Rectal Sparing by Balloon Catheter Immobilization during Intensity Modulated Radiation Therapy for Prostate Cancer

Abstract

Purpose: Escalating radiotherapy dose results in a substantial improvement in prostate cancer outcome. Obtaining an optimal plan and acceptable dose distribution would be limited by normal critical structures. Our study is to evaluate the effect of rectal balloon immobilization in reducing rectal dose during daily treatment of prostate cancer with intensity modulated radiation therapy (IMRT). Materials and Methods: From June 2005 to August 2005, 11 patients with prostate cancer underwent computed tomography (CT) simulations with and without rectal balloon (filled with 60ml air) before and at the end of IMRT course. TMS Helax treatment planning system was used to generate step-and-shoot five coplanar-field IMRT plans. Plans were separately designed with and without a rectal balloon. The prescription dose to clinical target volume (CTV) was 78 Gy in 39 fractions of 2 Gy each. CTV was determined according to the patients' risk category. The dose to 95% of planning target volume (PTV) was 78 Gy. Cumulative dose-volume histograms (DVHs) were analyzed for the CTV, PTV, rectum, and bladder. Inter-fractional prostate displacement was measured on two separate CT images with the distance between pubic bone rim/sacrum and anterior/posterior borders of prostate, respectively, at the cranial level of pubic symphysis. To assess the dosimetric difference and inter-fractional variation of balloon setup, the paired Student t-test was used. Results: Patients included in this study had the TNM stage distribution of T1cN0M0 in 4 patients, T2bN0M0 in 4 patients, T3bN0M0 in 2 patients, and T4N0M0 in 1 patient. CTV was prostate alone in 8 patients and prostate plus bilateral seminal vesicles in 3 patients. Minimum dose to CTV with and without a rectal balloon were both 79 Gy in average. Average segment number in plans with and without a rectal balloon were 43 and 42 (P=0.7), respectively. Plans with a rectal balloon showed a significant reduction in rectal volume fraction at 65 Gy (13% vs 17%, P=0.007), and at 70 Gy (9% vs 13%, P=0.005). Plans with a rectal balloon did not show difference in bladder volume fraction at 65 Gy (18% vs 16%, P=0.07). Inter-fractional prostate positions before and at the end of IMRT were similar in the distance from pubic bone to anterior border (0.90cm vs 0.96cm, P=0.6), and posterior border of prostate to sacrum (3.69cm vs 3.73cm, P=0.8). All patients tolerated their whole IMRT course with daily placement of rectal balloon. There was no grade Ⅲ acute toxicity. Conclusion: Rectal balloon immobilization during daily fractionated IMRT for prostate cancer is a feasible and reproducible procedure. It would be beneficial to reduce high dose to the rectum. The effect of prostate immobilization is acceptable with minimal inter-fractional setup variation.