Background: Residual masses on follow-up surveillance imaging are frequently detected in paediatric patients with Hodgkin's lymphoma and non-Hodgkin's lymphoma. The residual mass may consist of inflammatory, fibrous or necrotic tissue, or it could represent residual tumor. In most cases, positron emission tomography (PET) with 2-[fluorine-18]-fluoro-2-deoxy-D-glucose (FDG) is useful for distinguishing tumor from fibrosis. However, FDG is not tumor-specific, and increased accumulation of the tracer may be seen in a variety of benign entities which can give rise to false-positive or equivocal FDG PET findings. Alternatively, the uptake of 3'-deoxy-3'-[fluorine-18]-fluorothymidine (FLT) reflects cellular proliferation, and may prove to be a reliable method in resolving equivocal FDG PET findings. Indeed, several studies have demonstrated that FLT can be safely administered to children, and in some cases be more useful than FDG PET in differentiating between infection or inflammation and malignancy. This study hypothesizes that FLT PET can be used as an adjunct imaging modality in paediatric lymphoma patients with equivocal interim or post-therapy FDG PET findings, and that this technique can provide additional diagnostic information which will be useful in distinguishing fibrotic or necrotic residual mass lesions from those that may be harbouring malignancy.

This is a prospective pilot study evaluating the clinical use of FLT PET in paediatric patients with Hodgkin's lymphoma (HL) or non-Hodgkin's lymphoma (NHL). The overall objective of this trial is to assess the feasibility of using FLT PET as an adjunct imaging modality to follow-up paediatric lymphoma patients whose interim or post-therapy FDG PET scan is interpreted as being equivocal. The primary outcome measure is to obtain a preliminary estimate of the diagnostic performance (including the sensitivity, specificity and accuracy) of adjunct FLT PET. This pilot study will yield sufficient preliminary data to help justify and design a subsequent larger study of FLT PET in paediatric lymphoma patients.

This study is open only to HL and NHL patients whose interim or post-therapy FDG PET scan is interpreted as being equivocal and in whom it is not possible resolve the exam results using other conventional imaging techniques such as CT or MRI. All FLT PET image findings will be presented to the treating physician(s) responsible for managing the care of any patient enrolled in this trial. Treating physicians will continue to use routine practices to resolve equivocal FDG PET findings.

Inclusion Criteria:

• SickKids Hospital patient of any gender or race
• Participants who are able to undergo imaging procedures without general anaesthesia or sedation
• Patient's or the patient's parents'/guardians' written informed consent prior to participation
• Previous FDG PET scan with at least one documented equivocal finding (i.e. SUV ≥ 2.0, but < 3.5) and no other finding(s) that is strongly suggestive of malignancy. The lesion(s) must have a minimum size of 1 cm in diameter by any CIM in order to address the spatial resolution limitations of the PET scanner.

Exclusion Criteria:

• Patients who are pregnant or nursing
• Medically unstable or critically ill
• Lack of informed consent

Pediatric lymphoma