Relative reduction of residual tumor improves outcome prediction of PET after effective chemotherapy for patients with advanced stage Hodgkin lymphoma

Objectives PET after chemotherapy is established to guide consolidating radiotherapy in advanced stage Hodgkin Lymphoma (HL). This analysis aims to compare its prognostic usefulness to conventional imaging methods (CIM), to evaluate consistency in interpretation, and to assess if prediction may be improved using additional criteria.

Methods 739 patients from the HD15 trial were included. The central panel reviewed PET and CT scans to give a recommendation for further treatment. Prognosis was evaluated using progression free survival; groups were compared with the log rank test. Potential prognostic factors were investigated using ROC analysis and logistic regression.

Results 548 of 739 patients (74%) were PET negative with a 4-year PFS after panel of 91.5%. 191 PET positive patients (26%) had a 4-year PFS of 86.1% (p=0.022). In contrast, CIM was unable to separate patients by risk of recurrence both for all patients and in the PET negative and PET positive subgroups (p=0.7). Concordance between local and central PET-review was 90%. When investigating further potential factors, ROC analysis showed that only relative reduction of the largest residual tumor diameter was significantly associated with outcome (AUC 0.65, 95% CI 0.57 to 0.73). Comparing the 54 PET-positive patients with a reduction of at most 40% to the 135 PET-positive patients with a larger reduction revealed an odds ratio for recurrence within a year of 5.6 (95% CI 2.1 to 15.2).

Conclusions While morphological imaging alone does not prove to be helpful in the prediction of progression free survival, the percentage change of tumor shrinkage may help to identify PET positive patients with a higher risk for progression or relapse despite additional radiotherapy.