Online CBCT-based adaptive radiation with reduced PTV margins for prostate cancer: Does reference plan comparison tell the whole story?

BACKGROUND

Online adaptive radiation (ART) is a novel treatment approach that allows for a new daily treatment plan based on cone beam CT (CBCT) imaging. This daily adaptation facilitates precise tumor and organ-at-risk (OAR) localization and minimizes the impact of interfractional motion, allowing for planning target volume (PTV) margin reduction. Isotropic PTV margins for localized non-stereotactic adaptive prostate radiation performed on an ETHOS linear accelerator with HyperSight have been reduced from standard 7 mm to 5 mm. This study assesses the impact of margin reduction by evaluating the dose metrics of patient reference plans, as well as daily treated plans, for 7 mm vs 5 mm PTV margins.

METHODS

Patients with prostate cancer receiving moderately hypofractionated adaptive radiation were initially treated with a 7 mm PTV margin (n = 10). This retrospective study generated 5 mm PTV margin treatment plans (n = 10) for these patients for comparison. In addition, a full adaptive 20 fraction treatment course was simulated with margin reduction to identify differences not recognized with reference plan comparison alone.

RESULTS

Bladder V40.8 and V48.6 but not V60 were significantly reduced in 5 mm treatment plans compared to 7 mm treatment plans. However, when daily treated plan data was examined bladder V60 was lower for the 5 mm PTV case. Similarly, rectum doses V24.6-V57 but not V60 were significantly reduced in 5 mm PTV margin treatment plans. Further differences were identified when looking at the daily treated plan data as opposed to simply comparing reference plans.

SIGNIFICANCE

In the era of online ART, with significant data available, such as daily treated plan dosimetry, analysis of reference plans alone may not be sufficient. PTV margin reduction, made possible due to the use of online ART, reduced the volume of bladder and rectum receiving <60 Gy, which may reduce toxicity and secondary malignancy risk.