Beam parameters, depth dose, and profiles are known to differ with the selection of detectors. This is especially true for large fields (magna fields) with photons and electrons used for Total Body Irradiation (TBI) and Total Skin Electron Irradiation (TSEI), respectively. Magna field treatments are performed at 3–5 m from the source to cover the entire patient’s body with an adequate margin. In addition, a beam spoiler is used to bring up the skin dose. The extended treatment distance in TBI (dosimetrically known as a nonreference condition) results in changes in the photon spectra inside the patient, which in turn leads to the use of a nonreference energy correction factor, kNR, at each point of measurement. On the other hand, TSEI requires a more involved beam configuration to produce a ring-shaped dose distribution with dmax at the skin surface and 50% dose ≤2 cm deep with a total bremsstrahlung background of <4%. Apart from the nonreference dosimetry, the reduced signal-to-noise ratio at large distances necessitates a careful choice of detectors for measurements. This chapter discusses and provides data on the techniques and selection of detectors for accurate dosimetry.