Session Item

The feasibility of using an MR-linac to fully adapt to anatomical changes observed prior to radiation delivery has been established. There is a need to assess the value of the additional resources invested for re-contouring and re-optimization. This study compared the estimated dose delivered using full adaptation versus three degrees of freedom (3DOF) correction for MR-guided prostate stereotactic body radiotherapy (SBRT).  

Twenty-five patients treated with focal HDR-brachytherapy and MR-guided SBRT with full adaptation were included. A reference plan was generated to deliver 3000cGy/5 to the prostate with a 5mm expansion for the planning target volume. Images acquired for localization and during radiation delivery from each of the 5 fractions were rigidly registered to the reference image for every patient using a soft tissue prostate match. Prostate (CTV), solid bladder and rectum were contoured on all images for either generation of an adapted plan or estimation of delivered dose. To compare the delivered dose between 3DOF and full adaptation, dose clouds of the reference plan and the 5 adapted plans were overlaid onto the corresponding images acquired during delivery for each patient. Pairwise comparisons were performed for dose/fraction to 95% of the CTV, 5cm3 of the bladder and 50%, 20% and  1cm3 of the rectum with values of ±10% being considered clinically significant. 

A total of 125 fractions were analyzed. Median time for treatment sessions was 50 minutes (range 36-81 minutes). Dose to 95% of the CTV was not significantly different between the two methods. There was a large range of variation in the bladder and rectum volume at the time of treatment from reference planning (Figure 1). Despite the large changes in bladder volume, dose to 5cm3 of bladder was comparable between full adaptation and 3DOF, with 81% of fractions having a difference of <10% (Figure 2). However, mean delivered dose (cGy) to 20% of rectum was significantly reduced from 327 (SD: 62) to 310 (SD: 77) (p = 0.0082), and from 587 (SD: 31) to 554 (SD: 79) to 1cm3 (p = 0.003), when a full adaptation was executed. A higher number of fractions benefited from full adaptation with respect to rectal sparing when compared to 3DOF (Figure 2). 

In the presence of large anatomical variation, full adaptation for prostate SBRT is beneficial for rectal sparing; particularly in the high dose region when compared to 3DOF. Dose to bladder D5cc correlating to the trigone of the bladder, on the other hand, was less sensitive to the full adaptation. Further investigation correlating the dosimetric advantage of full adaptation with toxicity and patient reported outcomes will help evaluate the role of this treatment workflow.