SU-E-T-230: Field Factor Verification in Small Fields

Small field is being commonly used in radiosurgery and IMRT, however dosimetry is challenging due to source size, electronic disequilibrium and detector size. Correction factor, K, has been published for a limited set of detectors and beam energies. Validity of K for several detectors is investigated. Also extrapolation ion chamber could be used using reciprocity theorem for small fields which is attempted in this study.

Various small volume detectors: IBA SFD, PTW TN60012 diode, and PTW Pinpoint N31006, Standard Imaging A16, along with Nuclear Associate N2339IT extrapolation chamber were used to measure output of square fields from 1×1 to 10×10 cm² for Varian 6 and 16 MV beams. Measurements were made in a water phantom at 1.5cm depth for 6MV photon and 3.0cm depth for 15MV photon with SSD 100cm. Extrapolation chamber was used in solid water. Published correction factors (Francescon et al, MP, 2011)1, K were used for the detectors used in this study. The K value was derived for extrapolation chamber by modeling based on the difference between its detector output and the average output factor from the diodes and pinpoints.

The variation in output as measured from different detector is significant (±7.0%) for field sizes = 3×3 cm² and converges after large field sizes for both energies. The output factor for the extrapolation chamber was out of range from diodes/pinpoints in both 6MV and 16 MV beams. After applying detector K, the field size factors agree each other within ±2.2% for all field sizes.

The value of K provides reasonably good data for field factors irrespective of detector and the field factors can be matched within 2.2%. A mathematical function for extrapolation chamber also provides field factors comparable to the other detectors.