Integrated molecular pathology and therapeutic outcomes in dermatofibrosarcoma protuberans: Ki-67, COL1A1::PDGFB, and efficacy of Mohs surgery with postoperative imatinib in 153 Chinese patients.

BACKGROUND: Dermatofibrosarcoma protuberans (DFSP) is a rare cutaneous sarcoma with a high local recurrence rate. In one of the largest single-center Asian cohorts (n = 153), we performed integrated pathologic, molecular, and long-term therapeutic analysis to define the prevalence of COL1A1::PDGFB fusion and evaluate the efficacy of Mohs micrographic surgery and postoperative imatinib. METHODS: We retrospectively analyzed 153 consecutive DFSP cases (2018-2023). COL1A1::PDGFB fusion was detected by FISH with reflex RNA sequencing in negative cases; Ki-67 expression was assessed by immunohistochemistry. Treatment outcomes were evaluated in 124 patients with a median follow-up of 26.9 months using descriptive statistics and risk ratio estimates. RESULTS: Fusion transcripts were identified in 145 of 153 tumours (94.8%). Fibrosarcomatous DFSP showed larger size and higher Ki-67 index than conventional DFSP (5.1 vs 4.1 cm, P = 0.023; 23.4% vs 9.1%, P < 0.001). Among 122 patients followed after MMS, five relapsed (4.1%). Twenty-eight fibrosarcomatous cases received postoperative imatinib for ≥ 6 months and only two recurred (7.1%). Ki-67 ≥ 15% strongly correlated with fibrosarcomatous change. CONCLUSION: COL1A1::PDGFB fusion remains the molecular hallmark of DFSP. A Ki-67 index ≥ 15% indicates aggressive biology and should prompt meticulous surgical planning and consideration of postoperative imatinib in high-risk tumours.