Impact of respiratory motion on proton pencil beam scanning FLASH radiotherapy: anin silicoand phantom measurement study

Commissioning an ultra-high-dose-rate electron linac with end-to-end tests

Objective. The FLASH effect can potentially be used to improve the therapeutic ratio of radiotherapy (RT) through delivery of Ultra-high-dose-rate (UHDR) irradiation. Research is actively being conducted to translate UHDR-RT and for this purpose the Mobetron is capable of producing electron beams at both UHDR and conventional dose rates for FLASH research and translation. This work presents commissioning of an UHDR Mobetron with end-to-end tests developed for preclinical research.Approach. UHDR electron beams were commissioned with an efficient approach utilizing a 3D-printed water tank and film to fully characterize beam characteristics and dependences on field size, pulse width (PW) and pulse repetition frequency (PRF). This commissioning data was used to implement a beam model using the GAMOS Monte Carlo toolkit for the preclinical research. Then, the workflow for preclinical FLASH irradiation was validated with end-to-end tests delivered to a 3D-printed mouse phantom with internal inhomogeneities.Main results.PDDs, profiles and output factors acquired with radiochromic films were precisely measured, with a PRF that showed little effect on the UHDR beam energy and spatial characteristics. Increasing PW reduced theDmaxand R50by 2.08 mmµs-1and 1.28 mmµs-1respectively. An end-to-end test of the preclinical research workflow showed that both profiles in head-foot and lateral directions were in good agreement with the MC calculations for the heterogeneous 3D printed mouse phantom with Gamma index above 93% for 2 mm/2% criteria, and 99% for 3 mm/3%.Significance. The UHDR Mobetron is a versatile tool for FLASH preclinical research and this comprehensive beam model and workflow was validated to meet the requirements for conducting translational FLASH research.

Biological-equivalent-dose-based integrated optimization framework for fast-energy-switching Bragg peak FLASH-RT using single-beam-per-fraction

BACKGROUNDS

When comparing the delivery of all beams per fraction (ABPF) to single beam per fraction (SBPF), it is observed that SBPF not only helps meet the FLASH dose threshold but also mitigates the uncertainty with beam switching in the FLASH effect. However, SBPF might lead to a higher biological equivalent dose in 2 Gy (EQD2) for normal tissues.

PURPOSE

This study aims to develop an EQD2-based integrated optimization framework (EQD2-IOF), encompassing robust dose, delivery efficiency, and beam orientation optimization (BOO) for Bragg peak FLASH plans using the SBPF treatment schedule. The EQD2-IOF aims to enhance both dose sparing and the FLASH effect.

METHODS

A superconducting gantry was employed for fast energy switching within 27 ms, while universal range shifters were utilized to improve beam current in the implementation of FLASH plans with five Bragg peak beams. To enhance dose delivery efficiency while maintaining plan quality, a simultaneous dose and spot map optimization (SDSMO) algorithm for single field optimization was incorporated into a Bayesian optimization-based auto-planning algorithm. Subsequently, a BOO algorithm based on Tabu search was developed to select beam angle combinations (BACs) for 10 lung cases. To simultaneously consider dose sparing and FLASH effect, a quantitative model based on dose-dependent dose modification factor (DMF) was used to calculate FLASH-enhanced dose distribution. The EQD2-IOF plan was compared to the plan optimized without SDSMO using BAC selected by a medical physicist (Manual plan) in the SBPF treatment schedule. Meanwhile, the mean EQD2 in the normal tissue was evaluated for the EQD2-IOF plan in both SBPF and ABPF treatment schedules.

RESULTS

No significant difference was found in D2% and D98% of the target between EQD2-IOF plans and Manual Plans. When using a minimum DMF of 0.67 and a dose threshold of 4 Gy, EQD2-IOF plans showed a significant reduction in FLASH-enhanced EQD2mean of the ipsilateral lung and normal tissue by 10.5% and 11.5%, respectively, compared to Manual plans. For normal tissues that received a dose greater than 70% of the prescription dose, using a minimum DMF of 0.7 for FLASH sparing compensated for the increase in EQD2mean resulting from replacing ABPF with SBPF schedules.

CONCLUSIONS

The EQD2-IOF can automatically optimize SBPF FLASH-RT plans to achieve optimal sparing of normal tissues. With an energy switching time of 27 ms, the loss of fractionate repairing using SBPF schedules in high-dose regions can be compensated for by the FLASH effect. However, when an energy switching time of 500 ms is utilized, the SBPF schedule needs careful consideration, as the FLASH effect diminishes with longer irradiation time.

A review of the clinical introduction of 4D particle therapy research concepts

Background and purpose

Many 4D particle therapy research concepts have been recently translated into clinics, however, remaining substantial differences depend on the indication and institute-related aspects. This work aims to summarise current state-of-the-art 4D particle therapy technology and outline a roadmap for future research and developments.

Material and methods

This review focused on the clinical implementation of 4D approaches for imaging, treatment planning, delivery and evaluation based on the 2021 and 2022 4D Treatment Workshops for Particle Therapy as well as a review of the most recent surveys, guidelines and scientific papers dedicated to this topic.

Results

Available technological capabilities for motion surveillance and compensation determined the course of each 4D particle treatment. 4D motion management, delivery techniques and strategies including imaging were diverse and depended on many factors. These included aspects of motion amplitude, tumour location, as well as accelerator technology driving the necessity of centre-specific dosimetric validation. Novel methodologies for X-ray based image processing and MRI for real-time tumour tracking and motion management were shown to have a large potential for online and offline adaptation schemes compensating for potential anatomical changes over the treatment course. The latest research developments were dominated by particle imaging, artificial intelligence methods and FLASH adding another level of complexity but also opportunities in the context of 4D treatments.

Conclusion

This review showed that the rapid technological advances in radiation oncology together with the available intrafractional motion management and adaptive strategies paved the way towards clinical implementation.