Stereotactic Body Radiation Therapy (SBRT) Can Be Dosimetrically Equivalent to High-dose Brachytherapy as a Boost for Locally Advanced Cervical Cancer— A New Clinical Trial Concept

Abstract

Current standard of care for locally advanced cervical cancer consists of external beam radiation plus local boost therapy with intracavitary brachytherapy (BT). Intracavitary HDR procedures are invasive and require anesthesia to accommodate the insertion of intrauterine treatment devices. In addition to requiring technical skill, BT treatments lack reproducibility. Furthermore, BT uses sources, which result in rapid dose fall off therefore intracavitary treatment, may be ineffective in delivering dose to periphery of bulky advanced tumors; treatment of peripheral disease requires interstitial implants, which are technically challenging. Given the recent advances in conformal stereotactic therapy it is now possible to deliver high doses of radiation to confined volumes while sparing normal tissue. As the initial step towards a clinical trial, we performed a dosimetric comparison of SBRT to HDR BT for the definitive treatment of locally advanced cervical cancer. A total of 5 patients with FIGO stage IIIB cervical cancer were selected. Planning CT's from initial BT treatments were obtained, and were then imported for treatment planning. OARs and HR-CTVs were defined using GEC-ESTRO guidelines. Plans were created using 13 non opposed non coplanar intensity modulated beams (6 or 18 MV) for a total dose of 2550 cGy in 3 fractions (LQED2GY equivalent of 40 Gy). Contours and planning CT's were then imported into the Brachyvision treatment planning system for HDR BT planning using the same fraction size. HDR dose was optimized to point A and the HR-CTV. LQED 2 Gy of 2 cc doses to the bladder, rectum, small bowel, and sigmoid as well as the D90 of the CTV were then calculated and compared between plans per GEC-ESTRO guidelines. Given the ideal GEC-ESTRO tolerance dose for the rectum and sigmoid is 30 Gy and 45 Gy for the bladder, SBRT resulted in an average LQED 2 Gy to the bladder, rectum, small bowel, and sigmoid of 37.6 (29.1-45.5), 32.4 (16.8-51), 27.9 (21.3-36.7), and 27.9 (1.64-39.1) Gy respectively, while BT resulted in doses of 36.1 (28.3-52.1), 29.4 (16.2-50.4), 54.3 (24.4-88.6), 40.5 (24.8-77.8) Gy respectively. High Risk CTV D90 dose for SBRT was 41.1 Gy (40.8-41.7) and 35.4 (23.5-46.7) for BT. SBRT boosts result in higher more conformal dose to the tumor while maintaining similar doses to OAR in this subset of patients. SBRT is at the least dosimetrically equivalent to HDR BT in the treatment of locally advanced cervical cancer with respect to dose delivered to OAR and may exceed BT with regard to total dose delivered to the tumor. Given our small sample size further work is required. Using this data we hope to undertake a clinical trial to determine the feasibility of SBRT in the definitive treatment of locally advanced cervical cancer.