Improving delivery efficiency using spots and energy layers reduction algorithms based on a large momentum acceptance beamline.

BACKGROUND: Intensity-modulated proton therapy (IMPT) is a well-known delivery method of proton therapy. Besides higher plan quality, reducing the delivery time is also essential to IMPT plans. It can enhance patient comfort, reduce treatment costs, and improve delivery efficiency. From the perspective of treatment efficacy, it contributes to mitigating the intra-fractional motions and improving the accuracy of radiotherapy, especially for moving tumors. PURPOSE: However, there is a tradeoff problem between the plan quality and delivery time. We consider the potential of a large momentum acceptance (LMA) beamline and apply the spots and energy layers reduction method to reduce the delivery time. METHODS: The delivery time for each field consists of the energy layer switching time, spot traveling time, and dose delivery time. The larger momentum spread and higher intensity beam offered by the LMA beamline contribute to reducing the total delivery time compared to the conventional beamline. In addition to the dose fidelity term, an L1 and logarithm items were added to the objective function to increase the sparsity of the low-weighted spots and energy layers. After that, the low-weighted spots and layers were iteratively excluded in the reduced plan, which reduced the energy layer switching time and spot traveling time. We used the standard, reduced, and LMA-reduced plans to validate the proposed method and tested it on prostate and nasopharyngeal cases. Then, we compared and evaluated the plan quality, treatment time, and plan robustness against delivery uncertainty. RESULTS: Compared with the standard plans, the number of spots in the LMA-reduced plans was on average reduced by 13 400 (95.6%) for prostate cases and by 48 300 (80.7%) for nasopharyngeal cases and the number of energy layers was on average reduced by 49 (61.3%) for prostate cases and by 97 (50.5%) for nasopharyngeal cases. And, the delivery time of the LMA-reduced plans was shortened from 34.5 to 8.6 s for prostate cases and from 163.8 to 53.6 s for nasopharyngeal cases. The LMA-reduced plans had comparable robustness to the spot monitor unit (MU) error compared with the standard plans, but the LMA-reduced plans became more sensitive to spot position uncertainty. CONCLUSION: The delivery efficiency can be significantly improved using the LMA beamline and spots and energy layers reduction strategies. The method is promising to improve the efficiency of motion mitigation strategies for treating moving tumors.

Design and optimization of beam optics for a superconducting gantry applied to proton therapy.

PURPOSE: Proton therapy (PT) is a precise and effective radiotherapy method for tumors. To reduce the weight and footprint of normal conducting gantries applied to PT, a lightweight superconducting (SC) gantry with large momentum acceptance is studied at Huazhong University of Science and Technology. METHODS: To limit the frequency of field changing in SC magnets, a local-achromaticity beamline is designed with large momentum acceptance. Based on the analysis of high order aberrations, the lattice of the gantry beamline is composed of two symmetric bending sections to limit high-order aberrations, and sextupole fields are superimposed to further eliminate dispersion up to second order. RESULTS: We presented a second order beam optics design of a SC gantry. The optics fitting is completed with COSY Infinity and the result of particle tracking shows a momentum acceptance of ±8%. Alternating gradient canted-cosine-theta (CCT) magnets are applied to implement combined functions of beam bending and focusing. Some methods are proposed to minimize the field distortion in curved CCT magnets. CONCLUSIONS: The beam optics with high order considerations of a lightweight SC gantry with large momentum acceptance is presented.

Design of a light and fast energy degrader for a compact superconducting gantry with large momentum acceptance.

PURPOSE: Proton therapy is a precise radiation cancer treatment with low side effects. To reduce the cost and footprint of the facility, the superconducting gantry with large momentum acceptance becomes a potential solution. Benefit from this feature, beam delivery time depends largely on the energy-switching process and short time is helpful for increasing the number of volume repaintings. METHODS: This note introduces an energy degrader with lightweight moving parts and a new hybrid structure (wedge-block-block). The total energies are separated into three stages and are degraded at fixed rates in two boron carbide blocks. As only one pair of graphite wedges is used for energy modulation, the energy switching at each step reaches a 10 ms level. RESULTS: The transport process in the degrader was simulated in TOPAS. After the degradation, the maximum energy spread (1σ) was approximately 5.5%, and the distance between successive energy layers can be increased for treating non-sensitive tissues. Six configurations of the hybrid degrader achieved distinctly higher transmission efficiencies than the usual graphite multi-wedge degrader. Finally, the configuration that maximized the beam transmission in the lower-energy range (namely, the W-B1-B2 configuration) was chosen as the degrader. CONCLUSIONS: This new degrader not only improved the transmission efficiency, but also reduced the energy-switching time by virtue of its light and compact structure.