Session Item

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

Since June 2021 twenty patients with localized unfavorable intermediate or selected high-risk prostate tumor were enrolled to receive an ultra-high SDART of 24 Gy (BED1.5 = 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 the 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. The target minimum dose was defined by the OARs dose constraints, with a dose escalation to 24 Gy to the volume away from the HDAZ. A 10MV FFF single arc (140°-220°) was optimized using target penalties with the Monaco Monte Carlo TPS. CBCT matching ensured accurate patient setup alignment and target localization. Real-time 3D prostate motion was tracked with a novel electromagnetic tracking device. Treatment was interrupted when the signals exceeded a 2 mm threshold in any of the three spatial directions and couch position was corrected unless the offset was transient.

All the planning objectives were fulfilled. The median PTV volume was 68.5 cc [25.6 – 100.6]. The average MUs per plan were 6694 ± 520. All the treatment plans were quality assured using a 2D silicon diode array and fulfilled a 2%/2mm gamma passing rate >95% objective. The mean delivery time was 4.5 ± 0.6 minutes, while the mean overall treatment time was 16.0 ± 8.0 minutes. Intrafraction tracking was successfully carried out in all treatment sessions and beam interruptions due to target motion beyond limits were needed in 14 patients, with 1.5 [0 – 7] interruptions per patient on average. The mean value of the target average deviation was -0.21 ± 0.40 mm, 0.37 ± 0.65 mm, and -0.30 ± 0.52 mm in lateral, longitudinal, and vertical direction, respectively. Prostate displacement did not occur in a distinct direction. The prostate was found within 2 mm from its initial position in 88% of the treatment time, i.e. in 86% of the time during the setup phase and in 95% during the delivery phase (beam on + interruptions).  

The volume of rectal mucosa receiving critical doses was limited by the use of an HDAZ. The accomplishment of urethra sparing via negative dose painting to minimize GU 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 SDART in organ-confined prostate cancer.