Evaluation of effective field size characteristics for small megavoltage photon beam dosimetry

Background: Small photon beams are increasingly used in modern radiotherapy modalities. In small photon fields, the dosimetric field size will deviate from the nominal field size. An effective field size (FSeff) for use in small field dosimetry has been defined to overcome this issue. The present study aims to investigate the suitability of two ionization chambers and two semiconductor diodes in the measurement of 6MV photon beam profiles and to analyze the variations of FSeff in smaller fields. Materials and Methods: Measurements were made at 6 MV photon beams of a Siemens Artiste linear accelerator and transverse profiles were acquired for nominal square field sizes of side 1×1 to 10×10 cm2 via the irradiation of detectors and radiochromic film. Full width at half maximum (FWHM) at the 50% isodose level was used to calculate FSeff. Results: The uncertainty of the FWHM values derived from the in-plane and cross-plane profiles (ΔFWHM%) were below 6% for all the detectors were below 6% except for Semiflex in the 1×1 field size. In small field sizes (less than 3 × 3 cm2), larger differences occurred between the dosimetric and nominal field sizes in all detectors. No significant differences between nominal and effective field sizes were observed in a field rage of 4×4 - 10×10 cm2. Conclusion: In the acquisition of small field profiles, selection of an appropriate detector is influential in accurate measurements. The findings of present study support the argument that both the size and composition of detectors affect the small field profile measurements.