Needle‐free cervical cancer treatment using helical multishield intracavitary rotating shield brachytherapy with the 169Yb Isotope

Abstract

To assess the capability of an intracavitary 169 Yb-based helical multishield rotating shield brachytherapy (RSBT) delivery system to treat cervical cancer. The proposed RSBT delivery system contains a pair of 1.25mm thick platinum partial shields with 45° and 180° emission angles, which travel in a helical pattern within the applicator. A helically threaded tandem applicator with a 45° tandem curvature containing a helically threaded catheter was designed. A 0.6mm diameter 169 Yb source with a length of 10.5mm was simulated. A 37-patient treatment planning study, based on Monte Carlo dose calculations using MCNP5, was conducted with high-risk clinical target volumes (HR-CTVs) of 41.2-192.8cm3 (average±standard deviation of 79.9±35.8cm3 ). All patients were assumed to receive 25 fractions of 1.8Gy of external beam radiation therapy (EBRT) before receiving 5 fractions of high-dose-rate brachytherapy (HDR-BT). For each patient, 192 Ir-based intracavitary (IC) HDR-BT, 192 Ir-based intracavitary/interstitial (IC/IS) HDR-BT using a hybrid applicator with eight IS needles, and 169 Yb-based RSBT plans were generated. For the IC, IC/IS, and RSBT treatment plans, 38%, 84%, and 86% of the plans, respectively, met the planning goal of an HR-CTV D90 (minimum dose to hottest 90%) of 85 GyEQD2 (α/β=10Gy). Median (25th percentile, 75th percentile) treatment times for IC, IC/IS, and RSBT were 11.71 (6.62, 15.40)min, 68.00 (45.02, 80.02)min, and 25.30 (13.87, 35.39)min, respectively. 192 Ir activities ranging from 159.1-370GBq (4.3-10Ci) and 169 Yb activities ranging from 429.2-999GBq (11.6-27Ci) were used, which correspond to the same clinical ranges of dose rates at 1cm off-source-axis in water. Extra needle insertion and planning time beyond that needed for intracavitary-only approaches was accounted for in the IC/IS treatment time calculations. 169 Yb-based RSBT for cervical cancer met the HR-CTV D90 goal of 85Gy in a greater percentage of the patients considered than IC/IS (86% vs 84%, respectively) and can reduce overall treatment time relative to IC/IS. 169 Yb-based RSBT could be used to replace IC/IS in instances where IC/IS treatment is not available, especially in instances when HR-CTV volumes are ≥30cm3 .