Presenter(s)
C. R. Powers1, B. Wilson2, T. Mitin2, and C. W. Williamson2; 1Oregon Health and Science University, Portland, OR, 2Department of Radiation Medicine, Oregon Health & Science University, Portland, OR
Purpose/Objective(s):
Definitive radiation therapy for bladder cancer can be challenging due to variable bladder filling, impacting both target coverage and dose to OARs. Multiple mitigation strategies exist, with varying degrees of reliability, required resources, and added time. Our institution implemented an offline adaptive “plan of the day” protocol aimed at reducing unnecessary dose delivered to OARs without compromising target coverage or workflow efficiency. We report the implementation, validation, and early clinical outcomes.Materials/Methods:
20 consecutive patients with cT1-4a, cN0 bladder cancer underwent definitive chemoradiation, 55 Gy in 20 fractions to the entire bladder. Planning was done with empty bladder CT-based simulation. The entire bladder +/- prostatic urethra was contoured as CTV. 3 separate plans were generated with the following expansions: PTV_small: 0.5 cm uniformly; PTV_intermediate: 0.5 cm inf. and lat., 1.0 cm post., 1.5 sup and ant.; PTV_large: 0.8 cm inf. and lat., 1.2 cm post., 2.0 ant. and 2.5 sup. Daily CBCT was used for patient alignment, to approximate bladder filling size, and to select the smallest plan that fully encompassed the bladder each day at the treatment console. Three representative patients were selected to reflect best case, worst case, and average scenarios: 1 treated with predominantly small plans, 1 treated with predominantly large plans, and 1 treated with average distribution of all 3 plans. Fractional and cumulative delivered dose were retrospectively recalculated using daily CBCT images for dosimetric validation.
Results:
77% of fractions were delivered with the small plan. However, almost half of patients utilized all 3 plans and 80% received at least 2 of the plans during treatment. Treatment time was an average of 12 minutes 48 seconds per fraction. Target D99% values were 54.2 Gy, 54.3 Gy, and 55.5 Gy for the average, worst-case, and best-case patients, respectively. Compared to standard-of-care treatment volumes, small bowel V54.2 Gy was reduced from 18.8 cc, 14.2 cc, and 67.4 cc to 5.3 cc, 5.9 cc, and 9.7 cc for the average, worst-case, and best-case patients, respectively, representing a mean reduction of 72%. 45% of patients experienced acute grade 2 or 3 GU toxicity, and 45% had grade 1 or 2 GI toxicity. There was no grade 4 or 5 toxicity. At a median follow up of 9 months, organ preservation was 95%, distant metastatic free survival was 75%, and overall survival was 80%.Conclusion:
80% of patients treated for bladder cancer had significant variation in inter-fraction bladder filling and utilized at least two different plans. This “plan of the day” offline adaptive radiation therapy maintains target coverage and minimizes dose delivered to OARs, resulting in improved acute GI toxicity compared to historical controls. This protocol can be implemented without specialized equipment, training, or excessive burden to the regular treatment workflow. Further follow-up and continued implementation are warranted.