Breast Cancer Research and Treatment 32: 281-290, 1994. © 1994 Kluwer Academic Publishers. Printed in the Netherlands'. Report The Nottingham Prognostic Index applied to 9,149 patients from the studies of the Danish Breast Cancer Cooperative Group (DBCG) Ingegerd Balslev, ~ Christen Kirk Axelsson, 2Karin Zedeler, 3Birgitte Bruun Rasmussen, 4Bendix Carstensen 5 and Henning T. Mouridsen 6 Department of Tumour Endocrinology, Danish Cancer Society, Division for Cancer Biology, DK-2100 Co- penhagen; 2 Department of Surgery K, Vejle Hospital, DK-7100 Vejle; ~ The DBCG Secretariat, Rigshospitalet, DK-2100 Copenhagen; 4 Department of Pathology, Roskilde County Hospital, DK-4000 Roskilde; 5 Section of Biostatistics and Dataprocessing, Danish Cancer Society, Division for Cancer Epidemiology, DK-2100 Co- penhagen," 6 Department of Oncology 5074, Rigshospitalet, DK-2100 Copenhagen, Denmark Key words: breast cancer, histological grade, lymph-node staging, prognostic factors, multivariate prognostic index, survival, tumour size Summary In primary, operable breast cancer, the Nottingham Prognostic Index (NPI) based on tumour size, lymph- node stage and histological grade can identify three prognostic groups (PGs) with 10-year survival rates of 83%, 52%, and 13%. With the aim of defining a subset of patients having so good prognosis that adjuvant therapy can be withhold, the NPI was applied to a Danish population-based study group comprising 9,149 patients. As opposed to the British study, we used conventional axillary lymph-node staging. Histological grading was in both studies done by means of a similar slight modification of the Bloom and Richardson procedure, but in the Danish study only ductal carcinomas were graded. The 10-year crude survival was 68.1% for 4,791 patients with tumour size < 2 cm and 70.0% for 2,900 pa- tients with grade I tumours. For 4,761 node-negative patients, the 10-year survival was also 70.0%, the expect- ed survival being 89.3%. The relative mortality (observed:expected) was even at 10 years 2.1 demonstrating that more than 10 years observation time is necessary to estimate cumulated mortality. By application of the NPI, the Danish good PG comprising 27.3% of the patients had a 10-year survival of 79.0%. Thus, the index defined a subset with better survival than could be defined individually by each of its three components, but it did not succeed in defining a subset with survival similar to the expected; additional prognostic factors are therefore needed. The somewhat poorer survival of the Danish good PG may be ascribed to the British inclusion of non-ductal carcinomas, to interobserver variation present only in the Danish study, and to poorer expected survival of the Danish patients. The 10-year survival of the Danish moderate PG and poor PG was 56% and 25%, respec- tively. These improved survival rates are attributed to the administration of adjuvant therapies. There were virtually no node-positive patients in the good PG and no node-negative patients in the poor PG. Patients should therefore still be stratified initially by lymph-node status, but tumour size and histological grade are significant prognostic factors primarily within the node-negative group, and they should be included in future prognostication procedures. Address for offprints: I. Balslev, Department of Tumour Endocrinology, Danish Cancer Society, Division for Cancer Biology, Strand- boulevarden 49, Bldg. 7.1, DK-2100 Copenhagen O, Denmark