Minimal residual disease testing with radioisotopes

The aim of any therapy is to eradicate minimal residual disease (MRD) by studying the efficacy of different regimens. It allows us to design more aggressive therapy. It allows us to evaluate novel drugs, which target cellular mechanisms such as differentiation agents and apoptosis inducers and inhibitors. MRD is necessary to evaluate the efficacy of comparative treatments. It may be used to select patients and determine when and whom to treat. In the bone marrow transplant setting it may be used to assess the purity of the donor cells and to track the donor cells following transplantation. It allows the assessment of relevance of maintenance therapy. There are several techniques for detecting MRD with differing sensitivities. Cytology is very much the gold standard with haematologists and a leukaemia cell can be detected in 100 cells. In vitro progenitor assays involving the growth of cells [colony forming units (CFU)] following cytokine addition [myeloid progenitors include CFU-GM (for granulocyte-macrophage), CFU-G (for granulocyte), CFU-M (for macrophage); erythroid colonies are called burst forming units (BFU-E) and lymphoid colonies] have a similar sensitivity. Immunophenotyping using a panel of surface markers and flow cytometry can detect 1 abnormal cell in 10,000. Conventional cytogenetics by Giemsa staining can detect 1 in 30 cells. Using fluorescence is situ hybridisation on metaphase chromosome spreads or interphase nuclei increases the detection rate to 1 in 100 cells. By far the most sensitive of techniques is the polymerase chain reaction (PCR) based assays. PCR is highly specific, very sensitive, quantifiable, and can be standardized and automated. PCR has a sensitivity of 1 in 105-106. Reverse transcriptase-PCR (RT-PCR) using conventional reactions have sensitivities of 1 in 104-106 whereas real time PCR has a detection rate of 1 in 104-105. PCR followed by Southern blotting and hybridisation with radiolabelled probes are even more sensitive at 1 in 105-107 with detection of transcripts as low as 1fg. Radioactivity provides for very sensitive in situ hybridisations. For solid tumours imaging systems such as magnetic resonance imaging (MRI), computed tomographic (CT) and positron emission tomography (PET) is very useful for tracking disease bulk. Of these methods PET and the combination of PET-CT give the best results. (author)