Adult Fibrosarcoma (FS): A Reevaluation of 154 Putative Cases Diagnosed at a Single Institution over a 48 Year Period
A Bahrami, AL Folpe. Mayo Clinic, Rochester, MN.
Background: Adult FS, defined by the WHO as a “malignant tumor, composed of fibroblasts… and, in classical cases, a herringbone architecture” was once considered the most common adult sarcoma. Currently FS is regarded as a diagnosis of exclusion, representing only 1-3% of adult sarcomas. This trend is likely due to: 1) widespread use of immunohistochemistry (IHC) and molecular diagnostic techniques, 2) recognition of distinct FS subtypes, and 3) distinction of FS from undifferentiated pleomorphic sarcoma (UDPS). However, no recent series has critically re-evaluated putative FS, to estimate their true incidence.
Design: 178 cases diagnosed as adult FS in soft tissue locations were retrieved from
our institutional archives for the period 1960-2008. 24 cases with insufficient material were excluded. Based on the morphology of the final 154 cases, IHC was performed using some combination of: wide-spectrum CK, EMA, high molecular weight CK,
S100, Melan A, HMB-45, CD34, CD31, TLE1, smooth muscle actin, desmin, Myo-D1,
myogenin, c-kit, INI1, calretinin, WT1 and TTF1. Revised diagnoses were based on
clinical, morphological, and IHC findings.
Results: The original group of putative FS occurred in 81 M and 73 F (median 53
years, range 2-99 years), and involved the legs (50 cases), head/neck (30 cases), thorax/abdomen (22 cases), arms (21 cases), mediastinum (8 cases), abdomen/ retroperitoneum/pelvis (13 cases), and lung (10 cases). Only 22 cases met WHO criteria for FS. These occurred in 13 M and 9 F (median 49 years, range 6-74 years), and involved the legs (11 cases), head/neck (6 cases), thorax/abdomen (2 cases) mediastinum (2 cases), and arm(1 case). Non-FS (132 cases) were reclassified as: UDPS (38 cases), synovial sarcoma(21 cases), SFT (14 cases), myxo-FS (10 cases), MPNST (6 cases), DFSP, desmoplastic melanoma (4 cases each), LG fibromyxoid sarcoma, sarcomatoid CA, desmoid, RMS, myofibroblastic sarcoma, spindle cell liposarcoma (3 cases each), sclerosing epithelioid FS, fibroma-like epithelioid sarcoma, leiomyosarcoma, cellular fibrous histiocytoma (2 cases each), and other (9 cases).

Conclusions: Using modern diagnostic criteria and ancillary IHC we have been able
to reclassify 87% of putative FS. Exclusive of UDPS, the distinction of which from FS
is subjective, 61% of putative FS were reclassified, most commonly as monophasic
synovial sarcoma and solitary fibrous tumor. We conclude that true FS is exceedingly
rare, accounting for <0.1% of &#8764;18,000 adult soft tissue sarcomas seen at our institution during this time period, and should be diagnosed with great caution