The effects of air cavities on X-ray dose distribution at 6 and 25 MV.

Three different air channels were constructed in solid water. These resemble upper respiratory and larynx geometry. Central-axis depth-dose distributions were measured on and beyond the distal surface of the air cavities. The measurements were made with a parallel-plate Markus ionisation chamber and TLDs for 6 and 25 MV X-ray beams of field sizes 4 cm x 4 cm and 4 cm x 7 cm. Measured dose was then compared to calculated dose for the Clarkson Scatter Integration and Equivalent Tissue-air Ratio (ETAR) algorithms. Both algorithms show errors in dose calculation at the distal surface of air cavities mainly because they fail to account for the effect of electronic disequilibrium. The magnitude of prediction error is found to depend on energy, field size and cavity geometry. For a 4 cm x 4 cm field at 6 MV the dose difference between the calculated value and the measured value, at the distal surface of square cavity is 8.2% and 13.7% for the ETAR and Clarkson algorithm respectively while that for a 4 cm x 7 cm field, the dose difference is reduced to 1.4% and 7.1% respectively. In general the ETAR algorithm shows better performance than the Clarkson algorithm because the Clarkson method calculates scatter dose assuming a homogeneous water medium while the ETAR uses a weighted sum of scatter components which is density dependent. The measured data can be used as benchmark data in the development and testing of new photon dose calculation algorithms.