Radiographic Predictors for Malignant Transformation of Inverted Papilloma

Background Inverted papillomas (IPs) are benign tumors of the sinonasal tract with a propensity for recurrence, local aggression, and potential for conversion to malignancy of 5 to 10%. Biopsy can show what the anterior most aspect of the tumor is, but deeper transformation is often not picked up. The ability to predict which tumors have converted to malignancy preoperatively would allow for more targeted counseling of patients and more specific surgical plans. In this study, we aim to use radiographic findings of MRI and CT to help differentiate IP from IP that has converted to squamous cell carcinoma (IP-SCC).

Methods A retrospective analysis was performed at our institution comparing IP (n = 29) and IP-SCC (n = 27) tumors, evaluating preoperative radiographic imaging with corresponding operative reports. The prevalence of a convoluted cerebriform pattern (CCP) using T1 with contrast and T2-weighted MRI imaging was determined for each tumor. Using MRI diffusion weight imaging (DWI), we calculated the apparent diffusion coefficient (ADC) value of each tumor. We also determined the tumor origin, attachment sites, and presence of bony erosion using CT imaging and operative reports.

Results Benign IPs had a higher prevalence of CCP on MRI (13/14) compared with IP-transformed SCC (11/20), p = 0.024. Of tumor with CCP present, 12 out of 14 IPs showed diffuse CCP compared with only 6 of 20 IP-SCC tumors (p = 0.0019). The mean ADCs of malignant IP-SCC ADCb0,1000 = 1.12 × 10−3 mm²/s are significantly lower than those of benign IPs (ADCb0,1000 = 1.49 × 10−3 mm²/s, p = 0.0016). IP-SCC tumors were more likely to have an attachment to the orbital wall (p = 0.0011) and bony erosion (p = 0.0001) compared with IPs. IP tumors with carcinoma in situ also had a higher prevalence of orbital wall attachment (p = 0.029) and bony erosion (p = 0.004) compared with IPs.

Conclusion The evaluation of CCP and DWI with ADC values on radiographic imaging are promising qualitative and quantitative methods to help differentiate benign IP and IP that has converted to malignancy. The lower ADC values in malignant IP-SCCs correspond to increased cellularity and water diffusion restriction in these tumors. In addition, evaluating specifically for orbital wall attachment and bony erosion may help determine the likelihood that an IP has converted to malignancy.