Adjuvant Radiotherapy in Bladder Cancers: A Dosimetric Study Focusing on Ileal Conduit Sparing.

Purpose: To compare ileal conduit (IC) and other organ at risk (OAR) dosimetry between treatment techniques in a prospective cohort of patients planned for adjuvant radiotherapy (RT) after radical cystectomy and IC reconstruction.

Methods And Materials: Computed tomography (CT datasets of twenty patients who underwent adjuvant RT were obtained and used prospectively for delineation of target volumes (primary and nodal) and OARs, including IC, uretero-ileal anastomosis and ileal stoma using a specified protocol for simulation including a delayed CT to identify IC. Three RT plans were generated for each patient for a dose of 54 gray (Gy) in 27 fractions (PTV V95% >95%): 3-dimensional conformal radiotherapy (3DCRT) with (3DCRT_S) and without (3DCRT_N) stoma shielding, and volumetric modulated arc therapy (VMAT), with OAR constraints specified for VMAT plans (IC: Dmax<54Gy, V50Gy < 20 cc). Constraints were given for other pelvic OARs (bowel, rectum, femur heads) as per published literature. Plans were evaluated for target coverage as well as OAR doses; in particular, IC and ileal stoma). ANOVA test was used to compare medians of achieved doses, and a p-value <0.05 was statistically significant.

Results: The median IC volume was 63.34 (55.29-82.93) cc. The cranial end of IC was at L5 or L4 vertebral level in 95% of patients and caudal level at S2 or S3 in 80% of patients. In contrast, the ileal stoma spanned from L4 or L5 vertebral level cranially (100%) to L5 level caudally (80%). PTV V95% was similar for 3DCRT_N and VMAT plans while it was significantly lower for 3DCRT_S in areas of ileal stoma shielding (99.95% vs 99.01% vs 96.29%, p < 0.01). Median IC V50Gy was comparable in 3DCRT_N (38.81 cc) and 3DCRT_S (35.62 cc) while it was significantly lower in the VMAT plan (17.05 cc, p < 0.01). IC Dmax did not differ significantly between the three plans. On the other hand, when 3DCRT_N, 3DCRT_S, and VMAT plans were compared for ileal stoma doses, Dmean was comparable (11.93 Gy vs 7.41 Gy vs 9.54 Gy, p = 0.06) while Dmax was significantly higher for 3DCRT_N plan and least for VMAT plan (35.32 Gy vs 27.57 Gy vs 24.22 Gy, p < 0.01). VMAT plans fared significantly better than both 3DCRT plans for uretero-ileal anastomosis, bowel, and rectal dosimetry.

Conclusions: Ileal stoma shielding in 3DCRT compromises PTV coverage but does not spare IC effectively. Sparing IC with VMAT is feasible without compromising PTV coverage. Dosimetric gains with VMAT are expected to benefit patients needing higher pelvic RT doses and nodal RT by reducing the risk of anastomotic and mucosal complications. Clinical benefits should be evaluated in a prospective protocol.