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

Purpose To assess the capability of an intracavitary Yb-169-based helical multishield rotating shield brachytherapy (RSBT) delivery system to treat cervical cancer. The proposed RSBT delivery system contains a pair of 1.25 mm thick platinum partial shields with 45 degrees and 180 degrees emission angles, which travel in a helical pattern within the applicator. Methods A helically threaded tandem applicator with a 45 degrees tandem curvature containing a helically threaded catheter was designed. A 0.6 mm diameter Yb-169 source with a length of 10.5 mm 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 41.2-192.8 cm(3) (average +/- standard deviation of 79.9 +/- 35.8 cm(3)). All patients were assumed to receive 25 fractions of 1.8 Gy of external beam radiation therapy (EBRT) before receiving 5 fractions of high-dose-rate brachytherapy (HDR-BT). For each patient, Ir-192-based intracavitary (IC) HDR-BT, Ir-192-based intracavitary/interstitial (IC/IS) HDR-BT using a hybrid applicator with eight IS needles, and Yb-169-based RSBT plans were generated. Results 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 D-90 (minimum dose to hottest 90%) of 85 Gy(EQD2) (alpha/beta = 10 Gy). 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. Ir-192 activities ranging from 159.1-370 GBq (4.3-10 Ci) and Yb-169 activities ranging from 429.2-999 GBq (11.6-27 Ci) were used, which correspond to the same clinical ranges of dose rates at 1 cm 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. Conclusion Yb-169-based RSBT for cervical cancer met the HR-CTV D-90 goal of 85 Gy 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. Yb-169-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 >= 30 cm(3).