F Forghani*, M Schmidt, A Price, F Reynoso, N Knutson, M Prusator, T Mazur, Washington University School of Medicine, St. Louis, MO

• 
  F Forghani

TU-F115-IePD-F7-5 (Tuesday, 7/12/2022) 1:15 PM - 1:45 PM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 7

Purpose: Prostate SBRT treatment can depend on discretionary judgment regarding rectum and bladder filling. In this study, we evaluated anatomical variation from CBCTs for prostate SBRT patients toward providing rigorous criteria for acceptable setup deviation from simulation.

Methods: 132 CBCT were analyzed for 20 patients who underwent five-fraction SBRT prostate treatment. All patients were treated to the prostate plus setup margin (PTV) in 35 – 36.25 Gy, with 4 patients receiving simultaneous 40 Gy boost to the prostate. CBCT-based localization was based on fiducial markers. Hydrogel-based rectal spacer (SpaceOAR) was implanted in 18 patients. Bladder and rectum were contoured on all CBCTs. CBCTs were labeled as for-treatment (FT) if used for specifying treatment position and non-treatment (NT) if position was adjusted by subsequent CBCT before treatment. CBCT-derived structures were transferred to simulation CT by registration applied for treatment, and dosimetric criteria for bladder and rectum from a phase III clinical trial (NRG-GU005) were recorded.

Results: Of the 132 CBCTs, 29 were identified as NT and 103 were FT. Among NT and FT CBCTs, 13/29 and 19/103 did not achieve protocol criteria to within acceptable variation (evaluated per fraction). Excluding cases where the only unmet objectives are D0.03cc, 10/29 NT and 7/103 FT CBCTs did not achieve volume constraints. For these cases, only rectum objectives were unmet, and median (range) overlap volume between PTV and rectum were_4.2 (4.2)_and 3.2 (6.1) cc for NT and FT, with greatest overlap observed for one patient without hydrogel.

Conclusion: Preliminary data highlight that discretion can be effective in stratifying setup according to dosimetric criteria; however, sensitivity can be improved through rigorous guidelines. Cases where D0.03cc objectives are unmet can potentially be used for inferring population-derived PRVs. Instances where only volume objectives are unmet may provide geometric information for pursuing alternative strategies such as online adaptation

Keywords

Not Applicable / None Entered.

Taxonomy

TH- External Beam- Photons: extracranial stereotactic/SBRT

Contact Email