Biological Effective Dose (EBD) in Analysis of Rectal Dose in Prostate Cancer Patients Undergoing a Combination Therapy of EBRT and LDR With Hydrogel Spacer Insertion

Abstract

To evaluate rectal wall doses sparing in prostate cancer patients undergoing combination therapy of external beam radiotherapy (EBRT) and low-dose-rate (LDR) brachytherapy with hydrogel spacer insertion. Fifty-five patients were enrolled in this retrospective study, in which 25 patients were applied with injection of polyethylene-glycol hydrogel and others without the insertion as control. Patient was prescribed with EBRT in dose of 45Gy to primary tumor site and nodal regions over 25 fractions, and LDR in dose of 100Gy on prostate. Two weeks after spacer placement, patient underwent CT scan for VMAT planning. Intraoperative prostate seed (Pd-103) implant (PSI) was performed two weeks after EBRT completed. Doses to the target and rectal wall were examined in each plan. BED calculation was applied in evaluation of sum dose of the rectal wall in each patient according to AAPM TG-137. On average, 1.04cm interspace between prostate and rectum was achieved by spacer insertion. Rectal wall was defined as 0.4cm thickness inside of rectum contour, and the volume was in average of 13.9cm3 in patients with spacer and 12.6cm3 in those without spacer. Target dose coverage was not seen in significant difference in VMAT and PSI plans between two groups of the patients. In VMAT plan, rectal wall doses were in average of 44.8Gy, 43.6Gy, 42.3Gy, 40.0Gy and 28.8Gy to the volume of 0.1, 0.5, 1, 2 and 5cm3, respectively, in patients with no spacer. In patients with spacer, the doses reduced significantly in average of 42.7Gy, 39.9Gy, 37.7Gy, 34.2Gy and 24.5Gy to rectal volume of 0.1, 0.5, 1, 2 and 5cm3, respectively (p<0.01 or <0.05 in t-test). In postimplant dosimetry, in patient without spacer, rectal wall doses were in average of 129.1Gy, 84.2Gy, 64.8Gy, 43.8Gy and 15.3Gy to the volume of 0.1, 0.5, 1, 2 and 5cm3, respectively. In patient with spacer, the doses decreased to 46.3Gy, 35.3Gy, 28.8Gy, 21.1Gy and 9.3Gy to rectal wall volume of 0.1, 0.5, 1, 2 and 5cm3, respectively. Dose decrease in the volume 0.1 - 5cm3 was significant in patients with spacer, compared with those without insertion (p<0.01 or <0.05 in t-test). Calculation of BED was performed in order to evaluate sum dose of the rectal wall in combination of both VMAT and PSI plans. In patient without spacer, BEDsum was in average of 126.9Gy, 106.7Gy, 84.3Gy and 45.5Gy to rectal volume of 0.5, 1, 2 and 5cm3, respectively. In contrast, BEDsum was seen in decrease of 20% to 39% in patient with spacer, in which was 76.8Gy, 68.2Gy, 57.2Gy and 36.2Gy to the rectal volume of 0.5, 1, 2 and 5cm3, respectively. Insertion of hydrogel spacer significantly improved rectal dose sparing, about 20% to 39%, in prostate cancer patient in EBRT and LDR combination therapy.