Dosimetric impact of intrafraction motion on boosts on intraprostatic lesions: a simulation based on actual motion data from real time ultrasound tracking

Abstract

BackgroundIntrafraction motion is particularly problematic in case of small target volumes and narrow margins. Here we simulate the dose coverage of intraprostatic lesions (IPL) by simultaneous integrated boosts (SIB). For this purpose, we use a large sample of actual intrafraction motion data.MethodsFifty-three\u2009h of intra-fraction motion of the prostate were recorded in real-time by 4D ultrasound (4DUS) during 720 fractions in 28 patients. We simulate spherical IPLs with 3, 5, and 7\u2009mm radius and matching spherical SIBs with 0, 2, and 5\u2009mm safety margins. The volumetric overlap between IPLs and SIBs is calculated. Dose volume histograms (DVH) are estimated by Monte Carlo simulation.ResultsOn average, the distance of the prostate was 1.3\u2009mm from its initial position over all fractions and patients. Average volumetric overlap was 73, 82, and 87% of IPL volume in case of 3, 5, and 7\u2009mm IPLs and SIBs without safety margins. These improved to 95% or more in case of 2\u2009mm safety margins and 98% or more in case of 5\u2009mm safety margins. DVHs showed that 80% of the IPL volume received 60, 72, and 79% of maximum dose in case of 3, 5, and 7\u2009mm IPLs and SIBs without safety margins. These improved to 94% or more given moderately sized safety margins of 2\u2009mm.ConclusionsOn average over all fractions and patients, the dose coverage would have been acceptable even for small target volumes such as IPLs of radius 3 to 7\u2009mm and narrow fields. Moderate safety margins of 2\u2009mm could have ensured a delivery of 90% or more of the SIB dose to the IPL. In this case, SIB volume would have been considerably larger than IPL volume, but still considerably smaller than the overall PTV of the prostate.