ITV versus mid-ventilation for treatment planning in lung SBRT: a comparison of target coverage and PTV adequacy by using in-treatment 4D cone beam CT

Abstract

Background: The internal target volume (ITV) approach and the mid-ventilation (MidV) concept are the two main respiratory motion-management strategies under free breathing. The purpose of this work was to compare the actual in-treatment target coverage during volumetric modulated arctherapy (VMAT) delivered through both ITVbased and MidV-based planning target volume (PTV) and to provide knowledge in choosing the optimal PTV for stereotactic body radiotherapy (SBRT) for lung lesions. Methods and materials: Thirty-two lung cancer patients treated by a VMAT technique were included in the study. For each fraction, the mean time-weighted position of the target was localized by using a 4-dimensional conebeam CT (4D-CBCT)-based image guidance procedure. The respiratory-correlated location of the gross tumor volume (GTV) during treatment delivery was determined for each fraction by using in-treatment 4D-CBCT images acquired concurrently with VMAT delivery (4D-CBCT in-treat ). The GTV was delineated from each of the ten respiratory phase-sorted 4D-CBCT in-treat datasets for each fraction. We defined target coverage as the average percentage of the GTV included within the PTV during the patient's breathing cycle averaged over the treatment course. Target coverage and PTVs were reported for a MidV-based PTV (PTV MidV ) using dose-probabilistic margins and three ITVbased PTVs using isotropic margins of 5 mm (PTV ITV + 5mm ), 4 mm (PTV ITV + 4mm ) and 3 mm (PTV ITV + 3mm ). The intreatment baseline displacements and target motion amplitudes were reported to evaluate the impact of both parameters on target coverage. Results: Overall, 100 4D-CBCT in-treat images were analyzed. The mean target coverage was 98.6, 99.6, 98.9 and 97.2% for PTV MidV , PTV ITV + 5mm , PTV ITV + 4mm and PTV ITV + 3mm , respectively. All the PTV margins led to a target coverage per treatment higher than 95% in at least 90% of the evaluated cases. Compared to PTV ITV + 5mm , PTV MidV , PTV ITV + 4mm and PTV ITV + 3mm had mean PTV reductions of 16, 19 and 33%, respectively.Conclusion: When implementing VMAT with 4D-CBCT-based image guidance, an ITV-based approach with a tighter margin than the commonly used 5 mm margin remains an alternative to the MidV-based approach for reducing healthy tissue exposure in lung SBRT. Compared to PTV MidV , PTV ITV + 3mm significantly reduced the PTV while still maintaining an adequate in-treatment target coverage.

Identification of a potential source of error for 6FFF beams delivered on an AgilityTM multileaf collimator

“…Modern day radiotherapy frequently utilizes modulated treatment, either in form of static gantry dynamic multileaf collimator (MLC) treatments or volumetric modulated treatment (VMAT), 1 for which the gantry, MLC leaves, and dose rate constantly change during beam delivery 2 . Such advanced treatment techniques require a well‐controlled beam delivery during all stages of treatment, a stable beam, and a robust MLC model implemented in the treatment planning system (TPS), especially when consideration is given to reduction of standard planning target volume (PTV) margins in stereotactic body radiotherapy (SBRT) 3 …”

MLC tracking for lung SABR is feasible, efficient and delivers high-precision target dose and lower normal tissue dose

“…ITV-based planning will produce larger PTVs than respiratory gated or breath hold techniques, which provide similar PTVs to MLC tracking [33,34]. The mid-ventilation technique compares similarly to ITV-based approach [28,35]. Despite this larger PTV, the delivered dose with an ITV-based plan will be influenced predominantly by the tumor motion at treatment.…”

Feasibility of a TPS-integrated method to incorporate tumor motion in the margin recipe

“…This is opposite to the widespread use of the internal target volume (ITV) that encompasses the entire tumor motion and therefore considers respiratory motion as a systematic error. Thanks to the large penumbra in the lung, irradiation of the tumor at its mid-position allows for good dose coverage even if the lesion is not fully within the high-dose region for a small part of the breathing cycle [8,10,11]. Based on this rationale, the mid-ventilation strategy (MidV) was developed [12].…”

“…Despite the potential benefits of the MidV/MidP approaches, their deployment in clinical practice is hindered by the lack of commercial solutions providing an integrated approach to 4DCT treatment planning [10,[14][15][16].…”