H-RSBT is a mechanically feasible delivery technique for use in the curved applicators needed for cervical cancer brachytherapy. S-RSBT and H-RSBT were clinically equivalent for all patients considered, with the H-RSBT technique tending to require less time for delivery.
Purpose To provide a fast computational method, based on the proximal graph solver (POGS) – A convex optimization solver using the alternating direction method of multipliers (ADMM), for calculating an optimal treatment plan in rotating shield brachytherapy (RSBT). RSBT treatment planning has more degrees of freedom than conventional high-dose-rate brachytherapy due to the addition of emission direction, and this necessitates a fast optimization technique to enable clinical usage. Methods The multi-helix RSBT (H-RSBT) delivery technique was investigated for five representative cervical cancer patients. Treatment plans were generated for all patients using the POGS method and the commercially available solver IBM ILOG CPLEX. The rectum, bladder, sigmoid colon, high-risk clinical target volume (HR-CTV), and HR-CTV boundary were the structures included in our optimization, which applied an asymmetric dose-volume optimization with smoothness control. Dose calculation resolution was 1 × 1 × 3 mm3 for all cases. The H-RSBT applicator had 6 helices, with 33.3 mm of translation along the applicator per helical rotation and 1.7 mm spacing between dwell positions, yielding 17.5° emission angle spacing per 5 mm along the applicator. Results For each patient, HR-CTV D90, HR-CTV D100, rectum D2cc, sigmoid D2cc, and bladder D2cc matched within 1% for CPLEX and POGS methods. Also, similar EQD2 values between CPLEX and POGS methods were obtained. POGS was around 18 times faster than CPLEX. For all patients, total optimization times were 32.1–65.4 s for CPLEX and 2.1–3.9 s for POGS. Conclusions POGS reduced treatment plan optimization time approximately 18 times for RSBT with similar HR-CTV D90, organ at risk (OAR) D2cc values, and EQD2 values compared to CPLEX, which is significant progress toward clinical translation of RSBT.
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: The authors present a novel paddle-based rotating-shield brachytherapy (P-RSBT) method, whose radiation-attenuating shields are formed with a multileaf collimator (MLC), consisting of retractable paddles, to achieve intensity modulation in high-dose-rate brachytherapy. Methods: Five cervical cancer patients using an intrauterine tandem applicator were considered to assess the potential benefit of the P-RSBT method. The P-RSBT source used was a 50 kV electronic brachytherapy source (Xoft Axxent™). The paddles can be retracted independently to form multiple emission windows around the source for radiation delivery. The MLC was assumed to be rotatable. P-RSBT treatment plans were generated using the asymmetric dose-volume optimization with smoothness control method [Liu et al., Med. Phys. 41(11), 111709 (11pp.) (2014)] with a delivery time constraint, different paddle sizes, and different rotation strides. The number of treatment fractions (fx) was assumed to be five. As brachytherapy is delivered as a boost for cervical cancer, the dose distribution for each case includes the dose from external beam radiotherapy as well, which is 45 Gy in 25 fx. The high-risk clinical target volume (HR-CTV) doses were escalated until the minimum dose to the hottest 2 cm 3 (D 2cm 3) of either the rectum, sigmoid colon, or bladder reached their tolerance doses of 75, 75, and 90 Gy 3 , respectively, expressed as equivalent doses in 2 Gy fractions (EQD2 with α/ β = 3 Gy). Results: P-RSBT outperformed the two other RSBT delivery techniques, single-shield RSBT (S-RSBT) and dynamic-shield RSBT (D-RSBT), with a properly selected paddle size. If the paddle size was angled at 60• , the average D 90 increases for the delivery plans by P-RSBT on the five cases, compared to S-RSBT, were 2.2, 8.3, 12.6, 11.9, and 9.1 Gy 10 , respectively, with delivery times of 10, 15, 20, 25, and 30 min/fx. The increases in HR-CTV D 90 , compared to D-RSBT, were 16.6, 12.9, 7.2, 3.7, and 1.7 Gy 10 , respectively. P-RSBT HR-CTV D 90 -values were insensitive to the paddle size for paddles angled at less than 60• . Increasing the paddle angle from 5• to 60• resulted in only a 0.6 Gy 10 decrease in HR-CTV D 90 on average for five cases when the delivery times were set to 15 min/fx. The HR-CTV D 90 decreased to 2.5 and 11.9 Gy 10 with paddle angles of 90• and 120• , respectively. Conclusions: P-RSBT produces treatment plans that are dosimetrically and temporally superior to those of S-RSBT and D-RSBT, although P-RSBT systems may be more mechanically challenging to develop than S-RSBT or D-RSBT. A P-RSBT implementation with 4-6 shield paddles would be sufficient to outperform S-RSBT and D-RSBT if delivery times are constrained to less than 15 min/fx.
