ESTRO 2024 - Abstract Book

S3553

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

From each plan the PTV D95% was recorded as well as D0.1cc for the OARs: bladder, cauda equina, colon, rectum, sacral plexus, small bowel, vessels.

Results:

The median PTV D95% for the standard re-irradiation plans was 30.2Gy, table 1. The median PTV D95% for the isotoxic re-irradiation plans was 32.8Gy, a 2.8Gy median increase from the standard plans.

Dose escalation was feasible for seven plans, with a median dose increase of 4.9Gy (interquartile range (IQR) 1.1 20.2). The maximum dose escalation possible was 52Gy. An example is shown in figure 1. For the three plans that could not be dose escalated, two required lower dose levels (20Gy & 24Gy) at the outset to meet small bowel constraints. Of the 10 isotoxic plans, the limiting OARs were rectum (1), sacral plexus (3) and small bowel (3). Two plans exceeded the optimal bladder limit and one plan exceeded the optimal vessels limit which is acceptable as the constraints are ALARA but it was decided not to escalate the dose further. The maximum dose received by each OAR in the re-irradiation plans were converted into EQD2 for comparison, table 1. The greatest median difference was 3.2Gy for vessels, but some OAR had <0.5Gy median difference (bladder, cauda equina, rectum). Some OAR showed decrease in isotoxic plan compared to standard: bladder (4), small bowel (1), and vessels (1).

Table 1: Differences in median PTV D95% (Gy) and median OAR D0.1cc in EQD2 (Gy) between standard re irradiation plans and isotoxic re-irradiation plans.