Abstract
Single-fraction stereotactic ablative radiotherapy (SABR) is an effective treatment for early-stage lung cancer, but concerns remain about the accurate delivery of SABR in a single session. We evaluated the delivery of single-fraction lung SABR using magnetic resonance (MR)-guidance. An MR-simulation was performed in 17 patients, seven of whom were found to be unsuitable, largely due to unreliable tracking of small tumors. Ten patients underwent single-fraction SABR to 34Gy on a 0.35T MR-linac system, with online plan adaptation. Gated breath-hold SABR was delivered using a planning target volume (PTV) margin of 5mm, and a 3mm gating window. Continuous MR-tracking of the gross tumor volume (GTVt) was performed in sagittal plane, with visual patient feedback provided using an in-room monitor. The real-time MR images were analyzed to determine precision and efficiency of gated delivery. All but one patient completed treatment in a single session. The median total in-room procedure was 120min, with a median SABR delivery session of 39min. Review of 7.4h of cine-MR imaging revealed a mean GTVt coverage by the PTV during beam-on of 99.6%. Breath-hold patterns were variable, resulting in a mean duty cycle efficiency of 51%, but GTVt coverage was not influenced due to real-time MR-guidance. On-table adaptation improved PTV coverage, but had limited impact on GTV doses. Single-fraction gated SABR of lung tumors can be performed with high precision using MR-guidance. However, improvements are needed to ensure MR-tracking of small tumors, and to reduce treatment times.
Highlights
Stereotactic ablative radiotherapy (SABR) is the guideline-recommended treatment for medically inoperable early-stage non-small cell lung cancer (NSCLC) [1,2]
magnetic resonance (MR)-guided single-fraction lung stereotactic ablative radiotherapy (SABR) delivered during repeated breath-holds was generally well tolerated by patients
SABR was delivered with a high level of precision, as the average beam-on GTVt coverage by the planning target volume (PTV) in sagittal plane was 99.6%
Summary
Stereotactic ablative radiotherapy (SABR) is the guideline-recommended treatment for medically inoperable early-stage non-small cell lung cancer (NSCLC) [1,2]. Delivery of SABR in a single fraction is a potentially more convenient approach for patients, and the safety and efficacy of singlefraction SABR has been demonstrated for both early-stage NSCLC and pulmonary metastases [5,6,7,8,9]. One approach to improve accuracy is by using internal fiducial markers as a surrogate for x-ray based gating or tumor tracking [10]. Fast delivery of single-fraction lung SABR can be performed using flattening-filter-free (FFF) volumetric modulated arc therapy (VMAT), using an internal target volume (ITV) approach [15]. Single-fraction stereotactic ablative radiotherapy (SABR) is an effective treatment for early-stage lung cancer, but concerns remain about the accurate delivery of SABR in a single session. Conclusions: Single-fraction gated SABR of lung tumors can be performed with high precision using MR-guidance. Improvements are needed to ensure MR-tracking of small tumors, and to reduce treatment times
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