Bone Marrow Transplantation, (1999) 23 , 1297–1301  1999 Stockton Press All rights reserved 0268–3369/99 $12.00 http:/ / www.stockton-press.co.uk/ bmt Immunohistochemical detection of breast cancer cells in paired peripheral blood progenitor cell specimens collected after cytokine or cytokine and myelosuppressive chemotherapy MB Kleinman 1 , EL Wiley 2 , M Guo 1 , AW Rademaker 3 , M Villa 1 , MS Tallman 1,3 , SB Newman 1 , LI Gordon 1,3 and JN Winter 1,3 1 Division of Hematology/Oncology, Department of Medicine, 2 Department of Pathology, and 3 Robert H Lurie Cancer Center, Northwestern University Medical School, Chicago, IL, USA Summary: Mobilized peripheral blood progenitor cells (PBPC) from 30 patients with advanced breast cancer were studied for the presence of tumor cell contamination using a highly sensitive immunohistochemical technique with the capacity to detect one tumor cell in one million mononuclear cells. Aliquots of PBPC were obtained after 4 days of G-CSF and/or GM-CSF and again dur- ing G-CSF-stimulated recovery from myelosuppressive doses of cyclophosphamide. The overall incidence of tumor cell contamination was 23%, occurring in PBPC specimens from seven of 30 patients. All four cases in which tumor cells were detected after mobilization with cytokine alone also had tumor cells detected in PBPCs collected following chemotherapy and G-CSF. There were three cases in which malignant contamination was detected only in the specimens collected after cyclophos- phamide. There was a greater frequency of tumor cell contamination in aphereses performed during G-CSF- stimulated recovery from cyclophosphamide than in col- lections primed by cytokine alone (13% vs 23%; P = 0.08), although this did not reach statistical significance. This trend suggests that collection of PBPC during cyto- kine-stimulated recovery from myelosuppressive chemotherapy may be associated with a greater risk of contamination with malignant cells than apheresis dur- ing mobilization with cytokines in the steady state. Keywords: breast cancer; minimal residual disease; apheresis Malignant cells have been detected in the peripheral blood of patients with solid tumors such as breast and lung cancer and may contaminate peripheral blood progenitor cell (PBPC) collections in individuals undergoing high-dose chemotherapy with autologous stem cell reinfusion. 1,2 The likelihood of tumor cell contamination of the apheresis pro- duct may be related to the particular mobilization protocol. Brugger and colleagues 1 have shown that breast cancer Correspondence: Dr JN Winter, 233 E Erie St, Suite 700, Chicago, IL 60611, USA Received 16 July 1998; accepted 17 January 1999 cells contaminate PBPCs collected during granulocyte col- ony-stimulating factor (G-CSF)-stimulated recovery from chemotherapy (chemomobilization). Similarly, Passos- Coelho et al 3 found immunohistochemical evidence of breast cancer in some patients undergoing apheresis during granulocyte–macrophage colony-stimulating factor (GM- CSF)-stimulated recovery from myelosuppressive doses of cyclophosphamide. Using historical controls, they com- pared the frequency with which tumor cells contaminated PBPC mobilized with cytokine alone in the steady state to that of PBPC mobilized with chemotherapy and growth fac- tor and found no difference. 4 In contrast, mobilization with cytokine alone resulted in a higher rate of tumor cell con- tamination of PBPC collections than mobilization with chemotherapy-based regimens in a large multi-institutional series of women with stage IV disease. 5 Using a highly sensitive immunocytochemical technique, we evaluated the incidence of tumor cell contamination of PBPC specimens from patients with advanced breast cancer and investigated its relationship to the mobilization method. In the past, patients at our institution have been enrolled on protocols which specified PBPC mobilization with G- and/or GM-CSF. 6 Patients with advanced breast cancer then proceeded to ‘chemomobilization’ with cyclophosphamide (CY) and G-CSF. Consequently, we have had the unique opportunity to compare the incidence of tumor cell con- tamination of PBPC mobilized by cytokine alone in the steady state with that of PBPC mobilized by chemotherapy plus cytokine in the same individuals. Materials and methods Patient population The subjects of this investigation were participants in a phase I/II study of high-dose chemotherapy and PBPC res- cue for the treatment of advanced breast cancer. Eligibility criteria included age less than 60 years, histologically pro- ven stage IIIB or IV breast cancer, and an Eastern Cooper- ative Oncology Group (ECOG) performance status of 0–1. Optimal cardiac (normal left ventricular ejection fraction by nuclear scan), pulmonary (corrected diffusion capacity 50%), hepatic, and renal (serum creatinine 1.5 mg/dl) function was required. Written informed consent was