Treatment Planning of Obese Prostate Cancer Patients on Halcyon/Ethos with 6 MV-FFF Using Raystation
Abstract
Purpose
To assess dosimetric impact of Halcyon/Ethos (HE) 6 MV flattening filter-free (6 MV-FFF) energy limitation when planning obese prostate patients with nodal involvement. Specifically, this study aims to develop and validate an optimization template tailored for large body habitus and compare the resulting plans against 10 MV flattening filter (10 MV-FF) TrueBeam (TB) plans, a current institutional standard.
Methods
Ten obese patients (body mass index >30) treated to 5040 cGy in 28 fractions for prostate with pelvic nodal irradiation were retrospectively replanned in RayStation v11A. Three plans were generated per patient: HE 6 MV-FFF (HE), TB 6 MV-FFF (TB6), and TB 10 MV-FF (TB10), using identical objectives and normalized to equivalent target coverage. To assess the impact of extreme patient size, body habitus was synthetically expanded by 10 cm anteriorly and laterally, followed by re-optimization. Doses to rectum, bladder, femoral heads, bowel bag, and skin, as well as target homogeneity and dose gradient indices were analyzed.
Results
Dose distributions were qualitatively similar across platforms. Mean doses (cGy) for HE vs. TB10 were comparable for rectum (1794.7 vs. 1792.7), femoral heads (896.7 vs. 899.5), and bowel bag (1194.7 vs. 1165.0). HE achieved the lowest bladder mean dose (2723.5) compared to TB6 (2772.3) and TB10 (2766.0). Mean skin doses (cGy) were higher for HE compared to TB (HE: 765.6, TB6: 713.6, 10: 616.8). Dose–volume histogram analysis demonstrated increased very low-dose exposure for HE plans below approximately 270 cGy (rectum), 120 cGy (femoral heads), and 60 cGy (bowel bag). Homogeneity indices (HE/TB6: 0.06; TB10: 0.05) and dose gradient indices showed no statistically significant differences (p<0.05).
Conclusion
The 6 MV-FFF energy on HE does not impose clinically significant planning limitations for obese prostate patients. With a tailored optimization strategy, HE achieves comparable dosimetry to 10 MV-FF delivery, even in extreme body habitus.