[Evaluation of dose distribution in the region of small air space in human body by Co-60 and 10 MV X-ray beams].

In many parts of human body, air-filled spaces are components of normal anatomical structures, such as nasopharynx and paranasal sinuses, and the inclusion of these spaces in radiation treatment fields may create inhomogeneous dose distribution, due to the loss of scattering electron equilibrium, the lack of backscatter and the alteration in primary beam attenuation. Accurate determination or measurement of dose in the region of irregular air spaces has been experimentally difficult. However, tumors growing on the surface of the air cavities do cause serious concern by conscientious radiation therapists and physicist about the possibility of underdose. Using the models of simulated air cavities, combined air cavity and air channel, and taking measurement with a small window parallel plate ionization chamber in solid phantoms, it is found that the air spaces in the human body will cause underdose or overdose at the exit air space surface for 10MV X-ray and Co-60 r-ray. Surface dose were evaluated by a gain/loss factors (G/L) which was defined as the ratio of exit air space surface dose and the dose measured in a same but homogeneous phantom. The G/L factors are greater than 1.0 (overdose) where the air spaces are much smaller than radiation field sizes and are less than 1.0 (underdose) in extreme situations where the air spaces are larger than radiation field sizes and where the scattered volume of cavity wall in the treatment fields in also small. In addition, these effects of overdose and underdose are enhanced by increasing air space thickness and higher photon energy. We suggest that the radiation therapists and clinical physicists be cautious in applying these useful guidelines for treatment planning.