Purpose/Objective(s) Stereotactic body radiotherapy (SBRT) in central lung tumors has been associated with higher rates of severe toxicity. We sought to evaluate toxicity and local control in a large cohort, and to identify predictive dosimetric parameters. Methods and Materials We identified patients who received SBRT for central tumors according to either of two definitions. Local failure (LF) was estimated using a competing-risks model, and multivariate analysis (MVA) was used to assess factors associated with LF. We reviewed patient toxicity, and applied Cox proportional hazard analysis and logrank tests to assess whether dose-volume metrics of normal structures correlated with pulmonary toxicity. Results One hundred twenty-five patients received SBRT for NSCLC (n=103) or metastatic (n=22) lesions using IMRT. The most common dose was 45Gy in 5 fractions. Median followup was 17.4 months. Incidence of grade ≥ 3 toxicity was 8.0%, including 5.6% pulmonary toxicity. Sixteen patients (12.8%) experienced grade ≥2 esophageal toxicity, including 50% of patients where PTV overlapped the esophagus. There were two treatment-related deaths. Among patients receiving biologically effective dose (BED) ≥ 80 Gy (n=108), 2-year LF was 21%. On MVA, gross tumor volume (GTV) was significantly associated with LF. None of the studied dose-volume metrics of the lungs, heart, proximal bronchial tree (PBT), or 2 cm expansion of the PBT (“no-fly-zone” or NFZ) correlated with grade ≥ 2 pulmonary toxicity. There was no difference in pulmonary toxicity between central tumors located inside the NFZ, and those outside the NFZ but with planning target volume (PTV) intersecting the mediastinum. Conclusion Using moderate doses, SBRT for central lung tumors achieves acceptable local control with low rates of severe toxicity. Dosimetric analysis showed no significant correlation between dose to the lungs, heart, or NFZ and severe pulmonary toxicity. Esophageal toxicity may be an underappreciated risk, particularly when PTV overlaps the esophagus.

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4576 Background: Evidence suggests that changes in unidimensional measurements (using RECIST criteria) may not accurately reflect actual changes in tumor size, and do not correlate well with other biomarkers of response, tumor progression, or patient outcome. We evaluated changes in tumor volume of target lesions with volumetric CT, and compared them with response assessments derived from RECIST criteria. Methods: We evaluated target lesions, including lymph node, liver, peritoneal and lung metastases in 25 patients with metastatic gastric cancer or gastroespohageal junction adenocarcinoma treated on a phase II clinical trial with irinotecan, cisplatin and bevacizumab. All patients underwent thin-section multidetector CT at baseline and 6-week follow-up for 10 cycles. Target lesions were measured unidimensionally and volumetrically, using validated automated and semiautomated segmentation algorithms. For initial analysis, correlation was made with the RECIST response using a cut-off value of 65% volume change. A ratio of change in RECIST measure to volume measure was calculated at time of maximal clinical response for each patient, as well as for responders versus non-responders. Results: 18 of 25 (72%) patients showed a clinical response. Of these 18 responders, 5, 6, 4, and 3 were identified as responders using RECIST criteria after cycles 1, 2, 3, and 4, respectively. Using a cut-off of 65% volume change, 14 of these responders were indentified after cycle 1, and 4 were identified after cycle 2. Using a cut-off value of 65%, volume measurements identified responders a mean of 50.3 days earlier than did RECIST criteria. There was a statistically significant (p<0.01) change in ratio of volume measurement change to RECIST measurement change for responding versus stable patients. Conclusions: Volumetric change in tumor size appears to predict clinical response earlier than do RECIST criteria in the majority of cases. Changes in volume are more sensitive because they exhibit a wider dynamic range. Ratios of size changes volumetrically appears to better discriminate RECIST responders from those with stable disease. Further work is needed to define the biologically relevant cut-off value for volume change, and to correlate volume change with other biomarkers. No significant financial relationships to disclose.

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