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Distance Matters --- the Minimum CTV-To-Bladder Distance Tells All in HDR Cervical Cancer Treatment

M Li

M Li*, J Wong, M Karim, Z Gao, Y Goldberg, Morristown Memorial Hospital, Morristown, NJ

SU-E-T-459 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Bladder/rectum sparing is always the challenge in HDR 3D CT-guided brachytherapy treatment planning for cervical cancer due to the tight separation between them and High Risk Clinical Target Volume (HRCTV). In this study, we analyzed the minimum CTV-to-Bladder distance (MCBD), the minimum CTV-to-Rectum distance (MCRD), dose to 2cc of bladder (BD(2cc)) and rectum (RD(2cc)) respectively. The potential application of MCBD and MCRD as plan optimization indicators was investigated.

Total 49 consecutive HDR plans for 11 patients with stage I-IIB cervical cancer were retrospectively reviewed. The prescribed dose (PD) was either 6 Gy/fraction x 5 or 7 Gy/fraction x 4 fractions. For each plan, MCBD and MCRD were measured on relevant transverse CT slices. BD(2cc), RD(2cc), the minimum dose cover 90% HRCTV (D90) were recorded. All plans were optimized with dose constrains BD(2cc)<80%PD, RD(2cc)<70%PD, D90>90%PD.

For 49 insertions, MCRD was 14±7mm. MCBD was 2±1mm. D90 was 100±15%PD. RD(2cc) and BD(2cc) were 57±12%, 74±10% of PD respectively. The correlation coefficient of MCBD and BD(2cc) was 0.31(p=0.03), indicating that bladder dose was significantly impacted by MCBD.
Among 49 insertions, 19 cases had MCBD<=1mm with BD(2cc)>75% PD and D90 96±13%PD. 13 cases had 1mm75% PD. 17 out of 49 insertions had MCBD>2mm with D90 105±15%PD. Only 2/17 cases had BD(2cc)>75% PD. For 46/49 insertions, MCRD was >=5mm, which caused RD(2cc) extremely low.

Our study shows that bladder sparing is correlated with MCBD. 65% of 49 insertions had MCBD<=2mm. Among them, 72% cases had BD(2cc)>75%PD. We believe that MCBD and MCRD can be used as plan optimization indicators. When any of them is <2mm, the target coverage would be comprised in order to spare bladder or rectum. This allows physicians have a reasonable expectation before plan is completed and guide further insertions.

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