Session Item

Volume-at-dose (VaD) criteria are commonly used in treatment planning as dose-response surrogates for OARs. For exploration of adaptive workflows, it is important to investigate the robustness of VaD measures under different conditions. The first objective of this study was to investigate the variability of rectum VaD for prostate cancer patients (PCa) by simulating non-adaptive and adaptive treatment workflows (WFs). In an adaptive MR-linac WF, accumulated dose (from previous fractions) can potentially be used as input to plan adaptation; the fraction doses need to be accumulated to a common anatomy via dose mapping. The second objective was to study the effect of dose mapping on rectum VaD.

We used data from 5 intermediate-risk PCa patients (A1-A5) treated on the Elekta Unity MR-linac (6.1Gy x 7). For each patient we used the datasets gathered from reference planning and from 7 fractions. Each dataset consisted of an MR image set and a manually delineated structure set.

We investigated two different treatment WFs, simulated in a research version of the RayStation TPS: Adaptive vs. non-adaptive Conventional. Different CTV to PTV margins were used for the two WFs: 3 mm vs. 6 mm prostate margin; 5, 7, 7 mm vs. 6, 9, 9 mm (LR, AP, SI) anisotropic seminal vesicles (SV) margin.

For each patient we evaluated 7 Adaptive (fully reoptimized) fraction doses and 7 recalculated Conventional fraction doses. In Conventional we optimized a reference plan for each patient’s reference image; this plan was then recalculated on each fraction image (figure 1a-c) with isocenter shifts based on prostate-to-prostate registrations.

Total treatment dose was accumulated to the reference anatomy by mapping the fraction doses via deformable image registrations (DIRs) (figure 1d-f). A hybrid structure/image DIR in the TPS was used with the prostate, rectum, bladder, and SV as guiding structures. The effect of dose mapping on rectum VaD was evaluated by comparing rectum VaD before/after dose mapping. We evaluated the rectum dose in terms of clinically used VaD plan evaluation criteria: V_33Gy<30%, V_38Gy<15%, V_41Gy<10%. For evaluation per fraction, fraction doses were scaled by a factor of 7 (i.e., the total number of fractions).

Rectum VaD evaluation criteria were violated in Conventional due to rectal volume changes (figure 2a-b); for one patient and fraction V_41Gy<10% was violated by a factor of 2.5. In Adaptive, rectum VaDs were held below the clinical evaluation criteria. Dose mapping was found to alter rectum VaD (figure 2c-d); for one patient and fraction V_41Gy changed by a factor of 1.8.

From these results, we conclude: 1) reference plan VaD may not be representative of the delivered dose in conventional WFs; dose-response relationships based on reference VaD may be confounded by dosimetric deviations from reference, and 2) the effect of dose mapping on rectum VaD should be carefully considered, especially if accumulated dose is to be used for plan adaptation.