Ion chamber response to kilovoltage x-rays in the presence of a contrast agent

Contrast enhanced radiotherapy is a relatively new treatment modality in which a radiological contrast agent is uploaded into a tumor that is subsequently irradiated with kilovoltage x-ray beams thus maximizing absorption of radiation through the photoelectric effect. As with other treatment modalities, experimental verification of absorbed dose distributions is a necessary step as part of the quality assurance process. In this paper, we address the question of how the response to kilovoltage x-rays exhibited by an ion chamber is modified when a radiological contrast agent is incorporated into the irradiated medium. Using a combination of Monte Carlo simulation and experimental validation of the different models used in such simulations, we determined the correction factors that allow for the use of an ion chamber with a N_D,W calibration factor at a given kilovoltage x-ray beam quality Q to be used when said beam irradiates a medium into which a radiological contrast agent is incorporated. The correction factors were derived as a function of the contrast agent concentration and for five different beam qualities. For a given concentration, the correction factor varies linearly with the beam quality as defined by the half-value layer, and for a given beam quality it varies linearly with the contrast agent concentration.