A region-of-interest (ROI) placed on the parotid abnormality in this image produced a Hounsfield unit (HU) reading of -48. This abnormality is most likely a:

A Hounsfield unit (HU) measurement of -48 indicates a region that is less dense than water, which has a HU value of 0. This negative value is characteristic of fatty tissues, which usually range from about -50 to -100 HU, depending on the exact composition and structure of the fat.

In this context, a fatty tumor is the most probable explanation for a reading of -48 HUs, as they tend to present with lower density due to their high lipid content. Such tumors include lipomas or other adipose-rich growths, which display this specific attenuation on CT imaging. Understanding Hounsfield unit measurements is crucial in differentiating various types of tissues or abnormalities within the body, and in this case, the negative HU supports the conclusion that the abnormality is indeed a fatty tumor.