Abstract
Peripheral lung lesions treated with a single fraction of stereotactic ablative body radiotherapy (SABR) utilizing volumetric modulated arc therapy (VMAT) delivery and flattening filter‐free (FFF) beams represent a potentially high‐risk scenario for clinically significant dose blurring effects due to interplay between the respiratory motion of the lesion and dynamic multi‐leaf collimators (MLCs). The aim of this study was to determine an efficient means of developing low‐modulation VMAT plans in the Eclipse treatment planning system (v15.5, Varian Medical Systems, Palo Alto, USA) in order to minimize this risk, while maintaining dosimetric quality. The study involved 19 patients where an internal target volume (ITV) was contoured to encompass the entire range of tumor motion, and a planning target volume (PTV) created using a 5‐mm isotropic expansion of this contour. Each patient had seven plan variations created, with each rescaled to achieve the clinical planning goal for PTV coverage. All plan variations used the same field arrangement, and consisted of one dynamic conformal arc therapy (DCAT) plan, and six VMAT plans with varying degrees of modulation restriction, achieved through utilizing different combinations of the aperture shape controller (ASC) in the calculation parameters, and monitor unit (MU) objective during optimization. The dosimetric quality was assessed based on RTOG conformity indices (CI100/CI50), as well as adherence to dose–volume metrics used clinically at our institution. Plan complexity was assessed based on the modulation factor (MU/cGy) and the field edge metric. While VMAT plans with the least modulation restriction achieved the best dosimetry, it was found that there was no clinically significant trade‐off in terms of dose to organs at risk and conformity by reducing complexity. Furthermore, it was found that utilizing the ASC and MU objective could reduce plan complexity to near‐DCAT levels with improved dosimetry, which may be sufficiently robust to overcome the interplay effect.
Highlights
Volumetric modulated arc therapy (VMAT) provides modulation via continuous multileaf collimator (MLC) motion and dose rate modulation during gantry rotation.[1,2] This complex delivery is derived through inverse planning, which provides the ability to tune the treatment plan dosimetry
The main benefits of VMAT in lung stereotactic ablative body radiotherapy (SABR) may be the use of variable dose rate and gantry speeds that are produced by the optimizer, rather than complex aperture shapes
The internal target volume (ITV) D2% was lower than the benchmark case for the dynamic conformal arc therapy (DCAT), monitor unit (MU) objective, and hybrid VMAT” (hVMAT) plans, with the maximum difference occurring in the DCAT plans
Summary
Volumetric modulated arc therapy (VMAT) provides modulation via continuous multileaf collimator (MLC) motion and dose rate modulation during gantry rotation.[1,2] This complex delivery is derived through inverse planning, which provides the ability to tune the treatment plan dosimetry. Modulated delivery is best utilized for complex target‐organ at risk geometry, VMAT is used for a range of treatment sites throughout the body.[3,4,5] As an example, peripheral lung stereotactic ablative body radiotherapy (SABR) volumes are typically not complex in shape and in many cases are not proximal to serial organs at risk, so there is little indication to create modulated plans. The main benefits of using VMAT in this context are highly efficient delivery and in general a more efficient planning process compared with 3D conformal treatments which require manual tuning of beam weights. The main benefits of VMAT in lung SABR may be the use of variable dose rate and gantry speeds that are produced by the optimizer, rather than complex aperture shapes
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