An evaluation of the mid-ventilation method for the planning of stereotactic lung plans

Abstract

a b s t r a c tBackground and purpose: Stereotactic ablative body radiotherapy for lung plans requires 4DCT. Most radiotherapy centres use this to determine an internal target volume (ITV), despite studies suggesting that planning on a mid-ventilation (Mid-V) phase can reduce target volumes. The purpose of this study is two-fold: to determine whether the Mid-V approach provides adequate coverage and to discuss methods to enable the Mid-V approach to be applied more widely. Method: 4D scans of 79 patients were outlined on every phase. The mid-V phase was identified. Margins were determined from the range of motion, and plans generated with a 55 Gy prescription. A grid-based method was used to get the probability of tumour coverage in the presence of systematic and random uncertainties, with and without blurring for breathing motion. Results: For the Mid-V plans with the margins calculated from the van-Herk formula, after blurring doses for breathing, the coverage (dose covering 95% of the CTV 95% of the time) was greater than for plans with isotropic 5 mm margins uncorrected for breathing (58.2 Gy v 57.3 Gy). Similar results were obtained for a linear margin chosen as 0.15 of the breathing range. Deformable contour propagation in a commercial outlining system (ProSoma) identified the same mid-V phase in the majority of cases. Conclusion: Our results confirm that a mid-V approach can be used to reduce the PTV size, with no loss of tumour coverage. We propose the use of a simplified margin formula equal to the margin ignoring breathing plus 0.15 of the range of motion.

Real‐time 4DMRI‐based internal target volume definition for moving lung tumors

“…By using the full information of the GTV motion over the whole duration of the reference imaging session, random uncertainties in the PTV definition could be reduced through the use of ITV 5% r . Studies by Ehrbar et al 47 and Thomas et al 48 concluded in their dosimetric analyses that the mid-ventilation approach can reduce the dose to the lung with no or only slightly reduced tumor coverage with respect to the ITV approach. Both studies however, recalculated the dose on the same 4D-CT images that were used for definition of the target volumes.…”

“…Studies by Ehrbar et al and Thomas et al concluded in their dosimetric analyses that the mid‐ventilation approach can reduce the dose to the lung with no or only slightly reduced tumor coverage with respect to the ITV approach. Both studies however, recalculated the dose on the same 4D‐CT images that were used for definition of the target volumes.…”

“…A similar approach was previously used by Thomas et al The midV was defined by determining the centroid of the centroids of all 157 GTV positions and then selecting the GTV position at the point in time with the smallest distance of its centroid to this point. This method is equivalent to the approaches previously applied by Ehrbar et al and Thomas et al…”

Geometric Uncertainties in Daily Online IGRT

“…It has recently been demonstrated [23][24][25] that the ITV concept leads to excessively large PTVs since clinically realistic breathing traces can be combined in quadrature with the penumbral width. This is particularly relevant in lung tissue as the penumbra is broadened by the increased range of secondary electrons in the lung tissue.…”

A Framework for the Robust Delivery of Respiratory Motion Adaptive Arc Radiotherapy

“…This has been shown to reduce the amount of normal lung tissue receiving a high dose, as compared to treatment using an ITV [34]. Another solution, which in this thesis is referred to as motion kernel optimization (MKO), is possible if the probability distribution of the target position is known.…”