Dosimetric Implications of Prostate Bed Deformability: An Analysis of the SCIMITAR Clinical Trial

The post-operative prostate bed is a dynamic target volume due to the deformable nature of the bladder and rectum. These changes can lead to incorrect dosing of the prostate bed and organs at risk (OARs). Our objective was to quantify the dosimetric impact of prostate bed and OAR deformation.SCIMITAR (NCT03541850) is a prospective phase II clinical trial evaluating stereotactic body radiotherapy (SBRT) in the post-prostatectomy setting. This analysis included a subset of patients who received 5 fractions of 6-6.8 Gy to the prostate bed under CT-based image guidance. The clinical target volume (CTV) and OARs were contoured on fractional CBCT images. Changes in volume, shape (via the dice similarity coefficient [DSC]), and dosimetry were quantified. Student's t-test was used to analyze the differences between planning and daily treatment outcomes.A total of 29 patients (145 fractional images) were analyzed. We found the CTV volume remained stable (median change 1.1%; IQR: -15.1% - 16.1%), whereas the CTV shape was deformable (DSC of 0.76 [IQR: 0.71 - 0.79]). The bladder and rectum exhibited changes with median volume change of 5.7% (IQR: -24.3% - 51.0%) and 5.5% (IQR: -8.7% - 21.9%), respectively and median DSC of 0.77 (IQR: 0.68 - 0.84) and 0.74 (IQR: 0.69 - 0.80) respectively. The CTV received less radiation dose than planned (volume receiving 95%: 93.2% actual vs 99.6% planned, p < 0.01). 39% (56/145) of total fractions and 52% (15/29) of patients met criteria for CTV under-coverage (volume receiving 95% of the prescription dose < 93%). The rectum received higher dose than planned on several parameters (e.g., V27.5 Gy increased from 15.4% to 21.0% [p = 0.009] and V32.5 Gy increased from 6.0% to 10.9% [p = 0.006]) (Table 1).We found underdosing of the prostate CTV and overdosing of the rectum in patients receiving CT-guided postoperative SBRT. While future work will correlate these dosimetric consequences with toxicity, these data suggest that approaches such as adaptive radiotherapy may be beneficial.