2074 - Retrospective Dosimetric Comparison of Cobalt-60 and Iridium -192 Sources for Vaginal Cuff Brachytherapy in Patients with Intermediate and High Intermediate Risk Endometrial Cancer

Purpose/Objective(s): Adjuvant Vaginal Cuff Brachytherapy (VBT) after surgery is the standard of care for Intermediate and High Intermediate Risk Endometrial cancer depending on all the risk factors. There is paucity of data comparing Co-60 and Ir-192 sources for this purpose. We are doing retrospective analysis of Dosimetric Comparison Of Cobalt-60 And Iridium -192 Sources For Vaginal Cuff Brachytherapy in this study.

Materials/Methods: CT Planning images of adult patients with Intermediate and High Intermediate Risk Endometrial cancer after surgery treated with VBT alone with a single-channel vaginal cylinder were retrieved. Various structures such as the CTV, Bladder, rectum and sigmoid were delineated as per the PORTEC 4a recommendations. Points ranging from A1-A6 were plotted. Planning was done on both Co-60 (Bebig) and Ir-192 (HDR-Microselectron) sources. Optimization was done as per the PORTEC 4a recommendations such that 100% of the dose was prescribed at point A2. It was also ensured that A1 received at least 90% of the dose. The dose received by Organs at risk (OARs), CTV and the plotted points was compared in both the groups

Results: Twenty-eight patients were included in the study. The median length of the vagina treated with VBT was 4 cm (IQR- 3.5-4.5 cm). The mean CTV D90 was 88.3 (±15.5) % in the Ir-192 group and 91.5 (± 15.1) % in the Co-60 group (p=0.08). The mean dose at A1 (central vaginal apex point) was 89.9 % (± 5.1) % in the Ir-192 group and 101.5 (± 9.8) % in the Co-60 group (p<0.0001). The mean dose at A3 (5 mm lateral to A1) was 102.3 % (± 5.8) % in the Ir-192 group and 96.8 (± 9.2) % in the Co-60 group (p<0.0001). The mean D2cc doses in the Ir-192 and Co-60 group were 87.1 (±15.1) % and 84.1 (±14.4) % for the bladder (p=0.114), 91.8 (±15.4) % and 85.2 (±11.1) % for the rectum (p=0.002) and 44.7 (± 21.5) % and 38.3 (±17.3) % for the sigmoid (p=<0.0001). The mean D0.1cc doses in the Ir-192 and Co-60 group were 124 (±29.2) % and 110.2 (±34.2) % for the bladder (p=0.008), 128.6 (±23.2) % and 117.3 (±14.9) % for the rectum (p=0.001) and 65.9 (± 34.2) % and 58.6 (±44.6) % for the sigmoid (p=0.09).

Conclusion: Though CTV D90 coverage is similar with both sources, VBT with Ir-192 leads to about a 10% under-dose in the region of the central vaginal apex (A1) and higher doses to the bladder, rectum and sigmoid compared to Cobalt 60. This may translate in to better local control rate in patients using Cobalt 60. The variation in the apical dose can be attributed due to the longitudinal anisotropy of the Ir-192 source.