Time Dependence of Coverage of the Prostatic Fossa: Implications for Daily Adaptive Radiotherapy

Purpose/Objective(s)

Online daily adaptive radiotherapy (ART) is an attractive modality for reducing PTV margins and improving the therapeutic ratio through elimination of inter-fractional uncertainty. However, daily adaptation is more time consuming than traditional image-guided radiation, and consequently, this increased time from initial CBCT acquisition until treatment delivery may introduce additional setup errors. For example, when delivering postoperative radiotherapy to the prostatic fossa, the adjacent bladder and rectum can change shape in the time required to adapt patients, resulting in deformation of the target and potentially a geographic miss. Therefore, we hypothesized that decreased time between scans would be associated with better coverage.

Materials/Methods

A single-center retrospective study was conducted from 03/2021 to 11/2021. Patients receiving conventionally fractionated radiotherapy to their prostatic fossa +/- elective nodes with pre- and post-treatment CBCT imaging before and after the same fraction (CBCTpre and CBCTpost, respectively) were included. Clinical target volumes (CTVpre and CTVpost) were contoured by a single observer on all CBCTs (only the prostatic fossa CTV was analyzed in this study). CBCTpre and CBCTpost images were registered using the same shifts from the day of treatment. Overlap of CTVpre and CTVpost was calculated and correlated with time with Pearson's correlation coefficient. The time threshold at which the maximum difference in overlap of CTVpre and CTVpost was identified iteratively. Subgroups above and below this threshold were compared, evaluating the percent overlap between CTVpre with 1, 2, and 3 mm expansions with CTVpost (here, overlap serves as a surrogate of dose coverage).

Results

When evaluating overlap between CTVpre and CTVpost, there was a trend towards more overlap with shorter intervals between scans (r=-0.091). A threshold of 13.5 minutes was identified with scans <13.5 minutes apart (n=210) having an average overlap of 91.4% vs ≥13.5 minutes (n=32) being 89.2% (p = 0.026). However, this difference lost significance when using 3 mm PTV margins (p=0.134), which was previously shown to be the optimal margin for this cohort. Still, there was a suggestion of better coverage for intra-scan times <13.5 minutes for all margins evaluated: 1mm (75.7% vs 59.4% achieved 95% coverage), 2mm (93.3% vs 84.4%), and 3mm (98.1% vs 90.6%).

Conclusion

Shorter times utilized for daily adaptation of the prostatic fossa may be associated with better coverage, likely secondary to less displacement of the target from patient movement and deformation from changes in adjacent normal tissue. Given image sets with intra-scan times >13.5 minutes were limited in this study, these findings will need to be validated in larger prospective cohorts.