/mL, only 27% were free of disease at 5 years.

Selecting High-Risk Patients Before Treatment

PSA doubling times before radiation therapy were found to be the strongest independent predictor of biochemical disease recurrence after external-beam irradiation in a multivariate analysis of variables that included radiation dosage, technique, and Gleason score.[54] Patients had at least three pretreatment PSA determinations obtained over 60 days or more (median = 9 months). Among the 108 patients with T1-3 prostate cancer, the variables that predicted time to biochemical recurrence included PSA doubling times less than 12 months, greater than 12 months, and not rising (P = .007).

FIGURE 7

Predicting Disease Recurrence After Radiation Therapy

Further analysis of data from 99 patients (Figure 8)[55] showed PSA doubling time (< 12 months vs > 12 months) to correlate best with biochemical disease recurrence among an even wider set of pretreatment variables tested in a regression analysis: Gleason score (2-4, 5-7, or 8-10), center of prostate dose, American Joint Commission on Cancer tumor stage (T1, T2, or T3), palpation stage (T1, T2, or T3), and technique (whole pelvis or prostate only). The only other variable that bore a relationship with biochemical recurrence was center of prostate dose.

Doubling times among the 67 patients whose PSA did rise before treatment ranged widely, from 4 to more than 33 months. At 18 months, only 53% of the men with very fast doubling times (< 12 months) had no biochemical evidence of disease, compared with 89% of men with slower doubling times and 97% of men whose PSA levels did not rise. Rapid PSA doubling times correlated significantly (P = .002) with high pretreatment PSA levels (> 10 ng/mL). Based on these findings, patients at Fox Chase Cancer Center (where the study was conducted) who have fast pretreatment PSA doubling times are considered at high risk and are offered androgen deprivation as additional therapy.

Selecting Appropriate Radiation Doses

FIGURE 8

Using PSA Doubling Time to Predict Biochemical Disease Recurrence

High-dose 3D CRT has been shown to have a greater survival benefit than low-dose treatment,[56,57] but pretreatment PSA levels, along with tumor characteristics, have proven to be critical in determining optimal radiation doses.

Hanks et al[57] analyzed data from 618 patients who had undergone this type of therapy from 1989 to 1997. Median follow-up was 6 years. Patients were stratified by pretreatment PSA level-less than 10 ng/mL, 10 to 19.9 ng/mL, and 20 ng/mL and higher. Within these groups, patients were further divided into six subgroups by favorable (stage T1/2a, Gleason score £ 6, and no perineural invasion) or unfavorable (stage T2b/3, Gleason score 7-10, or perineural invasion) tumor characteristics. Rates of 5-year freedom from biochemical recurrence (bNED) as a function of dose were compared among the subgroups.

Significant differences in response emerged among dosages in each PSA subgroup, except for patients with the lowest pretreatment PSA levels and favorable tumor characteristics and those with the highest PSA levels and unfavorable tumor characteristics. This has established the optimal radiation doses for six subgroups of prostate cancer patients based, in part, on their pretreatment PSA levels. Figure 9 illustrates the 5-year bNED for the group with pretreatment PSA levels ranging from 10 to 19.9 ng/mL.[57]

FIGURE 9

Dose-Response Functions After Three-Dimensional Conformal Radiation Therapy

These findings are important for the management of patients with prostate cancer, because the dose-response curve for this form of radiation therapy is steep. Attainment of the optimum dose (usually between 70 and 80 Gy) is absolutely critical. Thus, the more accurate the estimation of radiation dose for the individual patient, the greater the likelihood of cure with a minimum of morbidity.[57]

PSA Doubling Time After Radiation Therapy

PSA doubling time has proven to be the most valuable predictor of disease recurrence after radiation therapy, as well as the best means of identifying high-risk patients who are candidates for additional therapy.

D’Amico and Hanks[58] found that posttreatment PSA doublng time accurately predicted the time to clinically evident disease recurrence based on the correlation they demonstrated between these two variables. They analyzed data from 22 patients with T1-3 cancer who underwent either standard external-beam irradiation or 3D CRT at Fox Chase Cancer Center and who had an elevation in PSA as the only sign of disease recurrence. Clinical recurrence was defined as detection of disease by bone scan, physical examination, or computed tomography.

The relationship between PSA recurrence and clinically evident disease was directly linear. Because a direct correlation between pretreatment PSA levels and tumor volume had already been demonstrated,[6] D’Amico and Hanks suggested that a constant PSA doubling time, indicating an exponential rise in PSA, reflected posttreatment disease activity-the exponential growth of tumor cells.[58] This correlation may assist clinicians in making the decision to treat, rather than observe, a patient presenting with prostate cancer.

