2989 - Initial Experience with Adaptive Stereotactic Body Radiotherapy for Prostate Cancer

Purpose/Objective(s): Adaptive radiotherapy (ART) has been implemented for prostate stereotactic body radiotherapy (SBRT) at our institution. This study reports our clinical experience and evaluates the dosimetric impact of ART in a cohort of 50 patients (250 fractions).

Materials/Methods: Prostate ART-SBRT was delivered using daily online adaptive planning on the Varian Ethos platform. Fifty patients treated with this approach were retrospectively analyzed by comparing adaptive plans with non-adaptive (scheduled) plans. The non-adaptive plan was defined as the original treatment (reference) plan recalculated on the daily anatomy. The analysis focused on dosimetric performance, including the dose received by 95% of the planned target volume (PTV D95%), maximum point dose to the rectum and bladder, and compliance with predefined organ-at-risk (OAR) constraints. Fractions in which non-adaptive plans failed to meet target coverage or OAR criteria, particularly those with PTV D95% < 80%, considered clinically significant given their potential impact over a five-fraction SBRT course, were examined in detail.

Results:

In the majority of fractions, differences in target coverage between adaptive and non-adaptive plans were modest. For example, in 84% of fractions, the adaptive plan met the D95% prescription, while the non-adaptive plan was only 3% below prescription, a deviation that would be acceptable for treatment on non-adaptive machines. In 10% of non-adaptive plans, coverage deviations exceeded 5%, whereas no adaptive plan exceeded this threshold. Importantly, in 3.6% of non-adaptive fractions, target coverage was markedly reduced, with D95% < 80%. In some of these fractions, the apparent prostate volume was markedly increased, which could be ascribed to multiple factors, including clinical decisions to generously include more tissue due to imaging uncertainty, variations between physicians’ judgments on target coverage, prostate swelling, and deformation due to anatomical changes in surrounding organs. For organs at risk (OARs), failure to meet one or more constraints occurred in <4% of adaptive plans compared with 14% of non-adaptive plans. Although online plan dose renormalization is not currently supported by the vendor, we tested it offline and observed a potential benefit: applying a 1.6% factor increased the number of non-adaptive plans meeting planning criteria by over 60%.

Conclusion:

The dosimetric advantage of adaptive over non-adaptive planning has been quantified in our study cohort, including improvements in target coverage and OAR compliance.

Table 1. Fractions in which OAR doses were =5% lower in adaptive plans compared with non-adaptive plans, and in non-adaptive plans compared with adaptive plans.

OAR Metric	Adaptive plan lower by 5% (n)	Non-adaptive plan lower by 5% (n)
Rectum Dmax	42	21
Rectum D2cc	74	38
Rectum D10%	77	32
Rectum D20%	84	37
Rectum D50%	110	60
Bladder Dmax	1	3
Bladder D25%	102	58
Bladder D50%	59	59