WE-C-108-02: Current Methods for Calculating Air Density Can Overestimate Skin Dose.
PURPOSE
Following accepted procedures for configuring Image-Value to Density Tables (IVDT) can lead to overestimation of skin dose due to exaggerated buildup and scatter in air. Treatment planning systems (TPS), such as Tomotherapy, which calculate dose to air before entering the body, can overestimate air density, leading to buildup and high skin dose. The purpose of this study is to evaluate the magnitude of errors in planned skin dose from incorrect air density assignment on various TPS.


METHODS
Air in a sample patient CT scan was contoured and manually assigned mass densities. Planning was performed using Pinnacle, Eclipse, and Tomotherapy TPS. Air densities were overridden to values between 0.0013 and 0.02 g/cc. A single anterior 6 MV breast tangent was applied to a sample patient to deliver 20 Gy, half the dose under RTOG protocol 1005.


RESULTS
Pinnacle and Eclipse assume negligible dose delivered along the beam path in air, so varying air density did not affect dose with these TPS. Using Tomotherapy, a test site 1 mm under the skin received 168% more dose with an air density of 0.02 g/cc than with normal air density of 0.0013 g/cc. The dose to contralateral breast was 1.40 Gy with air density of 0.0013 g/cc, but rose to 3.05 Gy with a slightly elevated density of 0.003 g/cc, greater than the limit of 2.4 Gy under RTOG 1005.


CONCLUSION
Following procedure by adding a point at the average HU for air will assign half the air voxels excessive density. An IVDT that generated air density of 0.003 g/cc can increase the contralateral breast dose above the max dose limit of RTOG 1005. We recommend placing the IVDT point for air near the maximum image value for air to assure all air voxels are assigned accurate density.