Study on the effect of residual setup uncertainty on prostate cancer planning target volume margin definition with frame immobilization and IGRT strategy

Abstract

Objectives: With the advancement of technology, radiation therapy for prostate cancer has improved remarkably by treatment delivery technique, immobilization device and treatment verification method. These improvements made treatment delivery more accurate and precise than in the past. As a result, there is a potential margin reduction for prostate cancer treatment. Target margin depends mainly on the accuracy of the treatment which is technique and departmental protocol dependent. This study aims to examine the optimal magnitude of planning target volume (PTV) margin needed for prostate cancer treatment with frame immobilization and image guided radiotherapy (IGRT) adaptive strategy.

Method: Daily cone beam computed tomography (CBCT) data of 65 prostate cancer patients were analyzed to calculate the PTV margin needed. 8 patients (4 received 70Gy and 4 received 76Gy) with the highest setup uncertainty were replanned using the new estimated margin. New plans were then evaluated for the clinical target volume (CTV) coverage. Comparisons were made on rectum dose and bladder dose for old plans and new plans.

Result: The estimated PTV margin was 3mm in L-R direction, 4mm in S-I and A-P direction. By the margin reduction, the mean bladder dose was reduced significantly ranging from 21% - 37.9%. (p=0.009, 0.013) The genitourinary toxicity is expected to be reduced by applying the new margin.

Conclusion: With the use of local immobilization device and adaptive offline strategy, the PTV margin can be reduced to achieve better organ sparing and provide room for dose escalation.