Outcomes of Real-Time MRI-guided Lung Stereotactic Body Radiation Therapy

Abstract

Lung stereotactic body radiation therapy (SBRT) is an effective therapy that provides excellent local control for small primary and/or metastatic lung tumors. We report our experience using real-time MRI-guidance with respiratory breath hold gating for delivery of lung SBRT. Patients with primary or metastatic lung tumors ≤5cm were eligible for treatment with MRI-guided SBRT. Patients underwent simulation and treatment on a Cobalt-60 MRI-guided treatment planning system, with max inhale breath hold used to minimize normal lung dose. All patients underwent 4D-CT simulation as well. Gross tumor to PTV expansions were 3mm, with gross tumor tracked in real-time during treatment and the PTV, or similar numeric expansion, used as a boundary structure for gating. Thirteen of fifteen tumors were treated with 50 Gy in 5 fractions, with the remaining two treated with 45 Gy in 5 fractions and 40 Gy in 5 fractions. Constraints for radiation therapy were similar to RTOG 0813. For comparison, the 4D-CT was used to create an ITV, which was itself expanded 5mm to PTV as per institutional practice. The study population consisted of fourteen patients with a total of fifteen treated tumors. Eleven of fourteen patients had primary non-small cell lung cancer, while the remaining three had metastatic lung tumors (nasopharygeal adenoid cystic carcinoma, breast adenocarcinoma, and tonsil squamous cell carcinoma). Nine of the fourteen patients were female, with a median age of 69.5 years (range 60-79). The median follow-up at this time is 13.7 months (range 1.6-35.9 months). The median PTV volume using respiratory gating with MRI-guidance was 16.5cc (range 2.8-58.5cc), compared to a median PTV volume of 43.6cc (range 8.2-136.4cc, p= 0.01) when using an ITV to account for respiratory motion. Median Lung-PTV volume using respiratory gating with MRI-guidance was 4746.9cc (range 2742.5-6563.5cc) compared to 3967.8cc (range 2255.8-5190.0cc, p = 0.02) when using an ITV. Analysis of > 800 minutes of cine images shows patients were coached into repeatable tumor positioning (< 2 mm) for a median breathhold of 29 seconds, with a median duty cycle of 53%. No treatment-related toxicities were noted. Only one of the fifteen treated tumors, a right hilar metastasis from right tonsil primary, recurred at 5 months after treatment. This tumor was seen to have responded to therapy on initial post-treatment imaging; however, subsequent imaging identified local failure at the right hilum as well as progression of systemic disease elsewhere. Of note, this tumor was treated with 40 Gy in 5 fractions. MRI-guided SBRT to the lung is well-tolerated by patients and provides excellent local control similar to historic controls. Respiratory gating utilizing real-time tumor visualization allows for smaller PTV volumes and greater sparing of normal lung.