CHICAGO—Radical robotic radiosurgery, a treatment designed to eradicate both gross and peritumoral microscopic disease, results in a distinctive PET/CT imaging pattern following treatment, which is likely related to the high radiation doses delivered 1-cm or more around the tumor, according to researchers from Georgetown University Hospital in Washington, D.C.
Presented Sunday at the 94th annual meeting of the Radiological Society of North America (RSNA), the study results describe early PET/CT findings following radical robotic radiosurgery in patients with inoperable stage IA non-small cell lung cancer (NSCLC), according to by Saloomeh Vahdat, MD, study co-author and presenter.
Inoperable patients with biopsy-proven stage IA NSCLC were enrolled into the IRB-approved study.
To conduct the study, three to five gold fiducial markers for radiotherapy were implanted under CT guidance in or near tumors in 10 patients with an average tumor size of 2 cm. Gross tumor volumes (GTVs) were contoured using lung windows; margins were expanded by 5 mm to establish the planning treatment volumes (PTVs). Treatment plans were designed using hundreds of pencil radiation beams. Doses delivered to the PTV ranged from 45-60 Gy in three equal fractions. The 30-Gy isodose contour, biologically equivalent to 50 Gy in two Gy fractions, extended at least 1 cm from the GTV to treat microscopic disease. Whole body 18F-FDG PET/CT and contrast enhanced chest CT scans were performed at six-month intervals following treatment.
A mean dose of 55 Gy (ranging from 45 Gy to 60 Gy) was delivered to the PTV in one to two-hour fractions over a five- to nine-day period. The mean tumor SUVmax before treatment was 6.2. "Typically, at 12 months, we saw an increase in FDG uptake where the radiation took place, which we thought was due to radiation pneumonitis," Vahdat said. "Fortunately, at 18 months, typical PET/CT scans showed a decrease in FDG uptake where the tumor was treated, which we thought was due to radiation fibrosis."
At three to six months, the researchers found a decrease in the mean tumor SUVmax to 2.4. Conversely, at 12 months the mean SUVmax increased to 3.3 and involved a large peritumoral area, which correlated with radiation pneumonitis seen on CT. At 18 to 24 months, with the development of radiation fibrosis on CT, the mean SUVmax decreased to 2.2. There were no local failures or regional lymph node recurrences at a median follow-up of 24 months.
“Local control following radical robotic radiosurgery is excellent. Early transient increases in SUVmax are likely related to radiation pneumonitis, not local failure,” Vahdat and colleagues wrote.
“Radical robotic radiosurgery, a treatment designed to eradicate both gross and peritumoral microsocpic disease, results in a distinctive PET/CT imaging pattern following treatment, likely related to the high radiation doses delivered 1-cm or more around the tumor,” the authors said, adding that future research will require pathologic confirmation of PET/CT findings.