The effect of setup uncertainty on normal tissue sparing with IMRT for head-and-neck cancer

Purpose: Intensity-modulated radiotherapy (IMRT) is being evaluated in the management of head-and-neck cancers at several institutions, and a Radiatio n Therapy Oncology Group study of its utility in parotid sparing is under d evelopment. There is an inherent risk that the sharper dose gradients gener ated by IMRT amplify the potentially detrimental impact of setup uncertaint y. The International Commission on Radiation Units and Measurements Report 62 (ICRU-62) defined planning organ-at-risk volume (PRV) to account for pos itional uncertainties for normal tissues. The purpose of this study is to q uantify the dosimetric effect of employing PRV for the parotid gland and to evaluate the use of PRV on normal-tissue sparing in the setting of small c linical setup errors. Methods and Materials: The optimized nine-beam IMRT plans for three head-an d-neck cancer patients participating in an institutional review board appro ved parotid-sparing protocol were used as reference plans. A second optimiz ed plan was generated for each patient by adding a PRV of 5 min for the con tralateral parotid gland. The effect of these additions on the quality of t he plans was quantified, in terms of both target coverage and normal-tissue sparing. To test the value of PRV in a worst-case scenario, systematic tra nslational setup uncertainties were simulated by shifting the treatment iso center 5 mm superiorly, inferiorly, left, right, anteriorly, and posteriorl y, without altering optimized beam profiles. At each shifted isocenter, dos e distributions were recalculated, producing a total of six shifted plans w ithout PRV and six shifted plans with PRV for each patient. The effect of s etup uncertainty on parotid sparing and the value of PRV in compensating fo r the uncertainty were evaluated. Results: The addition of the PRV and reoptimization did not significantly a ffect the dose to gross tumor volume, spinal ord, or brainstem. In contrast , without any shift, the PRV did increase parotid sparing and reduce covera ge of the nodal region adjacent to the parotid gland. As expected, when the plans were shifted, the greatest increase in contralateral parotid irradia tion was noted with shifts toward the contralateral parotid gland. With the se shifts, the average volume of contralateral parotid receiving greater th an 30 Gy was reduced from 22% to 4% when a PRV was used. This correlated wi th a reduction in the average normal-tissue complication probability (NTCP) from 22% to 7%. Conclusions: The use of PRV may limit the volume of normal tissue structure s, such as the parotid gland, exceeding tolerance dose as a result of setup errors. Consequently, it will be important to incorporate the nomenclature of ICRU-62 into the design of future IMRT studies, if the clinical gains o f increased normal-tissue sparing are to be realized. (C) 2001 Elsevier Sci ence Inc.