ESTRO 2024 - Abstract Book

S1492

Clinical - Lower GI

ESTRO 2024

Results:

In the qualitative assessment, discrepancies between ROs were found for both the primary tumor (GTVT) and lymph node target (CTVN) (see Figure 1). Here we present some of them. For the GTVT, two ROs delineated the whole circumference, whereas three delineated only the visible tumor (clarification CL#1: if the tumor is semi circumferent and is clearly visible, do not delineate the whole circumference). For the CTVN, large discrepancies were found at the cranio-caudal border with differences of up to 1 cm (CL#2: the cranial border should end at the anterior part of S1 at the patient midline and the caudal border should include the entire mesorectum). Large discrepancies were also found for the lateral lymph nodes with up to 4 cm difference posterior-anteriorly in the cranial region, and 1 cm posterior-anteriorly in the caudal region (CL#3: follow the English guideline and do not distinguish between posterior and anterior lateral lymph nodes). Regarding OARs, 3 of 5 ROs did not exclude bowel loops within the CTVN structure; for CaudaEquinaS1-S2 only some ROs included nerve roots (CL#4: complement with figure). The total volumes of delineated structures are presented in Table 1 together with DSC relative to the clinical reference and DVH criteria based on the clinical treatment plan. With regards to DVH criteria, 3 of 5 PTVN structures did not meet the clinical target dose coverage criteria (D98>95%). Although all OAR criteria were met for all OAR structures, a difference in V18Gy of 40% was observed for the bowel delineated by RO5 (who did not exclude bowel loops within the CTVN structure).