2.2 The machine-specific reference field (fmsr)

For radiotherapy radiation emitters, where the reference beam size (fref) of 10 cm � 10 cm cannot be obtained, a new concept of machine-specific reference (msr) field size has been proposed. The dimensions of the msr field (fmsr) should be as close as possible to that of fref and must be at least equal to the sum of rLCPE and half of the detector's outer volume.

### 3. Detectors and equipment

### 3.1 Ionization chambers for reference dosimetry in small and non-reference fields

An ideal air-filled detector to be used for measurement of dose deposited in water must be equivalent to the water and not perturb the charged particle fluence; its response must not be affected by dose rate or directional dependence; it must show good sensitivity, in terms of signal to noise ratio and time taken to obtain the

Prospective Monte Carlo Simulation for Choosing High Efficient Detectors for Small-Field… DOI: http://dx.doi.org/10.5772/intechopen.89150

signal; the detector signal must be beam energy independent and directly proportional to the dose deposited in water; and it must show minimum fluctuations, leakage, and no effect of cable irradiation.

The detector size must be such that it fulfills rLCPE criteria. The positioning of the air-filled detector must be such that the charged particle fluence remains approximately uniform over the sensitive volume of the detector.
