To evaluate the radiographic (Computed Tomography, CT) and metabolic (Positron Emission Tomography, PET) response of lung tumors to stereotactic lung image-guided radiotherapy (IGRT).
Materials/Methods
20 patients with solitary lung tumors ≤5 cm were treated with stereotactic lung IGRT on a prospective phase II trial at William Beaumont Hospital between 11/2005–10/2006. Target volumes were delineated using a combination of free-breathing CT, 4D CT and PET. Four (T1, n = 13) or 5 (T2/metastatic, n = 7) 12-Gy fractions were prescribed to the PTV edge. Both a CT and PET were obtained for all patients at 6 weeks, 16 weeks, and 12 months post-IGRT. A CT alone was done 6 months post-IGRT and every 6 months after 1 year. To assess response, the tumor volume and maximum dimension in 3 planes were measured on follow-up CT scans fused to the initial planning CT. Response Evaluation Criteria in Solid Tumors (RECIST) criteria were used to assess radiographic response (CR = no measurable tumor; PR = ≥30% reduction in the sum of the longest diameters). The maximum standardized uptake value (SUV) on PET was measured and verified with a single nuclear medicine physician. The relationship between CT and PET response and the following variables were evaluated: tumor stage, maximum dimension, volume, and IGRT dose.
Results
Median follow-up time was 9.3 mo [range: 3–16 mo]. One patient with significant consolidation obscuring response at weeks 6 and 16 was excluded from analysis. Response results are shown in Table 1. 26% of cases exhibited a radiographic CR at a median time of 6 months, all of which demonstrated a reduction in max SUV to ≤2.75 (mean 2.0). By 6 months, 87% exhibited either a CR or PR. Tumor response appears to continue even beyond 6 to 12 months. Metabolic activity substantially decreased within 6 and 16 weeks post-IGRT and remained low 6–12 months thereafter. Smaller pre-treatment tumor dimensions (p = 0.01) and volume (p < 0.01) statistically correlated with CR on CT. A single patient treated for a metastasis has clinical evidence of radiographic/metabolic local progression 12 months post-IGRT. No cases of regional or distant recurrence or death have been observed.
Conclusions
Stereotactic lung IGRT using 48–60 Gy in 4–5 fractions exhibited significant radiographic and metabolic response during the first 6–16 weeks post-IGRT and effects appear to remain durable beyond 6–12 months. Smaller tumor dimensions and volume correlated with a greater likelihood of achieving radiographic CR.
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