Towards accurate dose accumulation for Step-&-Shoot IMRT: Impact of weighting schemes and temporal image resolution on the estimation of dosimetric motion effects.

Breathing-induced motion effects on dose distributions in radiotherapy can be analyzed using 4D CT image sequences and registration-based dose accumulation techniques. Often simplifying assumptions are made during accumulation. In this paper, we study the dosimetric impact of two aspects which may be especially critical for IMRT treatment: the weighting scheme for the dose contributions of IMRT segments at different breathing phases and the temporal resolution of 4D CT images applied for dose accumulation. Based on a continuous problem formulation a patient- and plan-specific scheme for weighting segment dose contributions at different breathing phases is derived for use in step-&-shoot IMRT dose accumulation. Using 4D CT data sets and treatment plans for 5 lung tumor patients, dosimetric motion effects as estimated by the derived scheme are compared to effects resulting from a common equal weighting approach. Effects of reducing the temporal image resolution are evaluated for the same patients and both weighting schemes. The equal weighting approach underestimates dosimetric motion effects when considering single treatment fractions. Especially interplay effects (relative misplacement of segments due to respiratory tumor motion) for IMRT segments with only a few monitor units are insufficiently represented (local point differences >25% of the prescribed dose for larger tumor motion). The effects, however, tend to be averaged out over the entire treatment course. Regarding temporal image resolution, estimated motion effects in terms of measures of the CTV dose coverage are barely affected (in comparison to the full resolution) when using only half of the original resolution and equal weighting. In contrast, occurence and impact of interplay effects are poorly captured for some cases (large tumor motion, undersized PTV margin) for a resolution of 10/14 phases and the more accurate patient- and plan-specific dose accumulation scheme. Radiobiological consequences of reported single fraction local point differences >25% of the prescribed dose are widely unclear and should be subject to future investigation. Meanwhile, if aiming at accurate and reliable estimation of dosimetric motion effects, precise weighting schemes such as the presented patient- and plan-specific scheme for step-&-shoot IMRT and full available temporal 4D CT image resolution should be applied for IMRT dose accumulation.

Werner R ，Ehrhardt J ，Schmidt-Richberg A ，Albers D ，Frenzel T ，Petersen C ，Cremers F ，Handels H ... -  《-》

Investigation of simple IMRT delivery techniques for non-small cell lung cancer patients with respiratory motion using 4DCT.

Reitz B ，Parda DS ，Colonias A ，Lee V ，Miften M ... -  《-》

The susceptibility of IMRT dose distributions to intrafraction organ motion: an investigation into smoothing filters derived from four dimensional computed tomography data.

Coolens C ，Evans PM ，Seco J ，Webb S ，Blackall JM ，Rietzel E ，Chen GT ... -  《MEDICAL PHYSICS》

A novel four-dimensional radiotherapy method for lung cancer: imaging, treatment planning and delivery.

Alasti H ，Cho YB ，Vandermeer AD ，Abbas A ，Norrlinger B ，Shubbar S ，Bezjak A ... -  《PHYSICS IN MEDICINE AND BIOLOGY》

Experimental validation of the van Herk margin formula for lung radiation therapy.

Ecclestone G ，Bissonnette JP ，Heath E 《-》