Estimation of peripheral dose from Co beam in water phantom measured in Secondary Standard Dosimetry Laboratory, Pakistan.

Background: Peripheral or scatter dose harms neighbouring normal tissues during administration of dose to cancerous tissues, therefore, knowledge of peripheral dose is an important consideration in radiotherapy.

Aim: In present study, absorbed dose measurements in a water phantom were performed for three field sizes, 7 × 7 cm, 10 × 10 cm and 15 × 15 cm.

Materials And Methods: For each field size, dose was measured at six depths below the front surface of the water phantom; 2.5-15 cm with an interval of 2.5 cm. Measurements were made at equal transverse distances along the horizontal axis, from 1 cm to 6 cm, on both sides of the central beam axis and normalized with central axis dose of each field. All measurements were made at the source to surface distance of 100 cm.

Results: Variation of peripheral dose with lateral distance was analysed and an appropriate parametric equation for each field size and depth was constructed.

Conclusions: The peripheral radiation dose showed a strong dependence on field size and distance from field boundary.