SU-F-E-06: Dosimetric Characterization of Small Photons Beams of a Novel Linear Accelerator

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Abstract

Purpose:

The aim of the present contribution was to measure the main dosimetric quantities of small fields produced by UNIQUE and evaluate its matching with the corresponding dosimetric data of one 21EX conventional linear accelerator (Varian) in operation at the same center. The second step was to evaluate comparative performance of the EDGE diode detector and the PinPoint micro-ionization chamber for dosimetry of small fields.

Methods:

UNIQUE is configured with MLC (120 leaves with 0.5 cm leaf width) and a single low photon energy of 6 MV. Beam data were measured with scanning EDGE diode detector (volume of 0.019 mm3), a PinPoint micro-ionization chamber (PTW) and for larger fields (≥ 4×4cm2) a PTW Semi flex chamber (0.125 cm3) was used. The scanning system used was the 3D cylindrical tank manufactured by Sun Nuclear, Inc. The measurement of PDD and profiles were done at 100 cm SSD and 1.5 depth; the relative output factors were measured at 10 cm depth.

Results:

PDD and the profile data showed less than 1% variation between the two linear accelerators for fields size between 2×2 cm2 and 5×5cm2. Output factor differences was less than 1% for field sizes between 3×3 cm2 and 10×10 cm2 and less of 1.5 % for fields of 1.5×1.5 cm2 and 2×2 cm2 respectively. The dmax value of the EDGE diode detector, measured from the PDD, was 8.347 mm for 0.5×0,5cm2 for UNIQUE. The performance of EDGE diode detector was comparable for all measurements in small fields.

Conclusion:

UNIQUE linear accelerator show similar dosimetrics characteristics as conventional 21EX Varian linear accelerator for small, medium and large field sizes.EDGE detector show good performance by measuring dosimetrics quantities in small fields typically used in IMRT and radiosurgery treatments.

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