The word TUMOR means SWELLING in Greek.
A tissue swelling (tumor) can be BENIGN (OK) or MALIGNANT (cancerous). Although the appearance usually suggests the diagnosis, it can never 100% of the time accurately predict it. A history of fast growth, ulceration, irregular borders, bleeding, crusting etc, SUGGESTS (not predicts) that a swelling may be malignant. But the ONLY way to know what "it" is is to get a tissue sample out and have a PATHOLOGIST look at it under a microscope.
The odds are that you have a familial syndrome associated with fatty tumor growth. But, in very rare cases, these may mascarade as such and could be cancerous. The only way to know is to biopsy them EITHER by removal or by getting a liver of tissue out.
MEDICARE allows removal of tumors which block essential orifices (eyes, nose, ears, moth etc), which bleed and which have other aspects of cancers. In your case, I would suggest you consider removing the large ellbow one before it deforms the elbow further and see what it is.