Katrine Smedegaard Storm1, Lina M Åström2,3, Patrik Sibolt4, Eva Serup-Hansen4, Claus P Behrens5, Gitte Persson4,6
1Herlev and gentofte Hospital, Oncology dept, Copenhagen, Denmark; 2Herlev and Gentofte hospital, oncology dept, Copenhagen, Denmark; 3Technical University of Denmark, Dept of Health Technology, Roskilde, Denmark; 4Herlev and Gentofte Hospital, Oncology dept, Copenhagen, Denmark; 5Herlev and Gentofte Hospital, Oncology dept, copenhagen, Denmark; 6Faculty of Health Sciences, University of Copenhagen, Dept of Clinical Medicine, Copenhagen, Denmark
In daily online adaptive radiotherapy (oART) of bladder cancer, the impact of inter-fractional anatomical variability is reduced, allowing for planning target volume (PTV) margin reduction. However, contouring and re-optimization requires time and may increase total treatment time and thus increase the risk of intra-fractional variation, especially filling of the bladder and bowel. Patient-specific PTV margins may be optimal, but for the initial fractions a population-based margin is needed. This study assesses the intra-fractional variation in bladder volume during CBCT-guided oART.
Seventeen consecutive patients with bladder cancer treated with curatively intended oART from August 2019 to February 2021 were included for analysis. Two patients had urinary catheter. In total, 132 adaptive fractions were analyzed, where the number of fractions ranged from 4 to 11 among the patients. Patients were instructed not to drink or eat two hours prior to treatment. One cone beam-CT (CBCT) was acquired prior to the adaptive process and a second prior to treatment delivery. The interval between the two scans was defined as the total adaptive duration (TAD). The bladder was retrospectively contoured on all CBCTs by a physician. Volume differences, Dice similarity coefficients (DSC) and center of mass shifts (CMS) between the two scans from each fraction were calculated.
The median number of adapted fractions per patient were 8 (range 4-10). Median [interquartile range (IQR)] TAD was 14 minutes [8, 17] minutes (Table 1). The median [IQR] increase in bladder volume from first to second CBCT was 8.5 cm3 [4.7–13.3] cm3(Figure 1). The median [IQR] DSC was 0.88 [0.85, 0.91]. Median [IQR] CMS was 0.02 cm [-0.05, 0.11] cm. Largest shift was observed in the cranio-caudal direction, ranging from 0.02 to 0.20 cm. No difference was observed in any of the analyzed parameters when excluding patients with urinary catheter.
The intra-fractional variations during oART of bladder cancer were limited, despite the added steps of the adaptive process. The introduction of a strict bladder filling regimen may be an explanation of the small volume changes. The use of urinary catheter did not show any difference in this study.