ESTRO 2023 - Abstract Book

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ESTRO 2023

Data from SCRT plans of 82 LARC patients treated from February 2018 to March 2022 were retrospectively collected. All of them were treated with Varian Halcyon 2.0 linac with 6FFF MV and VMAT technique. In house software was used to analyse the dose-volume histograms (DVH) Different dosimetric parameters were studied for the most representative OARs: bowel cavity, bladder and femoral heads. Patient characteristics and structure volumes were also studied. Results were compared with published data (1-2). Results Patient mean age decreased from 77 years in 2018 to 68 years in 2022. Treated PTV mean volume was 941 cc (range p25 795 cc – p75 1063 cc). Mean volume was 169 cc (range p25 87 cc – p75 210 cc) and 1472 cc (range p25 1937cc – p75 3476 cc) for bladder and bowel cavity, respectively. Figure 1 shows DVH statistics and boxplots of selected dosimetric parameters for bladder (V12.5y, V21Gy and V25Gy) and bowel cavity (V10Gy, V12.5Gy, V18Gy and V23Gy). As can be seen in the Figure 1, the mean V21Gy value for bladder was 5.2% and did not present great variability between different plans, meanwhile the mean V12.5Gy was 18% and presented greater variability between plans. The mean V18Gy value in bowel cavity was 39cc and did not present great variability, while the mean V12.5Gy was 83cc and presented greater variability between plans. For the femoral heads, volume at 50% of prescribing dose was analyzed; V12.5Gy was 11.4%, with an interquatile range from 4.6% to 16.4%.

Conclusion The DVHs of OARs do not show large variation, indicating robust plan optimization. Also, the variability observed could be attributed to the anatomical variability between patients. The results show values much lower than the SCRT in house constraint presented at UK protocol (1). Comparing with Appelt et al. (2), ours mean values and interquartile ranges for bladder and bowel cavity are similar to reported data, and slightly higher for femoral heads.

References (1) The Royal College of Radiologists. National rectal cancer intensity-modulated radiotherapy (IMRT) guidance.