Anatomic and Dosimetric Comparison of Supine Photon and Upright Proton Treatment Approaches for Prostate Cancer
Abstract
Purpose
Prior to clinical launch of the world’s first installation of MEVION S250-FIT combined with Marie® system we evaluated anatomical and dosimetric differences between conventional photon-based supine treatments and proton-based upright treatments for prostate cancer.
Methods
Six MRI scans were acquired from three healthy volunteers, with one upright and one supine scan per subject. Synthetic CTs were generated for each position and used for contouring and treatment planning. The clinical target volume (CTV) included the prostate and 1 cm of proximal seminal vesicles. The planning target volume (PTV) was defined as the CTV plus a 7-mm margin, reduced to 5 mm posteriorly. Supine VMAT plans were created in Eclipse using institutional autoplanning scripts with two full arcs and normalized to achieve 95% PTV coverage at 7020 cGy. Upright proton plans were generated in RayStation using two lateral beams with robust optimization (3.5% density and 5-mm setup uncertainty) and normalized so that 98% of the CTV received 7020 cGyRBE. Anatomical and dosimetric parameters were compared between approaches.
Results
Upright positioning reduced rectum–PTV overlap by 16.1% and increased the longitudinal distance between the PTV and penile bulb by 2.5 mm, while bladder overlap remained similar. Both approaches met all planning constraints. Compared with photon supine plans, proton upright plans reduced rectum V65Gy, V50Gy, and V31Gy by 2.3 cc, 2.8%, and 5.2%; bladder V50Gy and V31Gy by 3.9% and 7.0%; and penile bulb mean dose by 17%. The 50% isodose volume was reduced by 23.5%. No volume of small bowel and femoral heads received a dose of 40Gy or above for all the proton and photon plans.
Conclusion
Proton therapy with upright positioning reduced doses to all critical organs compared with conventional supine photon therapy. Further clinical validation is warranted to confirm these findings and assess the long-term impact on treatment outcomes.