A novel minimally invasive dynamic-shield, intensity-modulated brachytherapy system for the treatment of cervical cancer

Abstract

Purpose: To present a novel, MRI-compatible dynamicshield intensity modulated brachytherapy (IMBT) applicator and delivery system using ¹⁹² Ir, ⁷⁵ Se, and ¹⁶⁹ Yb radioisotopes for the treatment of locally advanced cervical cancer. Needle-free IMBT is a promising technique for improving target coverage and organs at risk (OAR) sparing.

Methods and materials: The IMBT delivery system dynamically controls the rotation of a novel tungsten shield placed inside an MRI-compatible, 6-mm wide intrauterine tandem. Using 36 cervical cancer cases, conventional intracavitary brachytherapy (IC-BT) and intracavitary/interstitial brachytherapy (IC/IS-BT) (10Ci ¹⁹² Ir) plans were compared to IMBT (10Ci ¹⁹² Ir; 11.5Ci ⁷⁵ Se; 44Ci ¹⁶⁹ Yb). All plans were generated using the Geant4-based Monte Carlo dose calculation engine, RapidBrachyMC. Treatment plans were optimized then normalized to the same high-risk clinical target volume (HR-CTV) D₉₀ and the D_2cc for bladder, rectum, and sigmoid in the research brachytherapy planning system, RapidBrachyMCTPS. Plans were renormalized until either of the three OAR reached dose limits to calculate the maximum achievable HR-CTV D₉₀ and D₉₈ .

Results: Compared to IC-BT, IMBT with either of the three radionuclides significantly improves the HR-CTV D₉₀ and D₉₈ by up to 5.2% ± 0.3% (P < 0.001) and 6.7% ± 0.5% (P < 0.001), respectively, with the largest dosimetric enhancement when using ¹⁶⁹ Yb followed by ⁷⁵ Se and then ¹⁹² Ir. Similarly, D_2cc for all OAR improved with IMBT by up to 7.7% ± 0.6% (P < 0.001). For IC/IS-BT cases, needle-free IMBT achieved clinically acceptable plans with ¹⁶⁹ Yb-based IMBT further improving HR-CTV D₉₈ by 1.5% ± 0.2% (P = 0.034) and decreasing sigmoid D_2cc by 1.9% ± 0.4% (P = 0.048). Delivery times for IMBT are increased by a factor of 1.7, 3.3, and 2.3 for ¹⁹² Ir, ⁷⁵ Se, and ¹⁶⁹ Yb, respectively, relative to conventional ¹⁹² Ir BT.

Conclusions: Dynamic shield IMBT provides a promising alternative to conventional IC- and IC/IS-BT techniques with significant dosimetric enhancements and even greater improvements with intermediate energy radionuclides. The ability to deliver a highly conformal, OAR-sparing dose without IS needles provides a simplified method for improving the therapeutic ratio less invasively and in a less resource intensive manner.

Keywords: Image-guided cervix brachytherapy; Intensity modulated brachytherapy; MRI-guided GYN brachytherapy; Monte Carlo based dosimetry.