Journal d'oncologie intégrative

An Assessment to Determine Total Scatter Factors for Photon Beam Linear Accelerators

Abstract

Barrington Brevitt, Barnswell Dennis, Pankaj Patel and Mitko Voutchkov

Objectives: To explore a simple and effective way to determine the total scatter of a C- Series linear accelerator.
Methodology: Measurements for this study were acquired using a Varian C-Series linear accelerator, with a 6MV photon beam, a Blue water phantom, 2 IBA CC13 ion chambers and an IBA CCU electrometer. Measurements were acquired for field sizes ranging from 5cm x 5cm to 40cm x 40cm, increasing field size by an increment of 5cm. Three readings were collected for each field size and averaged. All readings were done at a depth of 10cm to reduce the probability of measuring electron contamination in the photon beam. Measurements for Sc calculation were acquired in air using a build-up cap with a 3cm diameter.
Results: The average radiation dose measured increased as the field sizes increased. The maximum dose recorded for Sc was 2.33cGy while that for Scp was7.96cGy. There was a non-linear direct relationship between radiation dose measured, Scp and Sc calculated and field size. The maximum standard deviation in charge readings for Sc measurement was 1.18% which was recorded for the 35cm x 35cm field. The minimum standard deviation was 0.70% obtained with the 20cm x 20cm field. the proportion of the phantom scatter contribution to the total scatter, decreased exponentially with increases in field size. The largest contribution was identified with the 10cm x 10cm field while the lowest was identified with the 40cm x 40cm field.
Conclusion: It can be concluded that this method was effective in assessing the total scatter factor and its derivatives for field sizes ranging from 5cm x 5cm to 40cm x 40cm.