Poster - 45: The effect of plan modulation on VMAT liver SBRT treatments: A motion interplay study




To investigate whether increasing the degree of MLC leaf modulation in VMAT liver SBRT treatment plans increases the dose differences due to the interplay effect.


Two VMAT plans, delivering 54Gy in three fractions, with differing degrees of MLC aperture modulation were created for each of 10 patients. To simulate respiratory motion, an in-house program was used to shift the positions of each active MLC leaf at every 0.6° of gantry rotation, according to the amplitude of a respiratory trace. To isolate the interplay effect from dose blurring, motion was simulated using four different starting points in the respiratory cycle. The same starting point was used for each of the three fractions, representing a worst case scenario. The four resultant dose distributions from each plan were subtracted from each other, and dose differences in the GTV were quantified using the standard deviation of the differential DVH.


Dose differences up to 1Gy were found in the GTV of the dose subtractions, indicating the presence of interplay effects. A Wilcoxon Signed Rank test indicates a significant (p<0.05) increase in the standard deviations of the low-modulation plans to those with a higher degree of modulation. No planning constraints were exceeded with the introduction of respiratory motion.


VMAT liver SBRT plans with a high degree of modulation exhibit an increased susceptibility to interplay effects. The dose differences due to interplay are not large enough to markedly decrease the plan quality.