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Early Experience Comparing Reduced Planning Target Volume Margins Using Cone Beam Computed Tomography Scan Guided
Samuel B Hayworth1, Whitney S Hotsinpiller1, Joel Pogue1
1Department of Radiation Oncology, The University of Alabama at Birmingham Heersink School of Medicine, Birmingham, Alabama.
Purpose:
Radiation therapy (RT) for muscle-invasive bladder cancer (MIBC) requires substantial planning target volume (PTV) margins to accommodate bladder-filling variability. We hypothesized that cone beam computed tomography (CBCT) scan guided online adaptive (AD) RT (oART) improves target coverage while reducing normal tissue exposure compared with non-AD RT.
Methods And Materials:
Over the course of a year, 8 patients with MIBC received oART. Five patients received 55 Gy in 20 fractions to a clinical target volume (CTV) that was expanded from the transurethral resection of a bladder tumor bed and 46 Gy to the remaining bladder; in contrast, 3 patients had the entire bladder designated as the high-dose CTV. PTVs were 7 mm isometric expansions of CTVs for AD treatment and non-AD (scheduled [SC]) treatment plans versus 15 mm for conventional large margin (LM) treatment plans. Target coverage and organ-at-risk (OAR) dosimetry for AD treatment plans were compared with SC and LM treatment plans per fraction using the Wilcoxon paired test. Total treatment time and acute toxicities were assessed.
Results:
The AD treatment plan was selected for the delivery of all fractions. Daily bladder volumes differed from simulation by a mean of 60 (SD, ± 66) cc. The CTV D98% > 98% was met for 160 (100%) of fractions with AD treatment plans versus 140 (87.5%) for LM and 67 (41.9%) with SC treatment plans. The CTV_high V90% = 100% for all AD treatment plans. Besides rectum_V30, AD treatment plans reduced OAR exposure for all metrics. Target and OAR objectives were met for 62.3%, 55.4%, and 91.2% of SC, LM, and AD treatment plans, respectively. Median fraction time was 24.7 minutes. Acute toxicity included only 3 grade 1 toxicity events and 2 grade 2 genitourinary or gastrointestinal toxicity events, with no occurrences of grade 3 or higher.
Conclusions:
oART achieved acceptable dosimetry for target volumes with a reduced PTV margin of 7 mm, despite considerable daily bladder volume variation. AD treatment plans improved target coverage and OAR dosimetry. Future patient-centered studies should explore the long-term clinical impact of reduced margin of oART for MIBC.
