NeuroTalk Support Groups - Prominent occipital protuberance

In June of 2009 I was diagnosed with Chiari Malformation with a syrinx. The herniation was at 14mm. I had the decompressive surgery in August. Here is my question:

About 2 months ago, my 19 year old son noticed a lump on the back of his head. If you look at the back of his head, the top of the lump is about even with the top of his ears and positioned about equal distance from each ear. It seems to be growing slowly. It is about the size of a ping pong ball. We went to the doctor and he scheduled a CT scan with and without contrast. We just got the results but won't see the doc until late next week. I was wondering if anyone could tell me what the results mean and if I should be concerned. The results are: CT examination demonstrates a prominent occipital protuberance measuring approximately 12mm x 20mm. There is no overlying soft tissue mass...everything else is normal. I am just not sure if this means that he may also have Chiari, or does this sound like something else? Has anyone heard of this before? I have done some research online and can't seem to figure this out. Any input would be greatly appreciated! Thank you in advance!