Gallium 67 scintigraphy has been used for the evaljation of tumors, particularly lymphoma for last 2
P
Iecades, its use has recently declined somewhat with ,Se advent of hight resolution anatomic imaging. We glould realize that diagnostic imaging modalities are of competitive, but rather complement each other. T~herefore, we should understand the advantages and 6init tions of each diagnostic test and use them in I, - most appropriate clinical setting.
.In this presentation I would like to address the
.t frequently asked questions regarding Gallium X imaging in lymphoma
What do the true positives and true negatives um images represent?
What are the causes of false positives and false tives?
How do we avoid misleading findings?
Current challenges in clinical oncology include 1)
accurate initial staging 2) follow-up for treat
t response and selection of the best treatment opon 3) early detection of tumor relapse and recur
4) - 4) prediction of disease-free survival, and sur
al
Accurate Assessment of Initial Staging
e current role for the routine use of Ga 67
´ 1 ng in initial staging of lymphoma is not well blished. Ga 67 tumor imaging is less sensitive in
ting abdominal disease than elsewhere. Routine does not obviate surgical exploration nor dose it
.1 ter the choice of initial therapy.
Detection of Viable Rresidual Mass
Once patients receive initial treatment, effective ´ --tment response is guaged by radiological evalua,on of tumor volume reduction, even though volume
reduction is a late stage of treatment response. The most commonly used diagnostic tests such as chest X -ray, CT and MRI are sensitive but lack specificity with a range of only 21% to 55%.
Medistianal involvement ocurrs in at least 50% of patients with Hodgkin´s lymphoma(HL), and only approximately 20% in non-Hodgkin´s Lymphoma (NHL) where as an abdominal presentation such as mesenteric and retroperitoneal lymphadenopathy is more common.
In a retrospective analysis of the chest radiographs in 65 patients with Hodgkin´s lymphoma, 88% showed mediastinal adenopathy with 40% showing a persistent abnormality up to 4 years following therapy. Residual mass may be seen even after successful treatment of tumor greater than 6cm.
European reseachers investigated patients with malignant lymphoma using Ga-67 and FDG scans during and after treatment. The Ga - 67 biodistribution was similar to FDG distribution in these lymphoma patients. The investigators found changes in Ga 67 uptake during chemotherapy exceeded volume changes seen on CT in magnitude. In Hodgkin´s lymphoma, normal gallium uptake after 2 cycles of chemotherapy suggested complete remission of the lymphoma. Mild uptake of tracer on the other hand indicates variable treatment response. Patients with moderate uptake had treatment failure. Kaplan et al reported similar trends in patients with diffuse large cell lymphoma. It is therefore evident that gallium uptake during or after treatement has prognostic value in predicting treatment response.
Planar and SPECT tumor imaging with gallium demonstrated a sensitivity of 90-96% and specificty of 80%-89%. Ga scanning was especially effective in about 27% of patients on whom CT was equivocal in detecting a residual viable tumor mass.
in Ga-67 imaging interpretation.
Tumor Size
Conventional planar Ga tumor imaging resolves 10 -13mm at best. Tumor size less than 1cm may result in a false negative Ga scan. Futhurmore, with a tumor size greater than 5cm central tumor necrosis may develop reducing Ga-67 tumor uptake. In order to avoid this limitation of spatial resolution, many attempts to increase sensitivity were made over the past years including : 1) increasing count density by ,injecting 10 mCi of Ga-67 : 2) Delayed scanning at least 72 hours and up to 2 weeks: and 3) use of SPECT to increase contrast.
Time of Ga Injection and Scaning
Both chemotherapy and radiotherapy have a profound effect on the biodistribution of Gallium. The Ga uptake is reduced especially with radiotherapy. .The optimal recommended time interval between
therapy and Ga injection should be at least 3 to 6 weeks.
Optimal time of Ga restaging
Kaplan et al recommended Ga 67 restaging at same time of other radiographic restaging during the mid-course of chemotherapy for diffuse large cell lymphoma.
However there are no scintigraphic data supporting the value of restaging Hodgkin´s lymphoma prior to the completion of chemotherapy.
Summary
Recently investigators have been looking into other tumor imaging modalities such as monoclonal antibodies, positron tomographic and somatostatin tumor imaging for the evaluation of lymphoma. There are no available scintigraphic data that these methods are superior to Ga-67 imaging when properly adminstered and interpreted.
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