Limited Benefit of Additional Contrast-Enhanced CT to End-of- Treatment PET-CT Evaluation in Patients with Follicular Lymphoma

BACKGROUND: Follicular lymphoma (FL) is generally characterized by a moderate metabolic activity (FDG-avidity) in nodal and extra-nodal sites and by the frequent detection of residual disease in post-treatment PET scans. Since median age at diagnosis is over 60 years, in many FL patients several conditions that affect the elderly often occur in combination with the tumor, thus complicating and limiting the use of conventional imaging for lymphoma staging and response assessment. For this reason, the choice of the imaging technique should take into account clinical needs (preservation of renal function, reduction of diagnostic radiation exposure) as well as the necessity to constrain health care costs.

OBJECTIVE: The aim of the study was to investigate if there is any advantage in the use of contrast-enhanced CT (ceCT) vs. unenhanced low-dose CT (ldCT) in routine protocols for end-of-therapy PET/CT evaluation of patients with FL.

METHODS: Thirty FL patients who underwent end-of-therapy PET/CT protocol with ldCT and ceCT were analyzed retrospectively. Two different observers evaluated PET/ldCT and PET/ceCT in a blinded manner. Number and sites of nodal or extra-nodal disease were compared, using PET/ceCT as gold standard in order to evaluate if the type of CT could result in changes of the DS and therapeutic strategy.

RESULTS: In 26 of 30 patients (87%; 95% confidence interval, 73%-98%), PET/ldCT showed the same number and sites of lesions highlighted by PET/ceCT. The inter-observer concordance and overall concordance between imaging procedures were excellent with a very high Cohen's kappa (respectively 0.82 and 0.83). 97% of lesions (103/107) were found by PET/ldCT and in 4 of 30 patients (13%) PET/ceCT provided additional nodal lesions in the mesenteric and iliac regions (3 mesenteric nodes and 1 iliac node; <1%). In these 4 patients, DS and consequently the therapeutic strategy were not changed after additional ceCT findings. PET/ldCT accuracy, sensibility, specificity, positive predictive value and negative predictive value and NPV were respectively 87%, 83%, 100%,100% and 60%.

CONCLUSION: Our results indicate that the clinical impact of PET/ceCT in assessing end-therapy evaluation in FL is limited. The PET/ldCT could be suggested as primary imaging modality of choice, thus limiting the acquisition of PET/ceCT images only for doubtful cases of residual disease in mesenteric area (represented solely by 13 % in our series). This diagnostic approach would be less expensive, minimize diagnostic radiation exposure, and preserve renal function.