Analysis of Volumetric Dosimetry of Target Volumes and Organs at Risk on ICRU Point-Based Dose Planning in CT-Guided HDR Intracavitary Brachytherapy to Carcinoma Cervix

Abstract

Intracavitary brachytherapy (ICBT) has been historically planned based on orthogonal X-rays with point A as prescription point and surrogate rectal and bladder points to determine the approximate dose to organs at risk. This was standardized based on ICRU report 38. However, with the availability of better imaging modalities (CT, MRI), it became obvious that those points were poor surrogates for the tumor or the organ at risk. The current study analyzes the volumetric dose distribution to tumor volume and risk organs, from CT image-guided point-based planning done for intracavitary high-dose-rate brachytherapy using Fletcher–Suit–Delclos applicator in locally advanced carcinoma cervix patients. Fifty-one patients with locally advanced carcinoma cervix who were treated with ICBT were included. Point A-based dose planning was done based on CT images, on Oncentra™ treatment planning software. The doses to bladder point and rectal point were determined and were used for the purpose of dose prescription and optimization. Relevant target volumes and risk organ volumes were determined over the same plans, and dose measurements are taken. They were then analyzed for correlation and linear regression model using IBM SPSS Statistics version 23. The point A-based prescription dose did not adequately cover the target volume. CTV 90 obtained only 53.23% (95% CI 49.21–57.25) of the prescribed dose. The bladder point dose correlated well with all sub-volumes, especially D2cc bladder dose (r = 0.525, p = < 0.001). It satisfied linear regression model with standardized beta of 0.665. On the contrary, the correlation with rectal point and D2cc rectal dose was not strong (r = 0.284, p = 0.055). The point-based dose underestimates bladder dose by 18.24 ± 7.77% and overestimates rectal dose by 5.46 ± 4.55%, both being statistically significant. CT image-guided point A-based dose planning, without any volume-based optimization, has poor target volume coverage. There is disproportionate overestimation of dose to rectum and rectal wall while calculating from rectal point dose. The bladder point dose has good mathematical prediction for dose to bladder. However, volumetrically, it underestimates the actual dose. While point A-based planning on tomographic imaging can be a stepping stone toward image-guided brachytherapy, volume-based planning is necessary for optimizing the dose to primary tumor and managing risk organ dose properly.