Utrecht Intertitial Applicator Shifts and Organ Movements in 3D CT-Based HDR Brachytherapy of Cervical Cancer

Abstract

To investigate Utrecht intertitial applicator shifts, and its effects of organ movements on DVH parameters during 3D CT-based HDR brachytherapy of cervical cancer. 13 cervical cancer patients underwent brachytherapy after external beam radiation therapy for total dose to 45 Gy/25 fractions. The prescribed dose of HDR brachytherapy was 7 Gy×4 fractions. 6 patients received brachytherapy only for two fractions as their individual reasons. We got the permission of the patients for the investigation. The dose of brachytherapy were calculated and transformed to EQD2 values in Gyα/β, biologically weighted dose normalized to 2 Gy fractionation. The used α/β values for target volumes and OAR were 10 and 3 Gy, respectively. We adopted the Zephyr Patient Transport Sled included Lithotomy Stirrups. After the applicator implanted, CT imaging was achieved for oncologist contouring CTVhr, CTVir, and OAR, including bladder, rectum, sigmoid colon and small intestines, which were performed according to GYN GEC ESTRO recommendations. The treatment plan was optimized according to the CTVhr, and executed in the case of total dose of CTV-hr D90 controlled by 80 to 85 Gyα/β, and OAR dose constrained by 90 Gyα/β3 for bladder, 75 Gyα/β3 for others. After the treatment, CT imaging was repeated. Two CT imaging were matched by pelvic structures. In both imaging, we defined the tandem by the tip and the base as the marker point, and evaluated applicator shift, including X (left as negative and right as positive) , Y (head as negative and foot as positive) and Z (anterior as negative and posterior as positive). Based on the repeated CT imaging, oncologist contoured the target volume and OAR again. We combined the treatment plan with the repeated CT imaging and evaluated the total dose of the target and OAR. We evaluated the change range for the doses of CTV-hr D90, D2cc for OAR. The average applicator shift were 0.10 mm to -0.16mm for X, 1.49 mm to 2.14 mm for Y, and 2.3mm to 1.9 mm for Z. The change of average physical doses and EQD2 values in Gyα/β range for CTV-hr D90 decreased by 2.55% and 3.5%, bladder D2cc decreased by 5.94 % and 8.77%, rectum D2cc decreased by 2.94% and 4%, sigmoid colon D2cc decreased by 3.38% and 3.72%, and small intestines D2cc increased by 3.72% and 10.94%. Applicator shifts and organ movements induced the total dose inaccurately. Although we adopted the Zephyr Patient Transport Sled included Lithotomy Stirrups, which reduced the movement of patients, the doses of target volume and OAR were variated inevitably. Applicator shifts and organ movements during the process of treatment could not be ignored.