Offline adaptive radiotherapy for bladder cancer using cone beam computed tomography

Abstract

We investigated if an adaptive radiotherapy approach based on cone beam CT (CBCT) acquired during radical treatment was feasible and resulted in improved dosimetric outcomes for bladder cancer patients compared to conventional planning and treatment protocol. A secondary aim was to compare a conventional plan with a theoretical online process where positioning is based on soft tissue position on a daily basis and treatment plan choice is based on bladder size. A conventional treatment plan was derived from a planning CT scan in the radical radiotherapy of five patients with muscle invasive bladder cancer. In this offline adaptive protocol using CBCT, the patients had 10 CBCT: daily CBCT for the first five fractions and then CBCT scan on a weekly basis. The first five daily CBCT in each patient were used to create a single adaptive plan for treatment from fraction eight onwards. A different process using the planning CT and the first five daily CBCT was used to create small, average and large bladder volumes, giving rise to small, average and large adaptive bladder treatment plans, respectively. In a retrospective analysis using the CBCT scans, we compared the clinical target volume (CTV) coverage using three protocols: (i) conventional; (ii) offline adaptive; and (iii) online adaptive with choice of 'plan of the day'. Daily CBCT prolonged treatment time by an average of 7 min. Two of the five patients demonstrated such variation in CTV that an offline adaptive plan was used for treatment after the first five CBCT. Comparing the offline adaptive plan with the conventional plan, the CTV coverage improved from a minimum of 60.1 to 94.7% in subsequent weekly CBCT. Using the CBCT data, modelling an online adaptive protocol showed that coverage of the CTV by the 95% prescribed dose line by small, medium and large adaptive plans were 34.9, 67.4 and 90.7% of occasions, respectively. More normal tissue was irradiated using a conventional CTV to planning target volume margin (1.5 cm) compared to an online adaptive process (0.5 cm). An offline adaptive strategy improves dose coverage in certain patients to the CTV and results in a higher conformity index compared to conventional planning. Further research in online adaptive radiation therapy for bladder cancer is indicated.

Adaptive-Predictive Organ Localization Using Cone-Beam Computed Tomography for Improved Accuracy in External Beam Radiotherapy for Bladder Cancer

“…Foroudi et al used information from CBCT during the first five fractions to create a series of plans. In this small study of five cases, they also found the online plan selection method to provide better coverage than conventional plans or an offline adaptive process (18). Their method can be applied only to patients receiving daily fractionation.…”

Adaptive radiotherapy for muscle‐invasive bladder cancer: Optimisation of plan sizes

“…Another approach is to tailor adaptive plan expansions to the patient's natural bladder variation, i.e. an individualised method, as we have done . This may provide higher conformality than the fixed plan; however, one drawback is the need to start treatment with conventional radiotherapy for the first seven fractions.…”

“…an individualised method, as we have done. 5,8 This may provide higher conformality than the fixed plan; however, one drawback is the need to start treatment with conventional radiotherapy for the first seven fractions. There is the possibility that any gain in the individualised approach is offset by the less conformal conventional treatment at the start.…”

Adaptive radiotherapy strategies for pelvic tumors – a systematic review of clinical implementations

“…In addition to one in silico implementation of online compensation [58], these were the only studies where feedback from delivered dose triggered adaptations. Contributing to differences in applied strategies and workflows could be the additional manual contouring, which was a pre-requisite in over 70% of the simulated ART workflows [22,59,60,[62][63][64][65][66][68][69][70]72,74,78,79,[81][82][83]85,86,88,89,91,93,[95][96][97][98][100][101][102][103][104][105][106][107]. To reduce the need of manual contours, the segmentation could be propagated using deformable image registration.…”