Stability of Intracavitary Applicator Placement for HDR Brachytherapy of Cervix Cancer

Abstract

Cervical cancer is often treated with a combination of external beam radiation therapy (EBRT) and high-dose rate (HDR) intracavitary brachytherapy (ICBT). An intrauterine ring and tandem (R&T) are inserted through the vagina until they reach the cervix and uterus, where the dose is prescribed. The goals of this study were to investigate the stability of intracavitary applicator placement during patient transfer and to evaluate the dosimetric impact of displacement. 14 patients with cervical cancer were analyzed. Three sets of orthogonal fluoroscopic radiographs were obtained in the HDR suite after the insertion and prior to treatment: pre-CT fluoroscopic radiograph with patient in the lithotomy position, pre-CT fluoroscopic radiograph with patient in the legs down position, and post-CT fluoroscopic radiograph with patient in the legs down position. Applicator position post-CT was compared to the pre-CT radiographs to determine if the position changed during patient transfer. The displacement was measured in the anterior-posterior, medio-lateral, and superior-inferior directions, as well as the degree of rotation. To study the dosimetric impact of applicator shifts on dose to organs at risk (OARs), the R&T were shifted virtually in the BrachyVision treatment planning system. The OARs studied included the bowel, sigmoid, rectum and bladder. 5mm shifts were made in the superior-inferior, medio-lateral and anterior-posterior direction. 3 rotations were made in the pitch, yaw and roll directions. Applicator shifts were analyzed in only one direction at a time. The dosimetric impact on OARs was evaluated by comparing DVH-based criteria from the original and shifted/rotated plans. The average displacements were 1.90 ± 0.47mm laterally, 2.97 ± 0.58mm longitudinally and 9.50 ± 1.48mm anterior-posterior. The average rotation on the PA radiograph was 0.97 ± 0.21° and 2.55 ± 0.61° on the LAT radiograph. 5mm anterior-posterior shifts had the greatest effect on dose to OARs. On average, 5mm ANT shifts had the greatest effect on the D2cc values of the bowel. There was a 13.69% increase in dose. 5mm ANT shifts also affected bladder dose, with a 36.47% increase in dose. 5mm POST shifts increased rectal dose by 28.61%. Other directional shifts were minimal. The largest effect on OAR dose due to rotation was to the sigmoid when the applicator shifted in the POST pitch direction. As a result, the dose increased by 4.66%. Other changes in dose were minimal. Patient transfer resulted in applicator shifts and rotations that had a measurable effect on dose to OARs. The displacements were the result of either a direct shift or rotation of the applicator. However, the dose differences reported in this study represent the worst-case scenarios, since the surrounding tissues and organs may move with the applicators. Additional tracking of these shifts and rotations may clarify the sources of these unwanted motions and suggest possible mitigation strategies.