MRI-Based Surgical Margin Assessment in Dermatofibrosarcoma Protuberans: Correlation With Pathological Extent.

Dermatofibrosarcoma protuberans (DFSP) is a locally aggressive cutaneous tumor characterized by infiltrative growth and a high risk of local recurrence if incompletely excised. Although Mohs micrographic surgery is recommended, wide local excision (WLE) remains widely used in clinical practice; however, the clinical utility of preoperative imaging for surgical margin assessment has not been fully established. We retrospectively analyzed 42 patients with DFSP, including fibrosarcomatous DFSP (FS-DFSP), who underwent MRI-based surgical planning followed by WLE between 2009 and 2023. Lateral margins were defined based on preoperative MRI, and deep tumor extent was assessed according to the relationship with the fascia. Analyses were performed in 36 patients after excluding cases without identifiable pathological tumor. The median planned lateral margin was 20 mm, whereas the median pathological margin was 16.5 mm. The median difference (pathological - planned) was -5 mm (interquartile range -10.25 to -1 mm; range -18 to +15 mm), with planned margins significantly larger (p < 0.001). A weak positive correlation was observed between planned and pathological margins (ρ = 0.32), but it did not reach statistical significance (p = 0.060). Importantly, the maximum underestimation of tumor extent was 18 mm, indicating that the discrepancy between imaging-based planning and pathological findings did not exceed 20 mm in any case. Lesions classified as fascia-contacting or beyond on MRI showed a higher likelihood of deep invasion (OR 27.0, 95% CI 1.5-483). Residual tumor was identified in 71% of patients undergoing additional resection after unplanned excision. All patients achieved R0 resection without local recurrence, and distant metastases occurred only in FS-DFSP. Preoperative MRI provides clinically useful information for surgical margin assessment in DFSP. A lateral margin of approximately 20 mm appears sufficient to achieve complete resection, and fascia-based evaluation may help stratify the risk of deep invasion.