Assessment of the robustness of volumetric-modulated arc therapy for lung radiotherapy

Abstract

Volumetric-modulated arc therapy (VMAT) is increasingly popular as a treatment method in radiotherapy owing to the speed with which treatments can be delivered. However, there has been little investigation into the effect of increased modulation in lung plans with regard to interfraction organ motion. This is most likely to occur where the planning target volume (PTV) lies within areas of low density. This paper aims to investigate the effect of modulation on the dose distribution using simulated patient movement and to propose a method that is less susceptible to such movement. Simulated interfraction motion is achieved by moving the plan isocentre in steps of 0.5 cm and 1.0 cm in six directions for five clinical VMAT patients. The proposed planning method involves optimisation using a density override of 1 g cm 23 , within the PTV in lung, to reduce segment boosting in the periphery of the PTV. This investigation shows that modulation can result in an increase in the maximum dose of .25%, an increase in PTV near-maximum dose of 17% and a reduction in near-minimum dose by 46%. Unacceptable organ at risk (OAR) doses are also seen. The proposed method reduces modulation, resulting in a maximum dose increase of 10%. Although safeguards are in place to prevent the increased dose to OARs from patient movement, there is nothing to prevent the increased dose as a result of modulation in lung. A simple planning method is proposed to safeguard against this effect. Investigation suggests that, where modulation exists in a plan, this method reduces it and is clinically viable.

Portal dosimetry for VMAT using integrated images obtained during treatment

“…The apertures were therefore modulated but with limited variation in position between adjacent leaves, in accord with our clinical practice for lung tumors. 34,35 The multileaf collimator (MLC) leaf motion was constrained such that a gantry arc speed of 3.0 • s −1 could be maintained. 29 Dose calculation was a convolution method on a grid of 0.4 × 0.4 × 0.4 cm.…”

Dose prescription with spatial uncertainty for peripheral lung SBRT

“…This is more efficient to calculate and provides a more realistic result than the isocenter shift method, which has also been used in the context of lung radiotherapy. 32,33 The approach used in this study is to retain the normal calculation of primary fluence but then to use the PDF to model local spatial uncertainty. The uncertainty is considered to be in the ITV position, but the convolution approach implicitly applies the uncertainty distribution throughout the patient, so that other structures are also affected.…”

“…In reality, a contouring error or spatial registration error when registering conebeam CT images may result in a shift rather than a symmetrical blurring of the dose distribution. 32 Use of a worst-case (minimax) algorithm may enable this to be further investigated. [36][37][38] This type of algorithm focuses on the impact of the worst-case scenario with respect to spatial uncertainty, and can therefore better handle a systematic error.…”

A planning comparison of 3-dimensional conformal multiple static field, conformal arc, and volumetric modulated arc therapy for the delivery of stereotactic body radiotherapy for early stage lung cancer

“…16 Since then, recent reports regarding the use of VMAT during respiratory motion are conflicting regarding the interplay effects on dose delivery, especially during hypofractionation. [17][18][19][20][21] Patients in this study were selected to have T1-2 N0 M0, mostly peripheral lesions. All patient underwent a free-breathing 4D-computed tomography scan, from which an internal target volume (ITV) was defined from the maximum intensity projection followed by an expansion to a planning target volume (PTV).…”