3.1.1 The square equivalent msr field (fmsr) greater than 6 cm 6 cm

Modern radiotherapy linear accelerators are available in two general models, that is, with flattening filter (WFF) and flattening filter free (FFF). For WFF radiation emitters, air-filled detectors with sensitive volume range between 0.3 and 1 cm3 , since these detectors are often water resistant and easy to use for in-phantom measurements have negligible leakage effect and good signal to noise ratio [20]. In the case of FFF radiation emitters, air-filled detectors with sensitive volume lying in the range of 0.1–0.3 cm<sup>3</sup> are preferred over the commonly used Farmer type airfilled detectors [21]. In case the Farmer type air-filled detectors is used in FFF beam, then the beam profiles must be corrected for their non-uniformity; the factor for correction can be 1.5% for FFF photon beam of 6 MV [22, 23].
