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Institution and research group
Authors: Mika M Sampo, MD1,2, Maija Tarkkanen, PhD2, Erkki J Tukiainen, PhD3, Pentscho Popov, PhD3, Mikael Lundin, MD6, Carl P Blomqvist, PhD2, Pelle Gustafsson, PhD4, Tom O Böhling, PhD1, Johan Lundin, MD, PhD2,5,6

Affiliations: Department of Pathology, HUSLAB and University of Helsinki, Finland, PO Box 400, the Hospital District of Helsinki and Uusimaa1, Department of Oncology, Helsinki University Central Hospital (HUCH), Finland PO Box 180, the Hospital District of Helsinki and Uusimaa2, Department of Plastic Surgery, HUCH, Finland, PO Box 288, the Hospital District of Helsinki and Uusimaa3, Department of Orthopedics, Lund University, Sweden, Lund University Hospital SE-221 85 Lund, Sweden4, Division of Global Health, Karolinska Institutet, Stockholm, Sweden5, Institute for Molecular Medicine Finland, University of Helsinki, Helsinki, Finland, PO Box 20, 00014 Helsinki University6

Data set and variables
All patients referred for non-metastatic, primary or recurrent soft tissue sarcoma (STS) of the extremities or trunk wall to the Soft Tissue Sarcoma Group between August 1987 and December 2002 are included. Exclusion criteria comprised: extraskeletal osteosarcoma, chondrosarcoma, Ewing/PNET family tumour, angiosarcoma, alveolar soft tissue sarcoma, epitheloid sarcoma, clear cell sarcoma, atypical lipoma/grade I liposarcoma, dermatofibrosarcoma protuberans, preoperative radiotherapy or chemotherapy or both or postoperative chemotherapy (n=15). In 84 cases we were unable to retrieve the original histological slides leaving 294 cases for analysis.

Necrosis and vascular invasion were reported as defined by Gustafson and colleagues.1 They were re-assessed for the present study and classified as absent or present. Tumour size (cm) was recorded as the largest diameter of tumour in the surgical specimen reported by the original pathologist. The pathologist assigned the histological malignancy grade of the tumour based on a four-tiered grading scale modified from Broders and colleagues.2-3 Grades 1 and 2 are low and grades 3 and 4 are high grades. Subcutaneous tumours with or without cutaneous extension but without involvement of the deep fascia were defined superficial, all others deep.

1. Gustafson, P, Akerman, M, Alvegard, TA et al. Prognostic information in soft tissue sarcoma using tumour size, vascular invasion and microscopic tumour necrosis-the SIN-system. European journal of cancer 2003; 39: 1568-76.
2. Broders, AC, Meyerding, HW. Pathological features of soft tissue sarcoma. Surg Gynecol Obstet 1939; 69: 267-80.
3. Angervall, L, Kindblom, LG, Rydholm, A, Stener, B. The diagnosis and prognosis of soft tissue tumors. Seminars in diagnostic pathology 1986; 3: 240-58.

Statistics and system tools
Sarcoma-specific survival (SSS) was calculated from the date of the diagnosis to death from sarcoma. Deaths due to other causes than sarcoma were censored. A Cox proportional hazards model was fitted entering the following covariates: tumour size in millimeters (continuous), necrosis (absent vs present), vascular invasion (absent vs present), tumour depth (superficial vs deep), location (extremity vs axis of body), and histologic grade (four levels). Please see Table 1 for the final regression model.

Based on the fitted Cox regression models 10-year sarcoma-specific survival is estimated. With the β-coefficients of a Cox model a prognostic index (PI) is estimated. The survival curve corresponding to the average PI value is computed, which is similar to the average survival for the total group. If the average PI value is taken as baseline reference, the relative risk (RR) of an individual patient is given by

RR = eestimated PI/eaverage PI = eestimatedPI-average PI

With the average survival curve corresponding to the average PI value and the patient's relative risk, an expected survival curve for that patient can be computed. For example, in the current series the cumulative 10-year survival is 0.71, and the expected 10-year survival for a patient with the relative risk of RR is 0.71RR.

Disclosure and sources of funding
The funding source had no role in the study design, collection, analysis, interpretation of data, or writing of the report.
The project has been funded exclusively by competed research funds. The major supporters are governmental EVO funds of the Helsinki University Central Hospital, the Finnish Cancer Society, the Sigrid Juselius Foundation, the K. Albin Johansson Foundation, Finska Läkaresällskapet and Duodecim Foundation. These financial contributors are warmly acknowledged.