CALCULATION AND MEASUREMENT OF THE DOSE AT POINTS OUTSIDE THE PRIMARY BEAM FOR PHOTON ENERGIES OF 6-MV, 10-MV, AND 23-MV

Purpose: A generalized model is developed to calculate the dose at any point in the body outside the primary beam, using experimentally determined data. Methods and Materials: The contribution of leakage radiation, radiation scattered from the collimator, from the floor, and in the patient was measured for 6, 10, and 23 MV photons. Results: The radiation scattered in the patient and the radiation scattered from the collimator exhibit a strong dependence on field size and distance and are predominant only at short distances. At larger distances the leakage radiation makes the largest contribution. With appropriate factors, correcting for patient dimensions and field shape, the accuracy is better than +/- 50%. Conclusion: It is possible to calculate the contribution of radiation scattered in the patient at any point of the body outside the primary beam for photons with an energy up to 23 MV. At large distances this contribution increases with energy. Radiation leaking through, and scattering from the collimator has to be measured for each individual treatment machine.