CALCULATION AND MEASUREMENT OF THE DOSE TO POINTS OUTSIDE THE PRIMARY BEAM FOR CO-60 GAMMA-RADIATION

Purpose: In radiation therapy one sometimes needs to estimate the dose to points in the body outside the primary beam. Therefore a generalized model is developed to calculate this dose with reasonable accuracy. Methods and Materials: Measurements were made for a cobalt beam to determine separately the contribution of leakage radiation, radiation scattered from the collimator, scattered from the floor and radiation scattered inside the patient. Results: The radiation scattered in the patient shows a strong dependence on field size and distance to the beam axis and is predominant only at short distances. The radiation scattered from the collimator also depends strongly on distance and field size and is more important than the leakage radiation. With appropriate factors, correcting for patient dimensions and field shape, the total dose outside the primary beam can be calculated with an accuracy better than +/- 30%. The results are in accordance with published data. Conclusion: Using the measured data it is possible to calculate the dose at any point of the body outside the primary beam for Co-60 gamma radiation. The accuracy is considered to be adequate for risk assessment.