Pencil Beam Scanning Proton Bragg Peak Conformal FLASH in Prostate Cancer Stereotactic Body Radiotherapy.

Simple Summary: Prostate cancer is one of the most diagnosed cancers in men. While it can be successfully treated with radiotherapy (RT), patients often leave treatment with previously healthy organs affected by radiation. This work assessed a novel ultra-high dose rate FLASH proton therapy technique for the treatment of prostate cancers. We were able to generate clinically viable Bragg peak FLASH treatment plans for prostate cancer patients previously treated with conventional proton RT. Feasible prostate treatment quality was observed with all constraints for organs at risk (OARs) being met while maintaining an ultra-high dose rate. Bragg peak FLASH radiotherapy (RT) uses a distal tracking method to eliminate exit doses and can achieve superior OAR sparing. This study explores the application of this novel method in stereotactic body radiotherapy prostate FLASH-RT. An in-house platform was developed to enable intensity-modulated proton therapy (IMPT) planning using a single-energy Bragg peak distal tracking method. The patients involved in the study were previously treated with proton stereotactic body radiotherapy (SBRT) using the pencil beam scanning (PBS) technique to 40 Gy in five fractions. FLASH plans were optimized using a four-beam arrangement to generate a dose distribution similar to the conventional opposing beams. All of the beams had a small angle of two degrees from the lateral direction to increase the dosimetry quality. Dose metrics were compared between the conventional PBS and the Bragg peak FLASH plans. The dose rate histogram (DRVH) and FLASH metrics of 40 Gy/s coverage (V_40Gy/s) were investigated for the Bragg peak plans. There was no significant difference between the clinical and Bragg peak plans in rectum, bladder, femur heads, large bowel, and penile bulb dose metrics, except for D_max. For the CTV, the FLASH plans resulted in a higher D_max than the clinical plans (116.9% vs. 103.3%). For the rectum, the V_40Gy/s reached 94% and 93% for 1 Gy dose thresholds in composite and single-field evaluations, respectively. Additionally, the FLASH ratio reached close to 100% after the application of the 5 Gy threshold in composite dose rate assessment. In conclusion, the Bragg peak distal tracking method can yield comparable plan quality in most OARs while preserving sufficient FLASH dose rate coverage, demonstrating that the ultra-high dose technique can be applied in prostate FLASH SBRT. [ABSTRACT FROM AUTHOR]