SU-F-T-548: An Extraction Method for Correction of Detector Volume Effect in Small Field Profile Measurement.

PURPOSE: To investigate the appropriateness of Gaussian detector response function in SRS profile measurement.

METHODS: In-air profile data for a 6MV beam were collected by using ionization chambers and radiochromic film. Two ionization chambers (FC65-G(IBA) and PinPoint(PTWTW31009)) were used to scan the beam profile. Film was used to measure the beam penumbra. A half-blocked beam was acquired with stepping size of 0.2mm and gradient of the measured profile represents the detector response function. The field widths of 2cm and 3cm were measured at SAD. Two detector response functions were evaluated, i.e. a traditional single Gaussian and a three-Gaussian symmetric function. Two corresponding models were used to fit measured data. One was error functions with one shaping parameter. The other model was a symmetrical superposition of three sets of error functions with fixed separation and two different shaping parameters. In both fitting, resultant Gaussian kernels and corresponding σs could be calculated analytically. SNR of the fitted curves were obtained and compared.

RESULTS: Our measured data indicated that the ionization chambers had a three-peak response function. From the first modeling, SNR for FC65-G were 36.0dB and 38.3dB for 2cm and 3cm field respectively. The corresponding SNR for the PinPoint were 43.6dB and 42.5dB. From the second modeling, SNR for FC65-G were 42.5dB and 47.4dB for 2cm and 3cm field respectively. The corresponding SNR for the PinPoint were 44.0dB and 44.5 dB. Three-gaussian model had an observable improvement for FC65-G chamber. Film measured penumbra was 3-5 times smaller than that of ionization-chambers.

CONCLUSION: Superposition of three Gaussian kernels resembles the detector response due to central electrode and chamber wall and results in a better fitting to the measurement data than a single Gaussian kernel. Our study indicated that the Three-Gaussian model was more suitable for the detector with sigma comparable to penumbra width.