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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

Purpose:
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.

Methods:
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.

Results:
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.

Conclusion:
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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