ESTRO 2024 - Abstract Book

S5999

RTT - Treatment planning, OAR and target definitions

ESTRO 2024

online treatment. Remaining four fractions were treated using a conventional linear accelerator due to the MRL not being available.

Over 56 fractions the median (range) time to contour was 5:07 (2:08-09:25) (mm:ss). The median (range) time for RTTs to contour was 5:22 (3:38-9:25) (mm:ss) and 4:37 (2:08-7:30) (mm:ss) for clinicians. The median (range) total treatment time for 56 fractions was 27:43 (21:04-34:44) (mm:ss), 27:49 (22:07-33:22) (mm:ss) for RTT contoured fractions and 27:00 (21:04-34:44) (mm:ss) for clinician contoured fractions. Volumetric analysis was performed for 52 fractions. 29 clinician contoured fractions were contoured offline by RTTs, 23 RTT clinician-independent fractions were contoured offline by clinicians (limited due to clinician availability). The CTV median (range) DSC was 0.92 (0.86-1.00), the median (range) HD was 0.65 mm (0.39-1.40), the median (range) MDA was 0.11 mm (0.01-0.17). The PTV median (range) DSC was 0.95 (0.91-1.00). No significant difference between clinician and RTT clinical target volumes (cm 3 ) were observed (p=0.7882). In offline dosimetric analysis of the 23 online adaptive treatments plans contoured by RTTs reported on offline clinician contours, 65% (n=15) met optimal target coverage of V5225cGy > 98% and 100% (n=23) met mandatory target coverage of V5225cGy > 95%. For those that failed the optimal target coverage constraint (n=8), V5225cGy was median (range) 97.7% (96.9-98.0%) PTV coverage. For 7/8 of these fractions, clinicians were found to have contoured a larger CTV, with a median (range) increase of 3.17 cm3 (0.27-7.11), 1/8 was smaller. All 8 CTV and PTV contours, and treatment plans, were blindly reviewed by a clinician and deemed clinically acceptable with no feedback. There was no relationship between minor difference in dosimetric analysis and CTV/PTV DSC.

Conclusion:

Following effective training programmes for RTT online contouring for bladder MRIgRT on the MRL, initial service evaluation results demonstrate RTT target volume contours to be comparable to clinician contours. The time taken for RTTs and clinicians to contour was similar and overall treatment time for both groups was reasonable in reference to intra-fractional filling (1) . With further analysis to be completed, clinical implementation will release clinicians from MRL bladder treatments, requiring their attendance to contour the first fraction only.

Keywords: Bladder MRIgRT, MR Linac, Contouring

References:

1. Mitchell, A., Ingle, M., Smith, G., Chick, J., Diamantopoulos, S., Hafeez, S., et al (2022). Feasibility of tumour-focused adaptive radiotherapy for bladder cancer on the MR-linac. Clinical and translational radiation oncology, 35, 27–32. https://doi.org/10.1016/j.ctro.2022.04.008 2. Adair Smith, G., Dunlop, A., Alexander, S. E., Barnes, H., Casey, F., Chick, J., et al (2023). Evaluation of therapeutic radiographer contouring for magnetic resonance image guided online adaptive prostate radiotherapy. Radiotherapy and oncology: 180, 109457. https://doi.org/10.1016/j.radonc.2022.109457