Dosimetric Changes Resulting From Patient Rotational Setup Errors in Proton Therapy Prostate Plans
Received 3 March 2008; received in revised form 26 August 2008; accepted 26 August 2008. published online 05 December 2008.
Purpose
To evaluate the dose changes to the target and critical structures from rotational setup errors in prostate cancer patients treated with proton therapy.
Methods and Materials
A total of 70 plans were analyzed for 10 patients treated with parallel-opposed proton beams to a dose of 7,600 60Co-cGy-equivalent (CcGE) in 200 CcGE fractions to the clinical target volume (i.e., prostate and proximal seminal vesicles). Rotational setup errors of +3°, −3°, +5°, and −5° (to simulate pelvic tilt) were generated by adjusting the gantry. Horizontal couch shifts of +3° and −3° (to simulate longitudinal setup variability) were also generated. Verification plans were recomputed, keeping the same treatment parameters as the control.
Results
All changes shown are for 38 fractions. The mean clinical target volume dose was 7,780 CcGE. The mean change in the clinical target volume dose in the worse case scenario for all shifts was 2 CcGE (absolute range in worst case scenario, 7,729–7,848 CcGE). The mean changes in the critical organ dose in the worst case scenario was 6 CcGE (bladder), 18 CcGE (rectum), 36 CcGE (anterior rectal wall), and 141 CcGE (femoral heads) for all plans. In general, the percentage of change in the worse case scenario for all shifts to the critical structures was <5%. Deviations in the absolute percentage of volume of organ receiving 45 and 70 Gy for the bladder and rectum were <2% for all plans.
Conclusion
Patient rotational movements of 3° and 5° and horizontal couch shifts of 3° in prostate proton planning did not confer clinically significant dose changes to the target volumes or critical structures.
∗Division of Radiation Oncology, University of Texas M. D. Anderson Cancer Center, Houston, TX
Reprint requests to: Andrew K. Lee, M.D., M.P.H., Department of Radiation Oncology, University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Blvd., Unit 1202, Houston, TX 77030-4009. Tel: (713) 563-2348; Fax: (713) 563-6940
ABSTRACT