There are no large studies comparing the suitability of different imaging techniques in PTLD and the published work consists of case series or individual case reports. Much of the relative importance of the different imaging techniques available, therefore, needs to be inferred from work done within the field of non-transplant related lymphoma diagnosis and staging in general and adapted to the particular circumstances of the post transplant patient (Seam et al, 2007; Kwee et al, 2008).
Computerised tomography (CT). The role of CT scanning in PTLD can be helpful in identifying areas for biopsy, staging and treatment response. The diagnosis of PTLD can be difficult due to the rarity of the condition and the myriad sites of presentation, therefore, the possibility of PTLD must always be considered. In one centre, a review of abdominal CT scans carried out for obstruction post liver transplant showed PTLD to be an uncommon cause, found in only 3 of 44 patients (Blachar & Federle, 2001). Pulmonary CT findings need to be interpreted carefully, taking into consideration the patient’s clinical state, particularly where extra-nodal disease is suspected, to ensure that all other differential diagnoses eg. aspergillus, biopsy scar, drugs etc have been excluded, because ground glass changes, single nodules and effusions can all be found in these conditions as well as PTLD (Collins et al, 1998; Copp et al, 2006).
CT imaging is not just important for making an initial diagnosis, but also for assessing the extent of the disease. The role of staging abdominal CT scans was shown to be of benefit in a single centre series of 51 patients with PTLD. All patients had a staging CT with approx 50% of the initial diagnosis being from an extra-abdominal site. Twenty nine percent of patients had no evidence of abdominal involvement by CT, of the remaining 70%, 22% had nodal involvement and the remainder had extra-nodal involvement with liver commonest (53%) and spleen and small bowel next commonest. In addition, this group observed that over 90% of liver and heart transplants had evidence of intra-abdominal involvement as against only 50% of renal and lung recipients (Pickhardt & Siegel, 1999).
The role of CT in initial diagnosis and staging of lymphoma has long been accepted, although it has a number of problems. In particular, it is neither able to identify involved nodes that have not increased in size nor to distinguish enlarged nodes due to non-malignant causes from malignancy. In addition, CT is poor at identifying bone involvement, which would automatically put a patient into Stage IV disease. However, when used with contrast, where appropriate, it provides valuable information about the size and location of tissues involved with lymphoma and has become one of the major tools for lymphoma diagnosis and staging.
18F-fluoro-2-deoxyglucose positron emission tomography (FDG-PET) and FDG-PET/CT. The FDG-PET scan shows areas of increased metabolic activity and has advantages over CT in that it can identify areas infiltrated by lymphoma that have not yet increased in size, and is better at detecting bone involvement (Newman et al, 1994; Carr et al, 1998). It is poor at localising the precise area of involvement unless combined with CT (Marom et al, 2004; von Schulthess et al, 2006), but has a higher positive predictive value than CT alone (Seam et al, 2007). However, lymphomatous infiltration cannot be distinguished from any other cause of increased isotope uptake due to glycolysis, such as infection, inflammation, granulomatous involvement and bone marrow recovering from chemotherapy (Juweid & Cheson, 2006). In addition, not all types of lymphoma are FDG-positive with reports of PET-negative disease in some T cell non-Hodgkin lymphoma and low-grade B lymphomas. The precise role of FDG-PET, even in the diagnosis and management of non-transplant related lymphoma, remains controversial, apart from Hodgkin lymphoma where it is becoming accepted practice (Juweid et al, 2007).