Liver Stereotactic Body Radiation Therapy Using Real Time Magnetic Resonance Guidance

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Conference Proceeding

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Int J Radiat Oncol Biol Phys


Background: Stereotactic body radiation therapy (SBRT) is a proven and effective modality for treatment of hepatic primary and metastatic tumors [1–5]. However, these lesions are challenging for planning and treatment execution due natural anatomic changes associated with respiration. Magnetic resonance imaging (MRI) offers superior soft tissue contrast resolution and the ability for real-time image guided treatment delivery and lesion tracking. Objectives: To evaluate the plan quality, treatment delivery, and tumor response of a set of liver SBRT cancer treatments treated with magnetic resonance-guided radiotherapy (MRgRT) on a MR-linac. Methods: Treatment data from 22 consecutive patients treated with SBRT were reviewed. All deliveries were performed using a step and shoot technique to one or more liver lesions on an MR-linac platform. Patients received 45 to 50 Gy prescribed to at least 95% of the planning target volume (PTV) in five fractions except for two patients who received 27–30 Gy in three fractions. CT and MRI simulation were performed in the supine position prior to treatment in the free-breathing, end exhalation, and end inhalation breath-hold positions to determine patient tolerability and potential dosimetric advantages of each technique. Immobilization consisted of using anterior and posterior torso MRI receive coils embedded in a medium sized vacuum cushion. Gating was performed using sagittal cine images acquired at 4 frames/second. Gating boundaries were defined in the three major axes to be 0.3 to 0.5 cm. An overlapping region of interest (ROI), defined as the percentage volume allowed outside the boundary for beam-on to occur, was set between 1 and 10%. The contoured target was assigned a 5 mm PTV expansion. OAR constraints given by AAPM TG101 were used during optimization. Results: Twenty-two patients, with a total of 24 lesions, successfully completed the prescribed treatment with minimal treatment breaks or delays. Fifteen patients were treated at end-exhale, and seven were treated at end-inhale. One patient was treated free-breathing due to poor compliance with the breath-hold technique. The reported mean liver dose was 5.56 Gy (1.39–10.43; STD 2.85) and the reported mean liver volume receiving the prescribed threshold dose was 103.1 cc (2.9–236.6; STD 75.2). Follow up imaging ranged from 1 month to 9 months post treatment and confirmed either stable or decreased size of treated lesions in all patients. Toxicities included nausea/vomiting and abdominal pain in 2 patients which resolved without any intervention. Four patients died due to complications from liver cirrhosis unrelated to radiation effect. Conclusions: SBRT treatment using a gated technique on a MR-linac has been successfully demonstrated and offers potential to allow for tighter margins and reduced toxicity. Further follow up is needed to identify potential toxicities and to clarify the benefits and risks associated with MR-guided SBRT.





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