Potential Utility of Ctdna to Detect False Positive PET/CT in the Evaluation of Lymphoma Response
Authors: Martín-Muñoz Alejandro , Jiménez Ubieto Ana , Ruiz-Heredia Yanira , Rufián Laura , Rodríguez Margarita , Baumann Tycho , Rodriguez Izquierdo Antonia , Poza Santaella Maria , Gil Alos Daniel , Sarandeses Pilar , Revilla Enrique , Figaredo Gloria , Errazquin Ricardo , Rapado Inmaculada , Ayala Rosa , Barrio Santiago , Martínez-López Joaquin 
Published: December 2024

INTRODUCTION
18F-FDG-PET/CT is the current state-of-the-art diagnostic imaging, for the staging, restaging and evaluation of response to treatment of B lymphomas with avidity for 18F-FDG. Nevertheless, the use of 18F-FDG-PET/CT alone in the response assessment is hampered by its radioactivity, interpreter variability, limited sensitivity and specificity, and a high rate of false positives (e.g. isolated mesenteric masses, pseudo progression in the setting of immunotherapy, primary mediastinal lymphoma...). During recent years, circulating tumor DNA (ctDNA) has emerged as a promising non-invasive complementary technique to improve diagnostic accuracy, thereby optimizing the clinical management of lymphoma patients and improving decision making.
METHODS
This study investigates the use of CloneSight, an ultrasensitive ctDNA-NGS to detect minimal residual disease (MRD) in patients with follicular lymphoma (FL) after first line immunochemotherapy (IQT) (n=45) or CAR-T cell (N=11), primary mediastinal lymphoma (PMBL) (n=11) or large B cell lymphoma (LBCL) after first line IQT (n=14), who do not reach a complete response (CR).
Lymph node tumors from 81 patients were analyzed using a custom NGS panel targeting the 56 most relevant mutated genes in B-cell lymphomas. Following identification of an average of 5-8 somatic mutations per patient, ctDNA was detected by CloneSight, an ultra-sensitive and patient-specific NGS test. All patients had treatment response assessment by PET/CT scans at end of treatment and/or after 4 cycles of treatment.
RESULTS
Among the 81 total patients, 31% (25/81) did not achieve a complete response (CR) based on PET/TC results after treatment. Focusing on these positive PET-TC patients, Clonesight test was applied to assessed MRD by NGS. Fifty-six% of patients (14/25) had a positive ctDNA-NGS result and 44% /11/25) had negative results. Interestingly, all patients with positive results from both tests progressed within 7 months.
Regarding FL patients treated with IQT (N=45), 27% (12/45) had a positive PET/CT. Of these, 7/12 patients had a positive ctDNA-NGS result and 5/12 had a negative ctDNA result. All patients with both positive techniques progressed. While 4 of 5 patients with discordant results achieved CR and did not progress with a median follow-up of 5 months (2 isolated mesenteric masses, one lesion in the colon after surgery, one cervical adenopathy). Negativity of ctDNA-NGS was confirmed by tissue biopsy in 3/4 of the cases. There was only one patient with a positive PET/CT and negative ctDNA-NGS result who progressed.
Of the 11 patients with FL treated with CAR-T, 3 patients (27%) had a positive PET-CT; one patient also had ctDNA-NGS positive and eventually progressed within 7 months. The remaining 2 patients had negative ctDNA-NGS and remain in CR after 3-4 years of follow-up (one had a left cervical adenopathy that was biopsied excluding lymphoma and the other a mesenteric uptake that became negative on all the following PET/TC analysis)
In the case of primary mediastinal lymphoma, 4/11 patients had a positive PET/CT (36%). Only 2 of them presented a positive result for ctDNA-NGS test, and they eventually progressed after 4 and 7 months. The other 2 patients with negative ctDNA test never progressed after 3-4 years of follow up (and achieved CR after radiotherapy).
6 out of 14 patients with DLBCL had a positive PET/TC, of which 4 also showed a positive ctDNA-NGS test result and progressed after 4-6 months. The last 2 patients with discordant results (positive PET-TC and negative ctDNA test) never progressed after 4-5 years of follow-up.
CONCLUSION
This study demonstrates the potential of ctDNA-NGS as a non-invasive tool to assess treatment response and monitoring MRD in several B lymphoma patients. CloneSight, an ultra-sensitive ctDNA-NGS assay, correctly identified 40 % of false positive in PET-TC detection (10/25). Although larger cohort are needed, these findings suggest that combining ctDNA-NGS with PET/CT in clinical practice might improve patient management by refining response assessment.