When it comes to small fields, Nizin PS And Das et al defined that field size is smaller than the maximum range of secondary electron which can cause electronic disequilibrium. This phenomenon brings to the complex dosimetry for measurement in small field. Measuring penumbral width is one of the tough works need to be cautious considered especially in the small field situations due to presenting electronic disequilibrium conditions. Measuring penumbral widths systematically incorrectly is caused by the finite size of the detectors and the components made in the detectors. Pappas E et al. found that ionization chambers had a finite size affected to broadening of the penumbral widths and it would increase the effect when the detectors made of non-tissue equivalent material because it could change the electron transport.In this case, the ionization chamber contains air which makes the electron transport travel longer than in tissue as a result it makes the penumbral widths wider. Laub and Wong suggested that diamond was suitable for small field dosimetry due to its high spatial resolution and water equivalent material. In contrast, Westermark M et al. found that detector with small size and high density caused the penumbral width underestimated such as their high concentration of Si diode (double diode) compared with the plastic scintillation and the liquid ionization chamber.
For the Dawson’s model. The total Penumbra contains Photon fluence distribution and Lateral spread of the secondary electron. The detector can effect the penumbra in 2 way .If Detector material is higher density than tissue .It will be shorter electron range leading to the sharper penumbra and when detector’s size is big than it can cause perturbation leading to broader penumbra.
For diode measurement compared with film. PFD overestimate penumbra because even though it’s made of high density material but its size is quite large which the PFD obtained the largest penumbral width for almost all depths and field sizes compared with films. It is definitely due to its larger diameter of active area than films according to Table 4.2 It also obtains the same reason that PFD has wider penumbral widths compared with other diodes also.
EDGE obtain comparable penumbra because eventhough it has the smallest sensitive volume but it’s external dimension is large .However its surrounding by brass material. The EDGE has the active area smaller than PFD but bigger size than films. However its penumbral width is sharper than PFD and films especially at the shallower depths. It might because its active area attached to the shallow depths is less than that to the deeper depths. Also it is covered with brass which is alloy (mixed between copper (Z=29) and zinc (Z=30)), considering high density. It might contribute to make thinner penumbral widths.
SFD underestimate penumbra compare with film it contains not only small dimention and small sensitive volume but also high density material. The SFD obtains the smallest active area than other diodes. It gains the tightest penumbral width compared with diodes and films.There are many publications (Lárraga-Gutiérrez et al., Yang et al., Yin et al.) using SFD as a reference measurement for penumbral width measurement in small fields. It might be a better reference detector for measuring penumbral widths in small field situations.