Room: ePoster Forums
Purpose: Recently released AAPM TG180 presents the importance and methods of quantifying cumulative absorbed dose delivered to patient when using IGRT techniques such as orthogonal planar MV Portal images. In order to follow TG180 recommendations, MV portal image dose for 5 head and neck patients treated with 6MV 3DCRT were analysed. H&N patients were our first choice because of the high dose delivered to their spinal cord, an OAR of critical importance.
Methods: The total number of Portal MV images was acquired for 5 H&N patients. These orthogonal images were obtained with gantry at 0 and 270 degrees. The mean number of images and it's standard deviation were calculated, so it was possible to simulate the dose increment added by Portal images on TPS and it's level of accuracy. Impact on spinal cord DVH, maximun dose and dose reaching 1 cubic centimeter were evaluated.
Results: The mean value for the maximun absorbed dose to spinal cord PRV came from the acquisition of one pair of orthogonal MV Portal images were 3.96Â±0.15 cGy when using no more than 2 MU each. TPS simulation showed that to surpass 5% of prescribed dose limit delivered to patient, around 90 pairs of orthogonal Portal images have to be done, which is far above the mean number of Portal images done in our department for anterior (20.6Â±2.5) and lateral (15.6Â±2.7) incidences throughout all treatment. The mean maximun absorbed dose added to spinal cord PRV in a typical H&N 3DCRT treatment is around 0.83 Gy due only to Portal images.
Conclusion: The maximun mean absorbed dose delivered to spinal cord PRV were estimated for each pair of orthogonal images taken from the patient. Typically, the dose increment to each patient due to MV Portal images done during treatment is far less than the TG180 recommended limitation.