Session Item

Great emphasis on rigorous planning and delivery techniques must be placed when using extreme hypofractionated regimens to fully exploit their potential benefits in optimizing the therapeutic ratio, thus yielding excellent clinical outcomes. The aim of this study was to report the clinical treatment planning implementation for organ-confined linac-based prostate Single-Dose Radiation Therapy (SDRT) using electromagnetic tracking for real-time intrafraction organ motion management (NCT04831983).

Since June 2021 five patients with localized unfavorable intermediate or selected high-risk prostate tumors were enrolled to receive an ultra-high SDRT of 24 Gy (BED 1.5 = 408 Gy). Patients were simulated with empty rectum and bladder filled by a Foley catheter. Fused CT and T2W 3D MRI image sets were used to delineate target and OARs. The PTV consisted of the CTV with a 2-mm isotropic margin. A high-dose avoidance zone (HDAZ) was created by a 3-mm expansion around the rectum, bladder, and urethra. Patients were planned to a minimum dose defined by the OARs dose constraints with a dose escalation to 24 Gy to the target volume away from the HDAZ. A 10MV FFF beam energy single arc from 140° to 220° was optimized using target penalties with the Monaco Monte Carlo TPS. During the treatment delivery, CBCT matching ensured patient setup alignment and target localization, and any online tracking detected motion greater than 2 mm was realigned by repeating CBCT.

Figure 1 shows axial, sagittal, and coronal fused CT/MR slices representing the dose distribution for a patient treated with a single fraction of 24 Gy. Treatment goals and characteristics are summarized in Table1. All the predefined planning objectives were fulfilled. Average PTV volume was 57.1 cc (range 25.6-78.8). Average beam delivery time lasted 4.5 ± 0.6 minutes. Average total monitor units per plan were 6709 ± 525. All the treatment plans were quality assured using a two-dimensional array with silicon diodes and fulfilled the gamma (2%/2mm) passing rate >90% objective.

The use of an HDAZ during planning limited the volume of rectal mucosa receiving critical doses. The accomplishment of urethral sparing via negative dose-painting to minimize genitourinary toxicity is feasible through appropriate imaging procedures and online tracking during treatment delivery. Our preliminary findings offer encouraging perspectives on the feasibility and safety of 24 Gy SDRT in organ-confined prostate cancer.