Room: Exhibit Hall
Purpose: To evaluate the volumetric differences on target and dosimetric benefits on normal tissues by using MR imaging in addition to CT when planning tandem and ring brachytherapy treatments.
Methods: Six patients underwent MR scanning (3-Tesla) with Tandem-and-Ring applicators in place during one fraction of their treatment in addition to their CT scanning. The treatment CT volumes were compared with volumes drawn with MR assistance for each patient. The comparison was based on same-patient, same-fraction, and same-physicianâ€™s contours. Treatment plans were optimized to each target volume using a standardized inverse optimization weighting. Targets were compared using a similarity metric, the ranges of greatest extension, volumes, and superior/inferior lengths. Optimized plans were evaluated for their impact on target coverage, and the dose to 2cmÂ³ of normal structures.
Results: On average, the absolute difference between MR-guided and CT target volumes was 21.62cmÂ³ (range 1.3cmÂ³ to 64.3cmÂ³), and two patients had larger MR-guided volumes. The absolute difference in distance from the target edge to the tandem was 5.0mm on average (range 0.1mm to 13.9mm), with 3 patients having less extension on MR-guided plans. The absolute difference in superior/inferior length was 14.2mm on average, (range 7.2mm to 29.9mm), and five patients having greater length on MR contours. The conformity of the MR volume with the CT was on average 0.55 (range 0.32 to 0.75). For patients 2 and 3, the MR High Risk CTV coverage was not achieved with the CT volume optimized plan. Normal tissue doses generally increased slightly with MR optimization, except when MR volumes were substantially smaller.
Conclusion: MR imaging allows greater confidence in target volume delineation, roughly equivalent normal tissue sparing, and improved accuracy for inverse optimized techniques. Implementing MR imaging for structure contouring guidance is highly recommended to improve target control rates in patient undergoing high dose rate brachytherapy.