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SU‐E‐T‐431: Vertically‐Oriented Farmer‐Type Chamber for Small‐Field Applications
Author(s) -
Ahmad M,
Chu A,
Lincoln H,
Chen Z,
Deng J,
Nath R
Publication year - 2014
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4888764
Subject(s) - monte carlo method , photon , optics , field (mathematics) , diode , field size , electron , ionization chamber , materials science , eclipse , physics , optoelectronics , nuclear physics , mathematics , statistics , ion , quantum mechanics , astronomy , imaging phantom , pure mathematics , ionization
Purpose: To introduce a non‐conventional measurement setup using Farmer‐type chambers to accommodate several situations of small‐field dose measurements without compromising accuracy. The validation of this technique was demonstrated for photon small‐field output measurements, and electron small‐field percentage depth‐dose (PDD) measurements. Methods: Initial chamber alignment was performed using the conventional (horizontally‐oriented) chamber setup. A PDD was acquired for a 4×4 cm 2 field size using this arrangement. This PDD was used as a positional reference for the vertically‐oriented chamber (VOC) configuration. Next, a PDD was acquired for a 4×4 cm 2 field size with the VOC. The PDD's were superimposed to find the effective shift of the VOC. Using the shifted VOC setup, photon small‐field output factors were measured and compared to stereotactic diode output factor measurements. Additionally, electron smallfield PDD's were acquired using the VOC setup and results were compared to electron Monte Carlo (eMC) predictions in the Eclipse treatment planning system (TPS). Results: (1) For photon small‐field output factors field‐sizes 2×2 cm 2 and larger, the difference between the VOC setup and SFD measurements were less than 0.8%. For field sizes less than 2×2 cm 2 discrepancies ranged from 4.0 to 10.6%. (2) PDD's measured by VOC setup show better than 1.6% agreement as compared to eMC for all electron energies measured down to the 80% depth on the 2×2 cm 2 PDD curve. Disagreement between the VOC setup measurement and eMC calculations for depths down to the 50% depth on the PDD curve is 3.6% or less. Conclusion: Using the VOC setup, it is possible to use a conventional farmer chamber for small field‐size measurements down to 2×2 cm 2 field size without sacrificing the accuracy of measurements.