RSBT can increase PTV EQD or decrease urethral D relative to HDR-BT at the cost of increased treatment time. Source aging reduces RSBT benefit, but RSBT remains theoretically superior to HDR-BT by >20% after 1 half-life has elapsed.

Purpose To present a novel multisource rotating shield brachytherapy (RSBT) apparatus for the simultaneous precise angular and linear positioning of partially-shielded 153Gd brachytherapy sources in interstitial needles for the treatment of locally-advanced prostate cancer. It is designed to lower the dose to nearby healthy tissues, the urethra in particular, relative to conventional high-dose-rate brachytherapy (HDR-BT) techniques. Methods and Materials Following needle implantation through the patient template, an angular drive mechanism is docked to the patient template. Each needle is coupled to a multisource afterloader catheter by a connector passing through a shaft. The shafts are rotated about their axes by translating a moving template between two stationary templates. Shafts’ surfaces and moving template holes are helically threaded with the same pattern such that translation of the moving template causes simultaneous rotation of the shafts. The rotation of each shaft is mechanically transmitted to the catheter/source/shield combination, inside the needles, via several key/keyway pairs. The catheter angles are simultaneously incremented throughout treatment, and only a single 360° rotation of all catheters is needed for a full treatment. For each rotation angle, source depth in each needle is controlled by a multisource afterloader, which is proposed as an array of belt-driven linear actuators, each of which drives a wire that controls catheter depth in a needle. Results Treatment plans demonstrated RSBT with the proposed apparatus reduced urethral D0.1cc below that of conventional HDR-BT by 31% for urethral dose gradient volume within 3 mm of the urethra surface. Treatment time to deliver 20 Gy with the proposed multisource RSBT apparatus using nineteen 62.4 GBq 153Gd sources is 122 min. Conclusions The proposed RSBT delivery apparatus enables a mechanically feasible urethra-sparing treatment technique for prostate cancer in a clinically reasonable timeframe.

Purpose: To present and quantify the effectiveness of a method for the efficient production of 169 Yb high-dose-rate brachytherapy sources with 27 Ci activity upon clinical delivery, which have about the same dose rate in water at 1 cm from the source center as 10 Ci 192 Ir sources. Materials: A theoretical framework for 169 Yb source activation and reactivation using thermal neutrons in a research reactor and 168 Yb-Yb 2 O 3 precursor is derived and benchmarked against published data. The model is dependent primarily on precursor 168 Yb enrichment percentage, active source volume of the active element, and average thermal neutron flux within the active source. Results: Efficiency gains in 169 Yb source production are achievable through reactivation, and the gains increase with active source volume. For an average thermal neutron flux within the active source of 1 9 10 14 n cm À2 s À1 , increasing the active source volume from 1 to 3 mm 3 decreased reactor-days needed to generate one clinic-year of 169 Yb from 256 days yr À1 to 59 days yr À1 , and 82%-enriched precursor dropped from 80 mg yr À1 to 21 mg yr À1 . A resource reduction of 74%-77% is predicted for an active source volume increase from 1 to 3 mm 3 . Conclusions: Dramatic cost savings are achievable in 169 Yb source production costs through reactivation if active sources larger than 1 mm 3 are used.

Purpose: 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 1.25 mm thick platinum partial shields with 45°and 180°emission angles, which travel in a helical pattern within the applicator. Methods: A helically threaded tandem applicator with a 45°tandem curvature containing a helically threaded catheter was designed. A 0.6 mm diameter 169 Yb 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 AE standard deviation of 79.9 AE 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, 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. 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 (a/ b = 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. 192 Ir activities ranging from 159.1-370 GBq (4.3-10 Ci) and 169 Yb 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: 169 Yb-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. 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 ≥30 cm 3 .

Purpose To present a system for the treatment of prostate cancer in a single‐fraction regimen using 169Yb‐based rotating shield brachytherapy (RSBT) with a single‐catheter robotic delivery system. The proposed system is innovative because it can deliver RSBT through multiple implanted needles independently, in serial, using flexible catheters, with no inter‐needle shielding effects and without the need to rotate multiple shielded catheters inside the needles simultaneously, resulting in a simple, mechanically robust, delivery approach. RSBT was compared to conventional 192Ir‐based high‐dose‐rate brachytherapy (HDR‐BT) in a treatment planning study with dose escalation and urethral sparing goals, representing single‐fraction brachytherapy monotherapy and brachytherapy as a boost to external beam radiotherapy, respectively. A prototype mechanical delivery system was constructed and quantitatively evaluated as a proof of concept. Methods Treatment plans for twenty‐six patients with single fraction prescriptions of 20.5 and 15 Gy, were created for dose escalation and urethral sparing, respectively. The RSBT and HDR‐BT delivery systems were modeled with one partially shielded 999 GBq (27 Ci) 169Yb source and one 370 GBq (10 Ci) 192Ir source, respectively. A prototype angular drive system for helical source delivery was constructed. Mechanical accuracy measurements of source translational position and angular orientation in a simulated treatment delivery setup were obtained using the prototype system. Results For dose escalation, with equivalent urethra D10%, PTV D90% for RSBT vs HDR‐BT increased from 22.6 ± 0.0 Gy (average ± standard deviation) to 29.3 ± 0.9 Gy, or 29.9 % ± 3.0%, with treatment times of 51.4 ± 6.1 min for RSBT and 15.8 ± 2.3 min for 10 Ci 192Ir‐based HDR‐BT. For urethra sparing, with equivalent PTV D90%, urethra D10% for RSBT vs HDR‐BT decreased for RSBT vs HDR‐BT from 15.6 ± 0.4 Gy to 12.0 ± 0.4 Gy, or 23.1% ± 3.5%, with treatment times of 30.0 ± 3.7 min for RSBT and 12.3 ± 1.8 min for HDR‐BT. Differences between measured vs predicted rotating catheter positions (corresponding to source position) were within 0.18 mm ± 0.12 mm longitudinally and 0.07° ± 0.78°. Conclusion 169Yb‐based RSBT can increase PTV D90% or decrease urethral D10% relative to HDR‐BT with treatment times of less than 1 h using a single‐source robotic delivery system with treatment delivered in a single fraction. The prototype helical delivery system was able to demonstrate adequate mechanical accuracy.

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