Heterogeneity Effects on Dose–Volume Parameters Used for Planning Gynecologic High-Dose-Rate Brachytherapy: A Retrospective CT-Based Monte Carlo Study

Purpose/Objective(s)
To quantify the effects on dose-volume parameters of three assumptions commonly used in 3D gynecologic brachytherapy planning: Infinite geometry, water-equivalent geometry, applicator and contrast medium can be part of the planning volumes.
Materials/Methods
A novel CT-based Monte Carlo code (HDRMC) is developed to account for tissue and non-tissue heterogeneity effects in HDR brachytherapy planning with CT-compatible applicators. HDRMC is based on the general-purpose GEPTS system. A CT post-processing method is used to include the exact composition and density of the applicator and contrast medium in the patient 3D phantom. In this study, the HDRMC code is used to re-calculate the 3D dose distributions for 37 cervix HDR brachytherapy plans generated by a commercial treatment planning system (TPS). Two sets of CT-compatible cervical applicators are considered: (1) The Standard applicator (Intra-uterine Tandem + 2 Ovoids), and (2) The Ring applicator. Contrast medium is used for bladder but not for rectum. The individual and combined effects of the above assumptions on the dose volume parameters recommended by GEC-ESTRO (D90, D100, V100, V150, V200, and Bladder and Rectum D5cc, D2cc, D1cc, and D0.1cc) are quantified.
Results
D90 and D100 can be overestimated by 1.8 and 3%, respectively, because of the infinite medium assumption. Using water phantom instead of CT-based phantom can cause D90, D100, V100, V150 and V200 to be overestimated by 2.7, 4.4, 4.3, 5.6, and 6.2%, respectively. D90 can be overestimated by 4.5% when part of the applicator is included in CTV. Combining the three effects can result in 7.4% under-dosage of CTV and 25% overestimate of V100. Using CT-based dose calculations and excluding the contrast product from the bladder volume can reduce bladder D2cc by 17.5%. Using CT-based dose calculations reduces rectum D2cc by up to 4.2%.
Conclusions
The use of CT-based dose calculations with the exclusion of non-tissue objects from planning volumes is recommended for more accurate 3D gynecologic brachytherapy planning.