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.

PurposeThe objective of this work was to investigate the accuracy of AAA dose calculation algorithm for RapidArc volumetric modulated technique (VMAT) in the presence of anatomical heterogeneities in the pelvic region.Material and methodsAn anthropomorphic phantom was used to simulate a prostate case, delineating planning target volumes (PTVs) and organs at risk. VMAT plans were optimised in eclipse (v10·0) treatment planning system (TPS). The dose distributions were calculated by the AAA dose calculation algorithm. A total of 49 thermoluminiscent dosimeters were inserted into the anthropomorphic phantom and dose measurements were compared with the predicted TPS doses.ResultsThe average dose variation was −1·5% for planning target volume corresponding to the prostate and −0·3% for planning target volume corresponding to the pelvic nodes, −0·2% for the rectum, +2·4% for the bladder, −2·0% for the femoral heads and +1·0% for the intestinal package.ConclusionAAA is a reliable dose calculation for the treatment with VMAT in the anatomy of the pelvis.