ESTRO 2024 - Abstract Book

S5987

RTT - Treatment planning, OAR and target definitions

ESTRO 2024

determine if RTTs can contour the CTV as accurately as physicians, thereby alleviating the need to call the physician to the machine.

Material/Methods:

Seven RTTs and five physicians took part in this study. We evaluated 30 PC patients (150 treatment fractions) treated on the MRIdian system (ViewRay, CA). For each fraction, RTTs contoured the CTV (CTVRTT), and then, as per our standard of care, the physician on call contoured the CTV, blinded to the RTT contours. Treatment was delivered adapting the plan to the physician contoured CTV (CTVTreat). Offline, a single radiation oncologist, experienced in MR prostate contouring, contoured the CTV for each treatment fraction, blinded to both real-time and RTT contours. This CTV was designated CTVGT – the ground truth to which CTVRTT and CTVTreat were compared for volume and target coverage metrics. For each treatment fraction we generated PTVGT using a 3 mm uniform expansion of the CTVGT. We then created new plans optimized to 95% dose coverage to 95% of PTVGT volume. We evaluated the 95% and 93% isodose coverage for physician and RTT targets. Volume differences between the three CTVs were also investigated.

Results:

CTV volume evaluation. We used the Mann-Whitney Rank-Sum test to compare volume differences between both CTVs and CTVGT. For both CTVTreat and CTVRTT the test yielded p>0.5, not statistically significant.

Target coverage evaluation.

The ground truth generated plan covered CTVTreat and CTVRTT to 95% of the prescribed dose in 96.7% and 98% cases respectively.

PTV coverage by the 95% isodose was 46% of PTVTreat contours and 23.3% of PTVRTT contours. For 93% isodose coverage, 74% of PTVTreat contours and 48.67% of PTVRTT contours were covered.

Conclusion:

While CTV volumes and coverage were similar between physicians, RTTs, and ground truth, for PTV coverage the results were markedly worse for both physician and RTT contours. This result indicates that the differences are in the margins of the CTV contours, and are exacerbated by the 3 mm volumetric expansion to PTV. This could be due to inter-physician and/or inter-RTT variability, which is likely to be more pronounced at the boundaries of the prostate, which are less clearly defined by a 0.35T magnet.

Of note, RTTs did not receive dedicated training for contouring target volumes. We are currently initiating such training with a dedicated radiology specialist. Following this training we will reexamine RTT generated contours.

At this point in time our study results indicate that we cannot forego physician presence at treatment for CTV contouring in prostate patients treated adaptively.