Measurement of the interplay effect in lung IMRT treatment using EDR2 films

Intrafraction organ motion during the dynamic delivery of intensity-modulated radiation therapy (IMRT) treatment of lung tumors may cause unexpected hot/cold spots within the target volume, due to the interplay effect between tumor motion and multileaf collimator (MLC) leaf motion. In the past, this has been investigated through theoretical analysis, computer simulation, and experimental measurement using an ionization chamber dosimeter. In the work presented here, the interplay effect was studied experimentally in 2D, using Kodak EDR2 films. A five-field lung IMRT plan was delivered to a solid water phantom with embedded film. The phantom was placed on a motor-driven platform with a sinusoidal motion to simulate the respiration-induced tumor motion. The delivery of each field began at one of eight equally spaced initial breathing phases. The dose distribution for each treatment fraction was estimated by combining the dose distributions for all fields with randomly sampled initial breathing phases. The dose variation caused by the interplay effect was estimated by looking at the dose values from 1000 trials of 30 fractions. It was found that, on a day-to-day basis, the standard deviation of the dose to a given pixel in the high-dose region could be as high as 2% to 4% due to the motion interplay effect. After 30 fractions, the standard deviation in the dose to each pixel is reduced to 0.4% to 0.7%. However, compared to the static delivery, the dose distribution from a 30-fraction case in the presence of motion shows some underdosing in the region of interest. We found that the maximum dose in the target remains within 1% of the maximum dose in the static case, but the minimum dose in the target is most likely to be about 6% lower than the static case. Our results indicate that there can be some underdosing of the tumor due to the interplay effect in lung IMRT delivery over the entire course of a 30-fraction treatment.