Modulation of Dose Intensity in Aerodigestive Tract Cancers: Strategies to Reduce Toxicity

Dr. Rich and colleagues present a compelling argumentfor the manipulation of temporal and spatial treatment parameters inchemoradiation programs. In essence, the authors address the shielding of normaltissues from the effects of cytotoxic agents. With respect to radiotherapy, thiscan be achieved via physical shielding by computer-generated dose algorithmsusing elaborate new planning technology (eg, intensity-modulated radiationtherapy [IMRT]), chemical shielding with radioprotectants (eg, amifostine [Ethyol]),or temporal shielding by altered-fractionation schemes that exploit thedifferential alpha/beta ratios between tumor and normal tissue (eg,hyperfractionation).

Chronomodulated Administration

With regard to chemotherapy, the authors explore the concept of temporallyshielding normal tissue by chronomodulation of administration schedules based oncircadian cell-cycle rhythms. Potential benefits include dose escalation and acorresponding reduction in acute and late side effects by maximizing thetolerance of normal tissues to antineoplastic agents. As the authors point out,chronomodulated intravenous infusion of fluorouracil (5-FU) has been used in thetreatment of various gastrointestinal malignancies for some time.

Further exploitation of this principle to incorporate other cytotoxic agentsand tumor sites carries the potential for significant gain, and continuedinvestigation is certainly warranted. As Rich et al have noted, the prospectbecomes even more attractive when cost is considered, especially compared withthe exorbitant price tags associated with cutting-edge refinements in diagnosticand therapeutic radiology.

Hyperfractionated Radiation Schedules

Alterations in temporal dose sequencing have been explored in detail forradiation therapy fractionation schemes.[1-9] At the University of Florida,hyperfractionated radiation schedules have been used since 1978 to treatmalignancies of the head and neck and other sites.[10-14] Advances in physicalshielding have moved away from traditional parallel-opposed blocked fields tononcoplanar stereotactic setups using sophisticated planning software. We havealso gradually shifted to ipsilateral wedge-pair setups for certain lateralizedhead and neck malignancies, with a resultant sparing of contralateral salivarytissues.

Patient selection for this method continues to evolve, the critical conceptbeing that normal tissue excluded from the treatment volume is at low risk ofharboring subclinical disease.[15] More recently, IMRT has been used forselected lesions. Tumor volumes are carefully outlined using three-dimensional(3D) computed tomography (CT) planning with intravenous contrast. Typically, theclinical tumor volumes for 50 Gy and 70 Gy are defined with selective sparing ofthe spinal cord, salivary glands, and any other critical structures in which ahigh radiation dose is undesirable.

Spatial Manipulation in Radiation Treatment Planning

Strict adherence to anatomic landmarks and precise tumor mapping are criticalin ensuring adequate coverage of the primary tumor and nodal compartments atrisk. Numerous potential pitfalls have been addressed in recent publicationsdealing with this issue.[4,16-22] Although inherent possibilities of increasedtumor control with greater sparing of adjacent normal tissue abound with thisnew technology, we agree that its expense and complexity preclude it from rapidacceptance into many community radiation therapy practices.

The coming decade will likely witness tremendous refinements in this type ofspatial manipulation in radiation treatment planning. Corresponding advancesanticipated by Rich et al in temporal adjustments of systemic treatmentstrategies offer exciting possibilities when viewed in conjunction with theanticipated strides in IMRT and altered radiation fractionation schemes.

Our experience in using paclitaxel (Taxol) concomitantly with radiation totreat upper aerodigestive, lung, and chest wall malignancies has been mixed,especially when the radiation is administered twice daily. Mucositis,esophagitis, and skin desquamation have been noticeably more severe with thisstrategy, often to the point that treatment breaks become necessary. Althoughthese effects could potentially be ameliorated with amifostine, as noted by Richet al, the cost and complexity of daily intravenous administration have thus farprecluded its use in our department. We have found pilocarpine (Salagen) to beeffective in decreasing the severity of radiation-induced xerostomia for amodest percentage of patients, although a once-daily dosage schedule would makeits long-term use more feasible.

Conclusions

In summary, we salute the insightful perspective offered by Rich et al on thepresent and future possibilities of normal-tissue sparing with a variety oftemporal and spatial modifications of antineoplastic therapy. We share theirhope that continued advances in these areas will translate into more cures withfewer undesirable treatment sequelae.

References:

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