Low dose out-of-field radiotherapy, part 1: Measurement of scattered doses

Purpose: To measure out-of-field doses in a phantom model to better quantify this radiation.

Material and methods: The individual contribution of photons and neutrons to the total out-of-field dose for 6 MV and 20 MV photons at open beam were measured in a purpose-designed water phantom. Radiation doses were measured at seven separate points (P1-P7) in the phantom with thermoluminescent detectors (TLD 100, 600, and 700) and GAFchromic™ EBT films.

Results: At a prescribed dose of 75Gy to the isocentre, the photon dose level in the close-to-field area (P2) ranged from 2.0-2.5Gy for 6 MV and 1.5-2.0Gy for 20 MV; the total out-of-field doses at P2 and P7, respectively, were estimated to be as follows: for 6 MV: TLD 100 (<3.23% and<0.14%); radiochromic film (<2.52% and <0.03%); and for 20 MV: TLD 100 (<2.94% and <0.78%); TLD 700 (<2.02% and <0.14%); and radiochromic film (<1.73% and <0.01%). Although the dose decreased rapidly as the distance from the central beam axis increased, even distant doses could be as high as several centigrays. The neutron dose for 20 MV photons at a distance of 25cm from the isocentre was 4.0mSv/Gy.

Conclusion: Our results show that in the close-to-field area, the dose level could be as high as 1.5Gy assuming a prescribed dose of 75Gy to the isocentre. By contrast, the doses delivered to more distant areas from the planning target volume were much lower (centigrays). These findings show that both 6 MV and 20 MV photons could produce dosimetrically important dose levels outside of the field. The data reported here may be of value to study the potential impact of even very low doses of radiation on human tissues.