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Unsettled Issues, Distance and Depth of Measurements of Output Factor(fclin, Fmsr) in Small Field Dosimetry

I Das1*, P Galavis1 , L Hu1 , P Francescon2 , (1) NYU Langone Medical Center, New York, NY, (2)Ospedale Di Vicenza, Vicenza, Italy


(Monday, 7/30/2018) 1:15 PM - 1:45 PM

Room: Exhibit Hall | Forum 3

Purpose: Small field dosimetry is challenging due to the range of secondary electrons and detector size producing variable perturbations. TRS-483 (IAEA Vienna, 2017) provided valuable information, however there are subtle differences that are not addressed properly for small clinical fields (fclin) compared to machine specific reference field (fmsr). The discrepancies in field output factor, Ω(fclin, fmsr) is especially significant among various publications. This could be possibly due to differences in treatment setup, depth and detectors which are studied in this investigation.

Methods: Measurements were performed in a water phantom for Ω(fclin,fmsr) with 100 cm source to surface distance (SSD) and source to axis distance (SAD) setups at various depths (dmax-20 cm) for plastic scintillator W1 and microDiamond detectors presuming k(fclin, fmsr) closed to 1.0 for all fields. A 6 MV beam was used from a Varian Edge machine that has high reliability in small fields.

Results: The measured values of Ω(fclin,fmsr) are nearly identical for both detectors confirming that k(fclin, fmsr) =1.0 except at 0.5x0.5cm² field at dmax. The differences between SSD and SAD at same depth are <2% which is within limit of experimental accuracy, thus either of the setup techniques can be used. However, Ω(fclin,fmsr) varies greatly with depth. Compared to reference depth (10 cm), Ω differs by 28%, 10.1% and -13.5% for dmax, 5 and 20 cm depths, respectively, for 0.5x0.5 cm² field and decreases linearly to 0% for large fields. These differences are inherent and can be resolved using correction factor based on the ratio of percent depth dose (PDD) and output at dmax.

Conclusion: The Ω(fclin,fmsr) is independent of the SSD and SAD setup for the same depth. A large variation in OF is observed with depth that can be successfully corrected. Care should be taken in selecting data for proper depth in clinical use.


Small Fields, Dose, Dosimetry


TH- Radiation dose measurement devices: Development (new technology and techniques)

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