RESUMO
PURPOSE: Ionisation chamber based reference dosimetry in magnetic resonance linear accelerators (MRL) aimed for radiotherapy requires correction for recombination losses. Published studies have found that such corrections can be carried out using the two-voltage method. These studies have, however, not included comparison with recombination corrections based on the Niatel method, which can be seen as a robust reference method due to its clear separation of initial and volume recombination and its explicit account of the pulsed nature of the dose delivery. The primary objective of this work therefore was to carry out such a comparison. MATERIALS AND METHODS: Four Farmer-type chambers (PTW-30006 and PTW-30013) were placed in a water phantom in 1.5 T Elekta Unity MRL. The chambers were oriented antiparallel or perpendicular to the static magnetic field B0 and irradiated at a source-to-surface distance of 133.5 cm with a 10 × 10 cm2 field size. RESULTS: The two-voltage method gave results in agreement (within 0.1%) with the recombination corrections derived from the Niatel method. The recombination corrections from three Niatel parameter sets (one based on a Varian Truebeam and two obtained directly in the MRL) deviated less than 0.1% from each other. A systematic shift in the recombination correction of less than 0.05% was observed if polarity corrections were not applied. CONCLUSIONS: The study supports the use of the two-voltage method in MRLs based on its excellent agreement with the Niatel method. This work, therefore, complements existing knowledge as previous studies have not included a comparison with the Niatel method.
RESUMO
BACKGROUND: The standard in Denmark for treating breast cancer patients receiving loco-regional irradiation is tangential 3D Conformal RadioTherapy (3DCRT), treated in deep inspiration breath-hold (DIBH). Treating with Volumetric Modulated Arc Therapy (VMAT) may reduce the treatment time, which is particularly important for DIBH treatments. The VMAT should be performed without increased dose to the heart, lung, and contralateral breast. This study compares VMAT and 3DCRT for left-sided breast cancer patients with intramammary lymph node involvement. MATERIAL AND METHODS: Twenty left-sided breast cancer patients were included. VMAT and tangential plans were created for all patients, with a prescription dose of 50 Gy. The tangential plans used 6 MV and for larger breast combined with 18 MV. The VMAT plans utilised two 6 MV fields in a butterfly configuration. Dose planning was done in Pinnacle3 16.0 using the Auto-Planning module for the VMAT plans. Comparison of the plans was based on: mean doses, metrics provided by DBCG guidelines, dose-volume histograms and required number of breath-holds for treatment delivery in DIBH. RESULTS: For most OAR, the doses were similar for VMAT and 3DCRT. The target coverage was comparable, with VMAT having a statistically significant improved dose homogeneity of the target volumes. Less than half the number of breath-hold was required for VMAT compared to 3DCRT. Mean gamma pass rates (3 mm and 3%) from ArcCHECK of the VMAT plans was 98.4% (range 96.6-99.8%). CONCLUSION: Automatic VMAT planning of left-sided breast cancer patients with lymph node involvement can produce dose distributions comparable to those of tangential 3DCRT, while reducing the number of breath-holds in DIBH by more than a factor of two. The reduction in breath-holds is beneficial for patient comfort and reduces the risk of intra-fraction patient motion.
