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Validation of Inhomogeneity Correction in Small Field

L Hu*, P Galavis , I Das , NYU Langone Medical Center, New York, NY


(Wednesday, 8/1/2018) 9:30 AM - 10:00 AM

Room: Exhibit Hall | Forum 3

Purpose: Small field dosimetry is challenging but critical in modern radiation treatment. Various publications have provided k(fclin,fmsr) to convert detector readings to dose in homogenous medium. However, validity of k values with inhomogeneity is unknown which is presented in this study.

Methods: Plastic scintillator detector, PSD-W1 (Standard Imaging) is an ideal detector with k(fclin,fmsr)=1.0 in water. An specifically designed lung phantom consisting of 5 cm thick commercial grade lung slab sandwiched between cork and solid water slabs was used. A custom made central hole was provided in the slab accommodating the W1 detector. The lung phantom was scanned on a CT simulator. The images were sent to the Eclipse TPS for planning. A 6 MV beam was planned on this phantom in a series of single fields ranging from 0.5x0.5 to 10x10 cm² using AAA and Acuros algorithms with 100 MU delivered to the W1 center at 100cm SAD with 0.1x0.1 cm² calculation grid in both algorithms. Measurements with the W1 detector were performed in the same geometry and beam parameters on a Varian Edge machine.

Results: Dose calculated by AAA and Acuros deviates from that by the W1 measurement as the field size decreases, from below 2% difference at 10x10 cm² to above 10% difference at very small field (0.5x0.5 cm²) for 6 MV beam. The Acuros calculated dose approaches more closely to that of the W1 than the AAA at field sizes larger than 1 cm, beyond which point the deviation for Acuros continues to increase, while that for AAA is reduced.

Conclusion: Our study validates the accuracy of PSD-W1 in an inhomogeneous medium and indicates that, Acuros provides a superior inhomogeneity correction in small field down to 2x2 cm² within 3.6% which is limit of accuracy in small field measurements.


Small Fields, Dose, Scintillators


TH- Radiation dose measurement devices: scintillators

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