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Quantifying the interplay effect in prostate IMRT delivery using a convolution‐based method
Author(s) -
Li Haisen S.,
Chetty Indrin J.,
Solberg Timothy D.
Publication year - 2008
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2897972
Subject(s) - multileaf collimator , convolution (computer science) , population , dosimetry , medical imaging , nuclear medicine , probability density function , mathematics , radiation treatment planning , radiation therapy , computer science , artificial intelligence , statistics , medicine , radiology , environmental health , artificial neural network
The authors present a segment‐based convolution method to account for the interplay effect between intrafraction organ motion and the multileaf collimator position for each particular segment in intensity modulated radiation therapy (IMRT) delivered in a step‐and‐shoot manner. In this method, the static dose distribution attributed to each segment is convolved with the probability density function (PDF) of motion during delivery of the segment, whereas in the conventional convolution method (“average‐based convolution”), the static dose distribution is convolved with the PDF averaged over an entire fraction, an entire treatment course, or even an entire patient population. In the case of IMRT delivered in a step‐and‐shoot manner, the average‐based convolution method assumes that in each segment the target volume experiences the same motion pattern (PDF) as that of population. In the segment‐based convolution method, the dose during each segment is calculated by convolving the static dose with the motion PDF specific to that segment, allowing both intrafraction motion and the interplay effect to be accounted for in the dose calculation. Intrafraction prostate motion data from a population of 35 patients tracked using the Calypso system (Calypso Medical Technologies, Inc., Seattle, WA) was used to generate motion PDFs. These were then convolved with dose distributions from clinical prostate IMRT plans. For a single segment with a small number of monitor units, the interplay effect introduced errors of up to 25.9 % in the mean CTV dose compared against the planned dose evaluated by using the PDF of the entire fraction. In contrast, the interplay effect reduced the minimum CTV dose by 4.4 % , and the CTV generalized equivalent uniform dose by 1.3 % , in single fraction plans. For entire treatment courses delivered in either a hypofractionated (five fractions) or conventional ( > 30 fractions) regimen, the discrepancy in total dose due to interplay effect was negligible.