The small field dosimetry is very important in modern radiotherapy because it has been frequently used to treat the tumor with high dose hypo-fractionated radiotherapy or high dose single fraction stereotactic radiosurgery (SRS) with small size target. But, the dosimetry of a small field (＜3×3 cm²) has been great challenges in radiotherapy. Small field dosimetry is difficult because of (a) a lack of lateral electronic equilibrium, (b) steep dose gradients, and (c) partial blocking of the source. The objectives of this study were to measure and verify with the various detectors the output factors in a small field (＜3 cm) for the 6 MV photon beams. Output factors were measured using the CC13, CC01, EDGE detector, thermoluminescence dosimeters (TLDs), and Gafchromic EBT2 films at the sizes of field such as 0.5×0.5, 1×1, 2×2, 3×3, 5×5, and 10×10 cm². The differences in the output factors with the various detectors increased with decreasing field size. Our study demonstrates that the dosimetry for a small photon beam (＜3×3 cm²) should use CC01 or EDGE detectors with a small active volume. And also, Output factors with the EDGE detectors in a small field (＜3×3 cm²) coincided well with the Gafchromic EBT2 films.　

KeywordsSmall field, Output factors, EDGE detector, Gafchromic EBT2 film