Abstract
PurposeTo explore the benefit of adaptive magnetic resonance-guided stereotactic body radiotherapy (MRgSBRT) for treatment of lung tumors in different locations with a focus on ultracentral lung tumors (ULT).Patients & MethodsA prospective cohort of 21 patients with 23 primary and secondary lung tumors was analyzed. Tumors were located peripherally (N = 10), centrally (N = 2) and ultracentrally (N = 11, planning target volume (PTV) overlap with proximal bronchi, esophagus and/or pulmonary artery). All patients received MRgSBRT with gated dose delivery and risk-adapted fractionation. Before each fraction, the baseline plan was recalculated on the anatomy of the day (predicted plan). Plan adaptation was performed in 154/165 fractions (93.3%). Comparison of dose characteristics between predicted and adapted plans employed descriptive statistics and Bayesian linear multilevel models. The posterior distributions resulting from the Bayesian models are presented by the mean together with the corresponding 95% compatibility interval (CI).ResultsPlan adaptation decreased the proportion of fractions with violated planning objectives from 94% (predicted plans) to 17% (adapted plans). In most cases, inadequate PTV coverage was remedied (predicted: 86%, adapted: 13%), corresponding to a moderate increase of PTV coverage (mean +6.3%, 95% CI: [5.3–7.4%]) and biologically effective PTV doses (BED10) (BEDmin: +9.0 Gy [6.7–11.3 Gy], BEDmean: +1.4 Gy [0.8–2.1 Gy]). This benefit was smaller in larger tumors (−0.1%/10 cm³ PTV [−0.2 to −0.02%/10 cm³ PTV]) and ULT (−2.0% [−3.1 to −0.9%]). Occurrence of exceeded maximum doses inside the PTV (predicted: 21%, adapted: 4%) and violations of OAR constraints (predicted: 12%, adapted: 1%, OR: 0.14 [0.04–0.44]) was effectively reduced. OAR constraint violations almost exclusively occurred if the PTV had touched the corresponding OAR in the baseline plan (18/19, 95%).ConclusionAdaptive MRgSBRT is highly recommendable for ablative treatment of lung tumors whose PTV initially contacts a sensitive OAR, such as ULT. Here, plan adaptation protects the OAR while maintaining best-possible PTV coverage.
Highlights
Stereotactic body radiotherapy (SBRT) represents the standard treatment for inoperable early-stage NSCLC [1] and enables good local control for pulmonary oligometastases [2,3,4]
magnetic resonance-guided SBRT (MRgSBRT) offers the opportunity of online plan adaptation to correct for interfractional changes in thoracic anatomy
Previous investigations of adaptive MRgSBRT of lung tumors have demonstrated its clinical feasibility and suggested dosimetric benefits compared to nonadaptive SBRT and favorable clinical outcomes [7, 15, 16]
Summary
Stereotactic body radiotherapy (SBRT) represents the standard treatment for inoperable early-stage NSCLC [1] and enables good local control for pulmonary oligometastases [2,3,4]. Current state-of-the-art SBRT techniques mainly rely on CT-based image-guided radiotherapy (IGRT) [5]. This usually includes 4D-CT-derived internal target volume (ITV) approaches, where the ITV encompasses the whole tumor trajectory during breathing [5,6,7]. Outcomes of pulmonary SBRT using state-of-the-art techniques are favorable, with high local control rates after application of ablative biologically effective doses (a/b = 10, BED10) >100 Gy [8, 9] and low treatmentrelated toxicity [2, 4, 10, 11]. MRgSBRT with gated dose delivery already represents an elaborate innovation which becomes significantly more laborintensive and time-consuming with additional online plan adaptation [17]. Given the already favorable outcomes of standard non-adaptive pulmonary SBRT, targeted patient selection based on evidence of meaningful benefits is key to the reasonable clinical use of adaptive MRgSBRT
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