Predictors of early response and event-free survival in Hodgkin lymphoma (HL): PET versus CT imaging.

8006 Background: Early PET imaging has emerged as a predictor of EFS in HL, but comparisons of early response assessment by PET vs. CT are limited. Children's Oncology Group (COG) study AHOD0031 was designed to evaluate the paradigm of early-response-based treatment intensity. PET and CT scans were obtained after 2 cycles of ABVE-PC, allowing direct comparison of imaging modalities. We examined predictors of early response to 2 ABVE-PC as defined by CT scan and by PET. We also evaluated the 2-cycle CT and PET as predictors of EFS. METHODS Patients (< age 21) had intermediate risk HL (excluding non-bulky IA/IIA, IIIB/IVB). Rapid early response (RER) by CT was defined as >60% 2-dimensional tumor reduction. RER by PET was defined as a negative (vs. equivocal/positive) scan. Predictors of RER and EFS were derived from clinical and pathologic factors available in the COG database. Continuous variables were made categorical based on quartiles associated with abnormal values. RESULTS 1,136 patients with CT-based assessment of early response who had protocol-mandated, concurrent PET were included in an analysis of predictors of early response. 770 of these patients were assigned to treatment with 4 ABVE-PC/21Gy involved field radiation and included in the analysis of predictors of EFS. Multivariable predictors of RER by CT after 2 ABVE-PC included a large mediastinal mass (OR 0.41), NS (OR 0.45), age<13 (OR 1.48), and stage I (OR 2.31). Multivariable predictors of RER by PET included large mediastinal mass (OR 0.78), ESR<20 (OR 1.37), and albumen <3.5 (OR 1.36). A Cox regression including both early CT and PET found them to be independent predictors of EFS. CT was predictive (HR=0.49, p=0.01); PET was of borderline significance(HR=0.69 , p=0.10). Stage 4 (HR 2.41, p=0.0014) and B symptoms (HR=2.05, p=0.008) were strong predictors. CONCLUSIONS Predictors of CT (anatomical) response differ from predictors of PET (functional) response. Each modality may thus represent different aspects of the biology of response and perhaps should inform the decision re: therapeutic intensity. As a predictor of EFS, RER based on the 2-cycle CT was more significant than RER defined by 2-cycle PET scans.