Room: Exhibit Hall
Purpose: To assess the peripheral dose (PD) using OSLD with megavoltage photon beams as a function of distance from the edge of the radiation field, field size, depth and energy.
Methods: PD measurements were carried out at 13 locations ranging from 1.5 to 20.8 cm outside the geometric edge of the radiation field at three different depths (surface, 5 cm and 10 cm) with 6 and 18 MV photon beams for 5 cm x 5 cm, 10 cm x 10 cm and 20 cm x 20 cm fields. The percentage PD with respect to central axis of the beam was compared with the results of parallel plate ionization chamber placed in the same positions as the OSLD.
Results: The percentage PD obtained at surface with 6 MV for 10cm x 10cm field was 6.77%, 2.31% and 1.0% at 1.5 cm, 9.5 cm and 20.8 cm distance away from field edge respectively whereas with 18 MV the % PD was 10.24 %, 3.44 % and 1.24 % respectively at the above mentioned positions. For a position of 3.1 cm distance away from the field edge, PD was 2.01%, 3.73% and 4.83% for 5 cm x 5 cm, 10 cm x 10 cm and 20cm x 20 cm fields respectively for 6 MV photons whereas for 18 MV beam, PD was 2.52%, 3.6% and 4.92% for 5 cm x 5 cm, 10cm x 10cm and 20cm x 20cm fields respectively.
Conclusion: The percentage PD with respect to the central axis dose decreases exponentially with the increase in distance from the field edge. From the data obtained, that percentage PD is higher at the surface and falls to a minimum at depth of dose maximum and tends to become constant with depth. OSLD dosimetry system provided an adequate dose assessment in peripheral regions.
TH- External beam- photons: Standard field experimental dosimetry