FLASH Radiotherapy Using Single-Energy Proton PBS Transmission Beams for Hypofractionation Liver Cancer: Dose and Dose Rate Quantification

Abstract

PurposeThis work aims to study the dose and ultra-high-dose rate characteristics of transmission proton pencil beam scanning (PBS) FLASH radiotherapy (RT) for hypofractionation liver cancer based on the parameters of a commercially available proton system operating under FLASH mode.Methods and MaterialsAn in-house treatment planning software (TPS) was developed to perform intensity-modulated proton therapy (IMPT) FLASH-RT planning. Single-energy transmission proton PBS plans of 4.5 Gy × 15 fractions were optimized for seven consecutive hepatocellular carcinoma patients, using 2 and 5 fields combined with 1) the minimum MU/spot chosen between 100 and 400, and minimum spot time (MST) of 2 ms, and 2) the minimum MU/spot of 100, and MST of 0.5 ms, based upon considerations in target uniformities, OAR dose constraints, and OAR FLASH dose rate coverage. Then, the 3D average dose rate distribution was calculated. The dose metrics for the mean dose of Liver-GTV and other major OARs were characterized to evaluate the dose quality for the different combinations of field numbers and minimum spot times compared to that of conventional IMPT plans. Dose rate quality was evaluated using 40 Gy/s volume coverage (V40Gy/s).ResultsAll plans achieved favorable and comparable target uniformities, and target uniformity improved as the number of fields increased. For OARs, no significant dose differences were observed between plans of different field numbers and the same MST. For plans using shorter MST and the same field numbers, better sparing was generally observed in most OARs and was statistically significant for the chest wall. However, the FLASH dose rate coverage V40Gy/s was increased by 20% for 2-field plans compared to 5-field plans in most OARs with 2-ms MST, which was less evident in the 0.5-ms cases. For 2-field plans, dose metrics and V40Gy/s of select OARs have large variations due to the beam angle selection and variable distances to the targets. The transmission plans generally yielded inferior dosimetric quality to the conventional IMPT plans.ConclusionThis is the first attempt to assess liver FLASH treatment planning and demonstrates that it is challenging for hypofractionation with smaller fractional doses (4.5 Gy/fraction). Using fewer fields can allow higher minimum MU/spot, resulting in higher OAR FLASH dose rate coverages while achieving similar plan quality compared to plans with more fields. Shorter MST can result in better plan quality and comparable or even better FLASH dose rate coverage.

Advanced pencil beam scanning Bragg peak FLASH-RT delivery technique can enhance lung cancer planning treatment outcomes compared to conventional multiple-energy proton PBS techniques

“…FLASH dose rates were also achieved and demonstrated to be an effective treatment in a human treated with a cutaneous T cell lymphoma [16], with treatment sparing normal skin and maximizing tumor control. This potential increase in the therapeutic ratio offered by FLASH has also attracted researchers to explore a wide range of FLASH-relevant topics on FLASH delivery techniques [17][18][19][20][21][22], mechanisms [23][24][25][26][27], and clinical translations such as treatment planning [28][29][30][31][32][33][34] and dosimetry [35][36].…”

Dose rate and dose robustness for proton transmission FLASH-RT treatment in lung cancer

“…Here we chose a feasible cyclotron current for the Varian ProBeam system, and under this current, the FLASH coverage for both plans is close. We demonstrated earlier in (7,8) that a less minimum spot time in the FLASH spot map delivery will allow smaller MUs in the treatment plans at the fixed beam current, likely resulting in superior plan quality to ones with greater minimum spot times.…”

“…As proton beams shoot through the body instead of being stopped as in the conventional proton treatment plans, the dosimetric performances are also expected to be very different. Several studies have investigated proton pencil beam scanning (PBS) transmission planning in the lung ( 1 , 2 ), head and neck ( 4 , 5 ), brain ( 6 ), and liver patients ( 7 , 8 ). All of those studies have shown promising dosimetric outcomes using transmission beams, which helps to inform the characteristics of proton transmission beams for treatment planning.…”

Recent progress in pencil beam scanning FLASH proton therapy: a narrative review

“…Another important factor in treatment planning is the number of beams and beam arrangement. Wei et al (40) studied the FLASH treatment planning for hypofractionation liver cancer cases by considering machine-related factors including beam current, minimum MU/spot, spot reduction, minimum spot time, and realistic plan-related factors including beam angular arrangement and number of fields. The results suggest minimum spot time and the number of beams are important parameters for the liver plans' FLASH dose rate coverage, target uniformity and coverage, and OAR dose constraints.…”