Session Item

The purpose of this study is to compare different techniques of SBRT for localized prostate cancer (PCa), stereotactic MR-guided adaptive RT (SMART) versus cone beam CT(CBCT)-guided SBRT, in terms of toxicity and time costs.

Patient with localized PCa (clinical stage T1-2bN0M0) underwent SBRT using CT-LINAC system (True Beam STx, Varian) or MR-hybrid LINAC system (MRIdian, Viewray). SBRT was prescribed to a dose of 40 Gy (8 Gy/fr) and  36.75 Gy  (7.25 Gy/fr) to prostate and PTV, respectively, at 80% isodose, delivered on 5 days (3fr/week).  An anisotropic 5mm-margin (3mm posterior) was created around the prostate for the PTV. Treatment was delivered using two different image guidance (IG) strategies. CT-guided SBRT strategy consisted of a pre-treatment CBCT acquisition with  implanted fiducial markers and images matching using the ExacTrac® system. SMART consisted of indentification of target and OARs on pre-treatment MR images, with on-line calculation and delivery of a new plan for every fractions because of inter-fraction variation of bladder and rectal filling. Then an intra-fraction motion management strategy  was applied, consisting of a gating approach based on the real-time acquisition of a sagittal cine MRI during the whole delivery time (temporal resolution: 8 frames/s).   Mean time for each step of daily workflow was recorded. Common Terminology Criteria for Adverse Events was used to score gastrointestinal and genitourinary early toxicity during 3-month follow-up. 

Sixty patients (pts) treated with prostate SBRT were compared. Real-time adaptive MR-guided RT strategy was used for 30 pts (50%) . Pre-treatment CBCT was performed in 150 treatments. Using CBCT-IG, grade 2 acute rectal toxicity occurred in 3 pts ; 1 pts with Grade 2  and 1 pts with grade 3 urinary toxicity were observed . No grade 2 and 3 toxicity was recorded for SMART treatments. Mean total daily treatment time was about 13 (range 11-17) and 24 (range 22-30) minutes for CT-guided SBRT and SMART respectively. Treatment workflows are shown in table 1 and table 2..

SBRT for prostate cancer is a safe and effective treatment. CT-guided SBRT  involves acceptable toxicity , however it needs additional equipment (ie. fiducials placement, ExacTrac® system ) to assure interfraction organ-motion control and delivery precision. SMART for prostate cancer confirms efficacy and can reduce toxicity: on-line adaptive and real-tracking improve precision and safety , with a slight difference in the time of daily treatment. Further studies to evaluate the optimal balance between clinical advantages of SMART technologies and additional cost in terms of time and complexity are needed.