Abstract
Small field dosimetry is a challenging task. The difficulties of small field measurements, particularly stereotactic field size measurements, are highlighted by the large interinstitution variability that can be observed for circular cone collimator commissioning measurements. We believe the best way to improve the consistency of small field measurements is to clearly document and share the results of small field measurements. In this work we report on the commissioning and validation of a BrainLAB cone system for 6 MV and 10 MV flattening filter‐free (FFF) beams on a Varian TrueBeam STx. Commissioning measurements consisted of output factors, percent depth dose, and off‐axis factor measurements with a diode. Validation measurements were made in a polystyrene slab phantom at depths of 5 cm, 10 cm, and 15 cm using radiochromic film. Output factors for the 6xFFF cones are 0.689, 0. 790, 0.830, 0.871, 0.890, and 0.901 for 4 mm, 6 mm, 7.5 mm, 10 mm, 12.5 mm, and the 15 mm cones, respectively. Output factors for the 10xFFF cones are 0.566, 0. 699, 0.756, 0.826, 0.864, and 0.888 for 4 mm, 6 mm, 7.5 mm, 10 mm, 12.5 mm, and the 15 mm cones, respectively. The full width half maximum values of the off‐axis factors agreed with the nominal cone size to within 0.5 mm. Validation measurements showed an agreement of absolute dose between calculation and plan of ≤ 3.6%, and an agreement of field sizes of ≤ 0.3 mm in all cases. Radiochromic film validation measurements show reasonable agreement with beam models for circular collimators based on diode commissioning measurements.PACS numbers: 87.53.Ly, 87.53.Bn, 87.56.nk, 87.55.D‐, 87.55.km
Highlights
Flattening filter-free (FFF) treatment beams have become commercially available on several platforms, including the Varian TrueBeam (Varian Medical Systems, Palo Alto, CA)
A good deal of work has been done on the modeling, measurement, and characterization of flattening filter-free (FFF) beams in general[4,5,6,7,8,9,10,11,12,13,14] and in characterization of FFF beams on the TrueBeam.[15,16,17,18] there is a paucity of published data on the characterization of stereotactic radiosurgery (SRS) cones,(19-22) and of SRS cones coupled with FFF beams in particular
Measured dmax for the 10xFFF cones ranges from 11 mm for the 4 mm cone to 18 mm for the 15 mm cone
Summary
Flattening filter-free (FFF) treatment beams have become commercially available on several platforms, including the Varian TrueBeam (Varian Medical Systems, Palo Alto, CA). Flattening filter-free beams may offer a number of advantages compared to conventional flattened beams, such as increased dose rate, decreased out of field scatter dose, and decreased neutron production.[1] The increased dose rate of the FFF beams is the attribute that has garnered the most clinical attention. The high dose rate of the FFF beams may have the largest impact on stereotactic radiosurgery (SRS), stereotactic body radiotherapy (SBRT), and intensity-modulated radiation therapy (IMRT). These treatments generally utilize restrictive immobilization devices to limit patient motion and maximize setup reproducibility. Small field commissioning and verification measurements are technically challenging[23,24] and have been shown to vary among institutions.[25]
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