Role of positron emission tomography in indolent non Hodgkin lymphomas

Fluorodeoxyglucose positron emission tomography (FDG-PET) is a established functional imaging technique used in the staging and evaluation of response in Hodgkin lymphoma and diffuse large B cell lymphomas. Its role in other subset of non Hodgkin lymphoma (NHL) and in particular in low grade NHL has been poorly investigated. We retrospectively analysed 53 PET scans of patients affected by low grade NHL and we compared them with a concomitant standard contrast-enhanced computerized tomography (CT) in order to define the degree of concordance of this two imaging techniques. Our series was composed by 19 marginal zone (MZL), 26 follicular (FL) and 8 mantle cell lymphomas (MCL). We compared PET and CT scans at diagnosis for MZL, FL and MCL and at the end of first line therapy before high dose therapy (HDT) only for MCL. Among MZL, PET and CT were concordant in 7 cases. Considering discordant cases, 7 presented CT positive with PET negative (in 4 cases the difference was related to the presence of splenic involvement of disease) while in 5 patients the discrepancy consisted in more sites of disease detected by PET. Among FL, PET and CT were concordant in 14 cases. Considering 12 discordant cases, one presented only CT positivity; in the remaining PET was positive in a higher number of sites than that detected by CT. Among MCL patients, only in three cases PET and CT were concordant. In the remaining discordant cases PET was positive in more sites of disease than CT and in all of them the stage was modified by PET results. At the evaluation at the end of first line therapy before HDT, 7/8 cases were in complete remission (CR) considering both CT and PET. In conclusion, the sensitivity of PET seems to be higher than CT even in low grade NHL. In particular, in more than 90% of FL and MCL discordant cases PET was more sensitive than CT. In MZL the role of PET is not so evident because it frequently failed to detect diffuse splenic involvement. Anyway, even in this subset of NHL, it could be useful in the detection of disease in soft tissues.