Session Item

Intracavitary brachytherapy (ICBT) not only provides conformal dose delivery, but also delivers a high central dose (HCD) to the target which cannot be reproduced by IMRT or SBRT. The ABS and SGO do not recommend conformal external beam boost due to inferior local control and survival. The ideal HCD profile for tumor control is a matter of debate. It is well known that various treatment plans with different image-guided (IG) target definitions can equally conform to HR-CTV and limit dose to organs at risk (OARs) for medium-sized ICBT targets. However, HCD profiles in various treatment plans (TP) have not been compared in the past. Target definition of HS-CTV is the most critical for IG-ICBT. The purpose of this study is to compare HCD profiles between conventional Point A and IG-ICBT plans with different target definitions. 

Five ICBT TP with different target definition were used to compare HCD profiles. Each TP was run on three tumor sizes – small, medium, and large. A patient with FIGO stage IB2 cervical cancer was selected for the medium-sized HR-CTV. A large target was created by adding a 1.0 cm symmetric expansion and a small target by subtracting 0.5 cm symmetrically. The three Point A based plans were evaluated:  standard Point A, Point A with inverse optimization (Point A-IO), and Point A with dose shaping (Point A-DS). Two inverse optimization plans were evaluated:  inverse plan with entire tandem length (IP-ETL) and inverse plan with modified tandem length to fit HR-CTV (IP-MTL). Priority for each treatment plan was HR-CTV coverage by prescribed dose (800 cGy). Secondary priority was organ at risk constraints (rectum D2cc < 470 cGy, bladder D2cc < 650 cGy). Target coverage, V100%, V150%, V200%, V250%, and OAR doses were compared for each plan.

The table shows target coverage (%), HCD (V150%, V200%, V250%) and OAR doses for the five treatment plans for three target sizes. All five treatment plans were able to achieve adequate target coverage (98-100%) and OAR constraints for all three target sizes except the Point A plan for large target. Point A based plans had larger V100%, V150%, V200%, and V250% than the inverse plans, particularly for larger targets. IP-MTL plans had the smallest HCD volumes. However, volume of HCD (V150%, V200%, V250%) relative to volume of prescribed dose (V100%) was similar between all treatment plans and target sizes: 46-55% for V150%, 26-35% for V200% and 17-24% for V250%.

Adequate target coverage and OAR constraints can be achieved with both Point A based planning and inverse planning. Conventional Point A based plans have larger V100% and HCD volumes compared to more modern inverse planning. However, volume of HCD relative to volume of prescribed dose is similar in both Point A based and inverse plans. Therefore, selecting a target definition for ICBT is a personal choice. There is no data available which is the best Plan for high local control with low morbidities.