(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: Despite the availability of beam modulation on modern linacs, some clinics still make use of tertiary collimation for stereotactic brain lesions (SRS/SRT). This old-fashioned form of collimation has the geometric advantage of being in closer proximity to the patient’s head hence reducing the beam penumbra as compared to multileaf collimation. Results from commissioning measurements are reported for a Varian TrueBeam, 6-MV and 10-MV flattening filter free beams.
Methods: Dosimetric parameters for seven cones of diameters 4, 5, 7.5, 10, 12.5, 15 and 17.5?mm were obtained for both energies, jaws were fixed at 50mmx50mm. Depth doses and output factors (OF) for all collimators were measured with a stereotactic diode (IBA) for which a daisy-chain strategy for relative output correction was performed using a 40mmx40mm field against an ionization chamber. In addition, OF values were investigated using an Exradin W1 Scintillator detector (Standard Imaging), SSD of 95 cm and depth of 5 cm. TMR were calculated using the depth doses following a published conversion strategy by van Battum et al.
Results: During scanning, no reference detector can be positioned without casting significant shadow on the signal, hence the scans are obtained in step mode. Further the TPR option on the tank could be challenging for this scenario. A conversion of depth-dose into Tissue-Maximum ratios (TMR) was proven reasonably efficient and accurate. The choice of the detector is of critical importance. Stereotactic diode results provided acceptable agreement within 2-3% of the Varian data. OFs measured for the smaller cones differed beyond this value when using the Scintillator detector and require further investigation.
Conclusion: measurement of small fields remain challenging compared to regular field size but agreement was found between the manufacturer data and current measurements when similar detector, that is a diode detector was used.
Keywords
Not Applicable / None Entered.
Taxonomy
TH- External Beam- Photons: Small field experimental dosimetry
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