Purpose: To quantify the relative peripheral doses (PD) to healthy tissues outside the treated region for different intensity-modulated radiotherapy (IMRT) technologies.
Material and methods: On a linear accelerator (linac) Oncor Impression (Siemens OCS) with two photon energies (6 MV, 15 MV), point dose measurements were performed at different depths in a solid phantom at 29 cm off-axis distance inplane. PD associated with artificial fluence distributions were compared with open beam contributions, where intensity-modulated (IM) beams were generated by segmented multileaf-modulated (sMLM) IMRT, by tin+wax compensators (TWComp), and by lead-containing cerrobend compensators (CComp). The field size of the open field and the maximum area (isocenter distance) exposed with the primary beam for the IMRT fields was 20 x 22 cm2. Measurements were performed with two kinds of thermoluminescence dosimeters to quantify photon and neutron components separately. Furthermore, experiments were done with and without phantom material in the direct beam to separate different scatter dose components.
Results: The results for the photon components and the neutron components are reverse. For the open field, the photon components increase with decreasing photon energy. In comparison with the open field, the photon components are further (factor 1.2-1.8 depending on energy and depth) increased when delivering IMRT with sMLM. When using CComp or TWComp, this factor is even higher and reaches a maximum of 2.4. At depths beyond 20 mm, photon component values slightly decrease with increasing photon energy for all types of IMRT techniques. Near the surface (10 mm depth), photon component values are distinctly higher than those at larger depth, and they increase with increasing photon energy. As expected, neutron components could be detected only for 15 MV. For sMLM and compensators, neutron components increased by factors 4 and 1.5 relative to the open field. The experiments with different scatter conditions show that about 50-70% of the photon components and all neutron components NPD are caused by radiation emanating from the linac head.
Conclusion: PD in IMRT can be minimized by proper selection of treatment delivery method and photon beam energy. When selecting the IMRT technique in centers where compensator IMRT and MLC IMRT is available, PD burden should be taken into account. The large amount of photon components and neutron components caused by leakage radiation from the treatment head leads to the recommendation that radiation protection aspects for patients undergoing IMRT should be considered in linac design. For further clarification, additional experiments have to be carried out on other types of linacs.