ESTRO 2024 - Abstract Book

S241

Brachytherapy - Gynaecology

ESTRO 2024

gross tumour volume evident on the day, including the entire cervix and any grey areas identified 3 . Simultaneously, this process, requires precise contouring of the organs at risk (OARs) in order to minimise their toxicity. There are four critical OARs; bladder, sigmoid, rectum and bowel. The GEC-ESTRO guidelines recommend that all OARs should have a documented D2cm 3 , D1cm 3 and D0.1cm 3 dose parameters 4 . These parameters correspond to the minimum dose received by the most irradiated 2,1 and 0.1cm 3 volume for each of the OARs. At St Luke's Cancer Centre at The Royal Surrey County hospital in Guildford, UK, we follow a hyperfractionation schedule whereby our patients receive two fractions on the same day six hours apart. In addition, it is common practise in our department to use interstitial needles aiming for a highly conformal dose to the CTV HR . This processes contribute to the complexity of brachytherapy and demonstrate why time management is of paramount importance on the day. The aim of this project was to investigate various expansion margins around the CTV HR within which the rectum and bowel can be safely contoured, taking into account the D2cm 3 , D1cm 3 and D0.1cm 3 dose parameters. The study also questioned the accountability of ovoids into the dose constraints and parameters mentioned above.

Material/Methods:

This cohort included 21 patients who were treated in our department between January and December 2022. Seventeen of those had interstitial needles inserted. Four expansion margins were investigated - 15,20,25 and 30mm. The rectum and bowel were contoured in their entirety and the reference D2cm 3 , D1cm 3 and D0.1cm 3 values were extracted from the ARIA Oncology system of Varian onto Excel. For the final analysis SPSS Statistics 22 was used. As each expansion envelope was applied on each of the OARs, the D2cm 3 , D1cm 3 and D0.1cm 3 values were calculated. This process was repeated but this time with the addition of the ovoids. The differences between the mean reference and calculated doses were also computed as percentage differences. The picture below shows an axial CT slice indicating the four different expansion margins around the CTV HR shown in the middle.

Results: