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TH‐D‐224C‐07: Focal Source Size Measurement for Monte Carlo Simulations of Percentage Depth Doses in Very Small Photon Fields
Author(s) -
Sham E,
Devic S,
Seuntjens J,
Podgorsak E
Publication year - 2006
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 - 10.1118/1.2241919
Subject(s) - optics , linear particle accelerator , imaging phantom , monte carlo method , physics , beam (structure) , full width at half maximum , photon , dosimetry , perpendicular , nuclear medicine , geometry , mathematics , medicine , statistics
Purpose: To measure the focal spot of a 10‐MV Clinac‐18 linac for use in Monte Carlo (MC) calculation of percentage depth doses (PDDs) in a very small (1.5 mm diameter) field. Methods and Materials: A technique using a translational slit‐assembly system was developed to measure the linac focal spot. The assembly consists of two lead blocks fastened together to produce a 300 μm wide slit. A small field diode was centered below the slit, reading radiation signals transmitted through the slit while the system (mounted at a distance of 65 cm from the target) was translated linearly across an open beam. The linear translation was achieved through a screw driving mechanism and the position of the slit was measured with a digital caliper. The source dimension was estimated from the FWHM of the Gaussian fit to scanned profiles corrected for background transmission signal. Results: Scans in various angular orientations perpendicular to the beam central axis show that the linac focal spot is elliptical with principal axes of (1.1 ± 0.1) mm and (2.1 ± 0.1) mm. Isodose distribution measured on the solid water phantom surface in a plane perpendicular to the beam central axis for the 1.5‐mm diameter, 10 MV photon beam is also elliptical and oriented in accordance with the measured source shape. A circular Gaussian source model with a FWHM of 1.5 mm, approximating the measured focal spot, was used in the MC calculation of PDDs for the 1.5 mm beam. The MC‐calculated PDDs agree within 2% with measured data. Conclusions: In contrast to standard radiotherapy fields, MC‐calculated PDDs for very small fields show a strong dependence on source dimension. The good agreement between the measured and MC‐calculated PDDs for the 1.5 mm diameter 10 MV beam validates the MC simulation technique using appropriate focal source size.