Feasibility of Combined Intracavitary-Interstitial High-Dose-Rate Brachytherapy With a Novel MR-Compatible Split-Ring Applicator

Abstract

The purpose of this study is to demonstrate clinical feasibility and preliminary dosimetric evaluation of combined intracavitary-interstitial HDR brachytherapy for cervical cancer, using a novel split-ring applicator and three-dimensional (3D) volume based treatment planning. A 43-year old female with Stage IIB cervical cancer underwent external beam radiation therapy (EBRT) with a 4-field technique to 45 Gy followed by a left-sided parametrial boost to a total dose of 50.4 Gy. The EBRT was combined with high-dose-rate (HDR) brachytherapy to deliver an additional 30 Gy in six fractions. The HDR brachytherapy was delivered using a novel MR compatible split-ring applicator with needle guide-holes in the split-ring caps allowing placement of interstitial needles (Figure, panel A). MRI and CT were used for delineating high risk clinical target volume (HR-CTV) and intermediate risk clinical target volume (IR-CTV) and 3D volume based treatment planning was performed. The patient had persistent T2 signal abnormality on MRI within the left corpus immediately adjacent to the bladder. Due to the unfavorable organ-at-risk (OAR) topography and extent of disease, a single interstitial needle was placed at the site of the MRI T2 weighted signal abnormality and two treatment plans were generated one with intracavitary applicators alone and the other with combined intracavitary applicators and interstitial needle. The DVHs for HR-CTV and IR-CTV and OARs (rectum, bladder, and bowel) were compared between the intracavitary and the combined intracavitary-interstitial applications for a single HDR application. The placement of the split-ring applicator and interstitial needle was well tolerated by the patient and completed under conscious sedation in a dedicated HDR brachytherapy suite. The MR compatible split ring applicator allowed clear delineation of the target volumes, OARs and their spatial relationships with the applicator when using conventional MRI pulse sequences. Using the intracavitary split-ring applicator alone, the D90 to the HR-CTV was 5.7 Gy. When a combined interstitial and intracavitary approach was employed, the D90 of HR-CTV increased to 7.2 Gy, an increase of 26.3%, which allowed the total EQD2 dose of HR-CTV increasing to about 89Gy (ABS guideline is EQD2 80-90Gy). The dose to the IR-CTV improved marginally (1.5%). The dose to the bladder was reduced from 6.3Gy (EQD2= 99 Gy) to 4.6Gy (Eqd2=86Gy) achieving ABS recommended dose constraints of 90 Gy (EQD2). The Figure demonstrates in panel A the MR compatbile split-ring applicator. In panel B is the axial CT slice showing the titanium needle placed in the target volume. Panel C presents the prescribed isodose distribution and panel D depicts the 3D view of the applicators with respect to patient anatomy. This preliminary study demonstrates the dosimetric advantages and clinical feasibility of using a combined intracavitary and interstitial split-ring applicator in patients with significant residual disease after completing external beam radiation therapy. In this study, the addition of a solitary interstitial needle provided substantial reduction of bladder dose and improved the dose coverage to the high-risk target volume. Further clinical investigation in a larger patient cohort is warranted to explore the dosimetric advantages of this approach and evaluate if these advantages translate into improved clinical outcomes.