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Background: Whole-brain radiation therapy (WBRT) and stereotactic radiosurgery (SRS) are two treatment modalities commonly utilized to treat brain metastases (BMs). Aim: The purpose of this study is to analyze retrospectively the local control and survival of patients with BMs of breast cancer (BC) treated via radiosurgery using Volumetric Modulated Arc Therapy (VMAT-RS). Methods: 18 patients with 41 BMs of BC and treated by VMAT-RS were studied. They were classified according to the molecular subtype of BC and the modified breast graded prognostic assessment -GPA- index. Patients presented 1-4 BMs, which were treated with 5 non-coplanar VMAT arcs. The spatial distribution of BMs, the influence of receptor status on the location of the lesions and survival assessed via the Kaplan-Meier model were analyzed. Results: The median survival time (MST) was 19.7 months. Statistically significant differences were determined in the MST according to the Karnofsky performance status (p= 0.02) and the HER2 status (p= 0.004), being more prolonged in the HER2+ patients. Finally, our results showed that the cerebellum is the predominant site of breast cancer BMs, and also suggested that HER2+BMs had a predilection for some structures of the posterior circulation, such as the cerebellum, brainstem and occipital lobes (p= 0.048). Conclusions: The VMAT-RS is a technique with an overall survival compared to other radiosurgery techniques. The baseline situation at the time of treatment, the modified breast-GPA and the molecular subtypes are factors that significantly influence patient survival.

PurposeTo evaluate the dosimetric errors associated with the effect of the collimator angle error in volumetric-modulated arc therapy (VMAT) treatments.Methods and materialsFour patients with different planning target volume (PTV) and localisations treated using VMAT were analysed (high-risk prostate, low-risk prostate, head and neck (H&N) and holocranial with hippocampus protection) in terms of dosimetric variations when errors in the collimator angle were introduced. Original plans underwent modifications of the planned collimator angles of ±0·5°, ±1° and ±1·5°. These modified plans were re-calculated using the same original plan fluencies, and the resulting dose–volume histograms and homogeneity index (HI-ICRU) were compared.ResultsFor the high-risk prostate case, there was a noticeable loss of PTV dose coverage for collimator angle errors larger than ±1°, with HI-ICRU relative variations up to 75% in the range analysed. The low-risk prostate case did not present significant changes in organs at risk or PTV dose coverage. For the H&N case, the spinal cord presented changes around 4% forD0·1 cc. In the holocranial case, optic lens showed dose variations up to 5% for collimator angle errors larger than ±1°.ConclusionsThe effect of the collimator error in VMAT increased as the PTV increased.For selecting the position of the isocentre, one should be cautious, and whenever possible choose a position close to the geometrical centre of the PTVs in order to avoid or minimise errors from the calibration of the collimator angle.

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