Interaction of histologic type and grade in predicting survival from soft tissue sarcoma

Abstract

10530 Background: Histologic grade has traditionally been considered the paramount prognostic factor in predicting survival for soft tissue sarcoma (STS). We hypothesized that histologic type modifies the prognostic effect of grade. Methods: Utilizing the population- based California Cancer Registry, we identified 8,740 cases of STS diagnosed between 1994 and 2003. Grade was categorized according to the ICD-O-3 five-level system (1 = well differentiated, 2 = moderately differentiated, 3 = poorly differentiated, 4 = undifferentiated/anaplastic, 9 = unknown). Using survival as the outcome variable, grade and histologic type were grouped into three categories, respectively, based on non- overlapping 95% confidence intervals (low, intermediate, and high for grade and favorable, neutral, and unfavorable for histologic type). Because there was a significant interaction between grade and histologic type, we examined the effect of grade within categories of histologic type, and the effect of histologic type within categories of grade. Five-year actuarial disease-specific survival was estimated using the Kaplan- Meier method. Results: Among 114 histologic types, malignant fibrous histiocytoma (20.8%), leiomyosarcoma NOS (12.4%), sarcoma NOS (7.0%), and myxoid liposarcoma (5.3%) were the most prevalent. Sixty-eight histologies were diagnosed in 20 patients or fewer (0.2%). Grade distribution was as follows: 1, 9.9%; 2, 15.1%; 3, 11.6%; 4, 23.8%; and 9, 39.6%. Five-year survival was 25.3% higher for patients with low versus high grade tumors (95.1% versus 69.8%). However, within each grade category, there was a statistically significant gradient of survival associated with histologic type. In the low grade category, survival was 20.2% higher for favorable versus unfavorable types (97.0% versus 76.8%). In the intermediate grade category, survival was 5.6% higher for favorable versus unfavorable types (91.4% versus 85.8%). In the high grade category, survival was 36.7% higher for favorable versus unfavorable types (89.3% versus 52.6%). Conclusions: Histologic type modifies the effect of grade in survival from STS. Within each grade category, there are significant differences in survival based on histologic type. Histologic type is an important predictor of biologic behavior in STS. No significant financial relationships to disclose.