PSA doubling time was multiplied by 4.5 to calculate the time to clinical recurrence, allowing the investigators to identify candidates for additional therapy. Specifically, patients whose PSA values rose at 46 months could expect at least an additional 5 disease-free years, and therefore, could delay androgen-deprivation therapy. Those who had a PSA recurrence at 8.6 months could expect at least 1 additional year without clinical evidence of disease and were considered candidates for prompt intervention.[58]

Follow-up of patients at Fox Chase has confirmed the value of this method in identifying men for whom additional therapy is appropriate, as well as the effectiveness of this therapy and how long it may extend their lives. From 1989 to 1997, 246 men with T2 to T4 cancer underwent 3D CRT and had biochemical recurrence of disease. With a median follow-up of 4 years, no variables could yet predict cause-specific or overall survival, although a number of variables, including PSA nadir and PSA doubling time, were significant, independent predictors of whether distant metastases would appear in 5 years.

Androgen-deprivation therapy made a significant difference in delaying distant metastases in the men who had fast doubling times. Among men who did not receive additional therapy, 43% had distant metastases at 5 years, compared with only 22% of men who underwent androgen deprivation.

Predicting Survival

The follow-up of large trials of radiation therapy and radiation followed by hormonal therapy is now becoming long enough (8 to 10 years) for differences in survival to emerge, indicating that PSA characteristics can indeed predict survival. As with radical prostatectomy studies, these survival differences are found among the highest-risk patients.

• RTOG 8610-A Radiation Therapy Oncology Group trial that compared radiation only with radiation followed by total androgen ablation (protocol 8610) was initiated in 1987. All 456 analyzable patients had bulky tumors (³ 25 cm³) with no evidence of disease dissemination beyond the regional lymph nodes (stage T2b-4).[59] Patients were randomized to receive either radiation therapy alone (230 patients) or 4 months of additional androgen-deprivation therapy (226 patients), including goserelin acetate (Zoladex), 3.6 mg subcutaneously every 4 weeks, and flutamide (Eulexin), 250 mg orally 3 times daily for 2 months before and during radiation therapy. Patients had enrolled in the trial before the era of high-dose 3D CRT (from 1987 to 1991). Thus, all patients received external-beam radiation therapy, with total pelvic doses of 45 Gy and prostate target doses of 65 to 70 Gy.

Significant differences in the incidence of local progression and distant metastases emerged at 5 years. Among patients treated with radiation only, 71% had local progression and 41% had distant metastases; among those who also underwent androgen deprivation, 46% developed local progression, and 34% developed distant metastases.[59] A recently reported follow-up[60] showed significant differences in both cause-specific and disease-specific survival at 8 years (P < .05). Overall survival was 52% and disease-specific survival was 72% in patients who had radiation therapy plus androgen ablation vs a 41% overall survival and 62% disease-specific survival in those who underwent irradiation alone. In this trial, the delay in disease progression measured by a PSA end point was ultimately associated with a survival advantage.

• RTOG 9202-A similar RTOG trial (protocol 9202) included 1,520 analyzable patients with locally advanced (T2c to T4) prostate cancer.[61] Patients received radiation therapy plus 4 months of total androgen ablation (goserelin and flutamide) and were then randomized to no further therapy or 24 months of luteinizing hormone-releasing hormone agonist (goserelin) therapy. Median follow-up is now 5 years.

At 5 years, the difference in biochemical disease-free survival (by the ASTRO definition for rising PSA) is considerable-46% for patients who received the longer-term hormonal therapy vs 21% for those who did not (P = .0001). Significant differences in freedom from distant metastases have also emerged-17% for the patients who received long-term therapy vs 11% for those who received shorter-term therapy (P = .001). Disease-specific survival at 5 years favors the long-term therapy group (92%) over the short-term therapy group (87%), but has not yet reached statistical significance (P = .07).

Two subsets of patients with worse tumor characteristics, however, did show significant differences in disease-specific survival or in both disease-specific and overall survival. One subset of patients with worse tumor characteristics (stage T2-4 tumors with Gleason scores of 8-10) showed significant differences in disease-specific survival-90% vs 86% (P = .03). Another subset (any stage tumor but with Gleason scores of 8-10) showed differences in both disease-specific survival (90% for the long-term therapy group vs 78% for the short-term therapy group, P = .007) and overall survival (80% vs 69%, P = .02). With longer follow-up, these differences will likely become significant for the whole population. These results also relate PSA (as a measure of disease recurrence) to survival.

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