ESTRO 2024 - Abstract Book

S242

Brachytherapy - Gynaecology

ESTRO 2024

The D2cm 3 , D1cm 3 and D0.1cm 3 mean reference dose parameters for both the rectum and bowel demonstrated excellent matching with the corresponding D2cm 3 , D1cm 3 and D0.1cm 3 calculated dose parameters for the 25 and 30mm expansion margins. The table below shows the values calculated for the rectum, indicating that outlining the ovoids results in a much smaller difference between the value of the reference and calculated dose parameters. The percentage difference for the D2cm 3 was 2.55% for the 25mm expansion margin, which dropped significantly to 0.65% difference when ovoids were included. The p value for this was 0.001. Similarly, the percentage difference for the D2cm 3 for the 30mm expansion margin was 0.64 which reduced to 0.19 with the addition of the ovoids. The p value for this was 0.003. Similar results were shown for the bowel measurements. The percentage difference for the D2cm 3 for the 25mm expansion margin without and with the ovoids contoured, were 1.04 and 1.18 respectively. The p value for this was 1. For the 30mm expansion margin the values were 0.79 and 0.98 respectively with a p value also of 1. Although the percentage differences for the bowel were not statistically significant, the calculated D2cm 3 , D1cm 3 and D0.1cm 3 dose parameters for the 25 and 30mm margins were in closest approximation to the corresponding mean reference values. The ovoids made no difference to these values with regards to the bowel.

Conclusion:

This project demonstrated that contouring the ovoids and adding a 25mm expansion margin around the CTV HR is statistically and clinically appropriate for managing the time consuming task of rectum and bowel contouring in cervical brachytherapy. Currently there are no guidelines to define this or to address the importance of ovoids in the dose calculations. The practise to this point in our department, based on previous work (2013) has been to expand the CTV HR to 20mm.The findings of this project however, are changing the local protocol with the aim to refine contouring details further for the remaining OARs.

Keywords: cervix, OARs, D2cc

References:

1. Radiotherapy and Oncology 78 (2006) 67–77 www.thegreenjournal.com

2. Journal of the ICRU Vol 13, Report 89, Chapter 10