Quality assurance of IMRT treatment plans for a 1.5 T MR-linac using a 2D ionization chamber array and a static solid phantom.

MR-guided radiotherapy requires novel quality assurance (QA) methods for intensity-modulated radiotherapy treatment plans (TPs). Here, an optimized method for TPs for a 1.5 T MR-linac was developed and implemented clinically. A static solid phantom and an MR-compatible 2D ionization chamber array were used. The array's response with respect to the incident beam gantry angles was characterized for four different orientations of the array relative to the beam. A lookup table was created identifying the optimum orientation for each gantry angle. For the QA of clinical MR-linac TPs, beams were grouped according to their gantry angles and measured with up to four setups. The method was applied to n = 106 clinical TPs of 54 patients for different tumour entities. Reference plans and plans created in the online adaptive workflow were analysed, using a local 3%/3 mm gamma criterion for dose values larger than 30% of the maximum. Pass rates were averaged over all beam groups. The array's response strongly depends on the beam incidence angle. Optimum angles typically range from -10° to 80° around the phantom setup angle. Consequently, plan verification required up to four setups. For clinical MR-linac TPs, the overall median pass rate was 98.5% (range 88.6%-100%). Pass rates depended on the tumour entity. Median pass rates were for liver metastases stereotactic body radiotherapy 99.2%, prostate cancer 99%, pancreatic cancer 98.9%, lymph node metastases 98.7%, partial breast irradiation (PBI) 98%, head-and-neck cancer 97.7%, rectal cancer 94% and others 96.6%. 85% of plans were accepted straightaway, with pass rates above 95%. A single plan with a pass rate below 90% was subsequently verified with a modified method. Off-axis target volumes, e.g. PBI, were verified successfully using a lateral shift of the phantom. The method is suitable to verify reference and online adapted TPs for a 1.5 T MR-linac, including plans for off-axis target volumes.

Partial breast irradiation with the 1.5 T MR-Linac: First patient treatment and analysis of electron return and stream effects.

INTRODUCTION: External beam partial breast irradiation (PBI) provides equal oncological outcomes compared to whole breast irradiation when applied to patients with low risk tumours. Recently, linacs with an integrated magnetic resonance image-guidance system have become clinically available. Here we report the first-in-human PBI performed at the 1.5 T MR-Linac, with a focus on clinical feasibility and investigation of the air electron stream effect (ESE) and the electron return effect (ERE) in the presence of the 1.5 T magnetic field, which might influence the dose on the chin (out-of-field dose, due to the ESE), the skin and the lung/chest wall interface (in-field dose, ERE). METHODS: A 59 years old patient affected by a 15 mm unifocal grade 1 carcinoma not special type of the right breast staged pT1c pN0 cM0 was planned and treated at Unity 1.5 T MR-Linac. To investigate the ERE and the ESE, an MR-Linac treatment plan was simulated without considering the 1.5 T B field using a research version of Monaco (V. 5.19.03). In vivo dosimetry was performed using Gafchromic® EBT3 films placed on top and underneath a 1 cm bolus which was placed on the patient's chin. The plans with and without 1.5 T magnetic field were compared in terms of dose to the chin, to the skin and to the interface lung/chest wall. Finally, the dose on the chin measured with the in vivo dosimetry was compared with the dose calculated by Monaco. RESULTS: PBI using the 1.5 T MR-Linac was successfully performed with a 7 MV photon 7-beams IMRT step-and-shoot plan. The treatment was well tolerated, the patient developed a slight acute toxicity, i.e. breast skin erythema and breast oedema CTC V.4 grade 1. The plan with 1.5 T magnetic field documented a fractional dose of 0.17 Gy in the chin area (2.6 Gy in 15 fractions), which was reduced to 0.05 Gy (0.75 in 15 fractions) by the presence of 1 cm bolus. The simulated plan without magnetic field showed a dose reduced by 2.3 Gy in the chin area. With the in vivo dosimetry a fractional dose of, respectively, 0.12 Gy and 0.034 Gy on top and underneath the bolus were measured (1.8 and 0.51 Gy in 15 fractions). The plan with 1.5 T magnetic field showed a skin D2 of 40 Gy and a skin V35 of 40.2%, which were reduced to, respectively, 39.7 Gy and 24.9% in the simulation without magnetic field. At the interface lung/chest there were no differences in DVH statistics. CONCLUSION: PBI with the 1.5 T MR-Linac was performed for the first time. ESE is accurately calculated by the treatment planning system, can be effectively reduced with a 1 cm bolus and is comparable to dose of cone beam-CT based position verification. The additional dose caused by ERE is not associated with an increased risk of acute toxicity.