Purpose: To present a novel and practical brachytherapy technique, called multi‐helix rotating shield brachytherapy (H‐RSBT), for the precise positioning of a partial shield in a curved applicator. H‐RSBT enables RSBT delivery using only translational motion of the radiation source/shield combination. H‐RSBT overcomes the challenges associated with previously proposed RSBT approaches based on a serial (S‐RSBT) step‐and‐shoot delivery technique, which required independent translational and rotational motion. Methods: A Fletcher‐type applicator, compatible with the combination of a Xoft Axxent™ electronic brachytherapy source and a 0.5 mm thick tungsten shield, is proposed. The wall of the applicator contains six evenly‐spaced helical keyways that rigidly define the emission direction of the shield as a function of depth. The shield contains three protruding keys and is attached to the source such that it rotates freely. S‐RSBT and H‐RSBT treatment plans with 180° and 45° azimuthal emission angles were generated for five cervical cancer patients representative of a wide range of high‐risk clinical target volume (HR‐CTV) shapes and applicator positions. The number of beamlets used in the treatment planning process was nearly constant for S‐RSBT and H‐RSBT by using dwell positions separated by 5 and 1.7 mm, respectively, and emission directions separated by 22.5° and 60°, respectively. For all the treatment plans the EQD2 of the HR‐CTV was escalated until the EQD2cc tolerance of either the bladder, rectum, or sigmoid colon was reached. Results: Treatment times for H‐RSBT tended to be shorter than for S‐RSBT, with changes of −38.47% to 1.12% with an average of −8.34%. The HR‐CTV D90 changed by −8.81% to 2.08% with an average of −2.46%. Conclusion: H‐RSBT is a mechanically feasible technique in the curved applicators needed for cervical cancer brachytherapy. S‐RSBT and H‐RSBT dose distributions were clinically equivalent for all patients considered, with the H‐RSBT deliveries tending to be faster. Ryan Flynn has ownership interest in pxAlpha, LLC, which is a startup company developing a rotating shield brachytherapy system.
Purpose: To introduce a novel multi‐source rotating shield brachytherapy (RSBT) apparatus for the precise simultaneous angular and linear positioning of all partially‐shielded 153Gd radiation sources in interstitial needles for treating prostate cancer. The mechanism is designed to lower the detrimental dose to healthy tissues, the urethra in particular, relative to conventional high‐dose‐rate brachytherapy (HDR‐BT) techniques. Methods: Following needle implantation, the delivery system is docked to the patient template. Each needle is coupled to a multi‐source afterloader catheter by a connector passing through a shaft. The shafts are rotated by translating a moving template between two stationary templates. Shaft walls as well as moving template holes are threaded such that the resistive friction produced between the two parts exerts enough force on the shafts to bring about the rotation. Rotation of the shaft is then transmitted to the shielded source via several keys. Thus, shaft angular position is fully correlated with the position of the moving template. The catheter angles are simultaneously incremented throughout treatment as needed, 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 multi‐source afterloader, which is proposed as an array of belt‐driven linear actuators, each of which drives a source wire. Results: Optimized treatment plans based on Monte Carlo dose calculations demonstrated RSBT with the proposed apparatus reduced urethral D1cc below that of conventional HDR‐BT by 35% for urethral dose gradient volume within 3 mm of the urethra surface. Treatment time to deliver 20 Gy with multi‐source RSBT apparatus using nineteen 62.4 GBq 153Gd sources is 117 min. Conclusions: The proposed RSBT delivery apparatus in conjunction with multiple nitinol catheter‐mounted platinum‐shielded 153Gd sources enables a mechanically feasible urethra‐sparing treatment technique for prostate cancer in a clinically reasonable timeframe.
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