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

Abstract

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.PACS number: 87.53.Mr

Use of reduced dose rate when treating moving tumors using dynamic IMRT

“…It has been shown that certain combinations of MLC sequence and target motion can give large dose discrepancies ( 1 , 2 ) for individual fractions. These effects generally average out when multiple fractions are used, ( 3 – 9 ) but Seco et al ( 1 ) found that this is not always the case (e.g., for step‐and‐shoot IMRT) and, for some situations, a dose error could still remain after multiple fractions.…”

4D radiobiological modelling of the interplay effect in conventionally and hypofractionated lung tumour IMRT

“…From the figure, it is evident that dose delivery over multiple breathing cycles by reducing the dose rate would reduce the interplay effect. On average, the TCP pop reduction owing to interplay effect was reduced by approximately 1.5% with 300 MU min 21 compared with 600 MU min 21 . Although this is not a significant reduction for the range of motion assumed in this study, this indicates that reduced dose rate is beneficial for treating moving targets while no motion management techniques are used.…”

“…The effect of dose rate on TCP pop owing to interplay is studied for single-fraction treatments (as there is no significant difference for conventional fractionation) with 300 and 600 MU min 21 .…”

“…However, the conclusion derived from point-dose measurements by Jiang et al does not provide a complete picture of the interplay effect to the overall tumour volume. Using twodimensional (2D) film measurements, Berbeco et al 21 have shown that the SD of the dose to a pixel inside the target volume can be as high as 2-4% for single-fraction treatments, which corresponds to stereotactic radiosurgery, although the effect is reduced to 0.4-0.7% with 30 fractions. According to their measurements, the maximum dose in the target varies ,1%, while the minimum dose varies up to approximately 6%.…”

“…For the 20-fraction treatment, the drop in TCP pop was 21.05% 6 0.39%, whereas for the three-and single-fraction treatments, it was 22.80% 6 1.68% and 24.00% 6 2.84%, respectively. By reducing the dose rate from 600 to 300 MU min 21 for the single-fraction treatments, the drop in TCP pop was reduced by approximately 1.5%.…”