Evaluation Of Rectal Dose In Prostate Cancer Patients Having Hydrogel Spacer Insertion In Ldr Brachytherapy And Ebrt Radiotherapy

To evaluate hydrogel spacer for improved rectal dose sparing in prostate cancer patients in both LDR brachytherapy and EBRT radiotherapy. Eleven patients with prostate cancer treated using both Pd-103 seed implantation and VMAT plan were selected in this retrospective study. Patient was prescribed with LDR brachytherapy in dose of 100Gy on primary cancer site, and then treated using VMAT plan in dose of 45Gy to primary tumor and nodal regions over 25 fractions. Briefly, after completion of intraoperatively planned Pd-103 seed implantation, patient had hydrogel spacer placed between prostate and rectum. Postimplant dosimetry was analyzed using Variseed software on CT scan after identifying seeds and contouring target and OAR. VMAT plan was generated using the same CT scan, in optimization with gEUD objective for target and OARs. In plan evaluation, PTV dose coverage and OAR dose constraint were adequate to satisfy departmental planning directive following RTOG0526 and RTOG0815 guidelines. Dose to rectal wall was examined in both postimplant and VMAT plan. Rectum was outlined as a solid organ, and rectal wall was defined as 0.4cm thickness inside of rectum contour. Volume of each contour, i.e. cm3, was measured using a treatment planning system. On average, 1.06cm (SD 0.32) separation between prostate and rectum was achieved by insertion of the spacer. Rectal wall volumes were in average of 21.8cm3 and 24.1cm3 in patients with spacer and without spacer, respectively. In postimplant assessment, we found, in patient without spacer, that rectal wall doses were in average of 66.6Gy (SD 23.1), 43.4Gy (SD 12.8), 15.4Gy (SD 4.5) and 4.4Gy (SD 1.8) to the volume of 1, 2, 5 and 10cm3, respectively. In those with spacer, on average, doses were 25.4Gy (SD 7.1), 19.0Gy (SD 5.3), 9.1Gy (SD 3.4) and 3.4Gy (SD 1.1) to rectal wall volume of 1, 2, 5 and 10cm3, respectively. On the volume 1 - 5cm3, rectal wall doses decreased about 40-60% in patients with spacer, compared with those having no insertion (p<0.01 in t-test). In VMAT plan, doses were in average of 43.5Gy (SD 2.0), 41.7Gy (SD 2.6), 35.5Gy (SD 4.3) and 24.6Gy (SD 5.3) to rectal wall volume of 1, 2, 5 and 10cm3, respectively, in patients with no spacer. In patients having spacer, rectal wall doses were in average of 39.6Gy (SD 4.5), 37.2Gy (SD 4.8), 30.7Gy (SD 4.6) and 21.2Gy (SD 4.0) to the volume of 1, 2, 5and 10cm3, respectively. Decrease of rectal wall doses was about 10-15% in the patients having spacer insertion (p<0.01 in t-test). Injection of hydrogel spacer improved rectal dose sparing in prostate cancer patients in both LDR brachytherapy and EBRT radiotherapy. Analysis of biological effective doses (BED) combining EBRT and brachytherapy will be presented.