At present, new techniques use in the treatment of cancer patients in radiation therapy has been developed with the use of a small beam treatment. Therefore, the accuracy and precision of penumbral regions in small field dosimetry are significance because the treatment planning uses this information to calculate the dose.
However, the measurement of penumbra is complicated because it presents the effects of electronic disequilibrium and steep dose gradient.
There were many publications notified that different sizes of detectors causing different sizes of penumbral widths which directly affect the accuracy of treatment planning calculation.
Dawson DJ. et al. studied penumbra measurement using various ionization chamber sizes with a diameter from 0.3 to 1.4 cm at 2.5 to 25 cm depth of Co60, 6 and 31 MV photon beams. The results presented that penumbral widths between 20-80% dose (P 20-80) and 10-90% dose (P 10-90) increased linearly when diameters of ionization chambers increased at any depths and energies as shown in Figure 3.1.