The physical basis for the clinical dosimetry of a 4 MV linear accelerator

The absorbed radiation dose from cobalt-60 gamma rays has been successfully calculated, confirmed by phantom measurements and applied to clinical radiotherapy for patients requiring Irregular fields. The question arises whether the same method of calculation is applicable to high energy radiation generated by a 4 MV linear accelerator. This study was made to determine Just what the discrepancies would be, if any, and to present the data in the form of tables and charts. Primary and scattered radiation components of the 4 MV linear accelerator were measured using techniques designed to evaluate the scatter components and thus permit deduction of the effect of the primary component. These conponents were analyzed separately and are presented in tabular form as: tissue-air ratios and scatter-air ratios. The tables of tissue-air ratios relate the measured absorbed dose in a water tank simulating tissue (a phantom) to that measured in air with a phantom Just large enough to provide electronic build-up. The tables give the measured values of tissue-air ratios for field sizes from an extrapolated "zero field size" representing the primary component of the beam to a maximum field size of 40 cm x 40 cm for the brass flatness filter and 30 cm x 30 cm for the lead flatness filter for depths of overlying water of 1.2 cm to 25 cm. The scatter-air ratio tables give the amount of scattered radiation reaching the chamber for radii of the radiation field of 1 cm to 20 cm for the brass flatness filter and 1 cm to 17 cm for the lead flatness filter for depths of 1.2 cm to 25 cm. These data were then recombined to mathematically simulate irregular fields at different depths. Other parameters necessary for radiotherapy such as central axis depth dose tables, tissue-maximum ratio tables, and a field size dependence table are also presented. The depth dose tables give the percentage of absorbed dose from the depth of maximum build-up (1.2 cm) which is 100% for each field size to a depth of 30 cm for the lead flatness filter and 25 cm for the brass flatness filter. The field sizes range from 3 cm x 3 cm to 30 cm x 30 cm for the lead flatness filter at 80 cm target-skin distance and 3 cm x 3 cm to 40 cm x 40 cm for the brass flatness filter at 100 cm target-skin distance. These tables are used for radiotherapy when a constant target-skin distance is utilized in treatment. The tissue-maximum ratio tables are used for treatment when a constant target-axis distance is utilized. They relate the ratio of absorbed dose Treasured with depths of water from 1.2 cm to 25 cm added above the chamber to the absorbed dose at the depth of maximum build-up for each field size. The field sizes ranged from 3 cm x 3 cm to 30 cm x 30 cm for the lead flatness filter and to a maximum field size of 40 cm x 40 cm for the brass flatness filter. The field size dependence table tabulates the relative dose at 1.2 cm depth normalized to a 10 cm x 10 cm field for field sizes of 4 cm x 4 cm to 40 cm x 40 cm and may be used for both flatness filters. Several tests were made to check the validity of the calculative method for 4 MV x-rays. The calculated absorbed dose is compared to the measured absorbed dose in each case. In general, the method gives excellent results with errors less than 3%.