Surface and build-up region dose characteristics for high energy photons

OBJECTIVES

The surface doses of high energy photon beams show variability depending on the design of the treatment machine. Especially while using bolus that has a direct effect on surface and build-up region doses, defining correctly in the treatment planning system used for patient treatment is very important. It needs to be measured for each radiation beam used. In this study, the dose change of high energy beams surface and build-up region that were used in I.U. Oncology Institute according to the field size and the skin source distance were investigated.

METHODS

Co-60 teletherapy equipment, Orion 4 (MV), Saturn 42 (6-15 MV) high energy x-rays were used. Measurements were made for different field sizes and source skin distances by using Marcus–type parallel plate ion chamber at water equivalent solid phantom, at surface and build-up region. Measured surface and build-up doses were installed to CMS XiO treatment planning system.

RESULTS

As a result of this study, the surface dose increased linearly as the field size increased and the maximum dose point was moved toward the surface by 1-2 mm as predicted. Surface doses for a 10x10 cm field size are 24.3%, 17.4%, 12.6% and 10.2% respectively for Co-60, 4MV, 6MV and 15MV. With the increase in skin source distance, a decrease in surface doses is observed.

CONCLUSION

The use of correct information is achieved by downloading surface doses measured for different field sizes to the treatment planning system.

Keywords: Build-up region; high energy photons; surface dose; radiotherapy dosage