Diagnosed plasmacytoma, second opinion is "most probably" osteosarcoma

Hello,

I was diagnosed with "well defined plasmacytoma" a week a go. That was the biopsy's result in a private clinic here in Portugal. The doctors told me they suspected of osteosarcoma, but were "happy" to tell me that that wasn't the case (I did x-ray, pelvic, abdominal and toraxic CAT scans, bone scan, blood tests and biopsy). Yesterday I went to a specialist in plasmacytoma in the Portuguese Cancer Institute (IPO) and he asked the clinic for the samples of my biopsy to analize them in the IPO. He said that, although not impossible, it's almost certain that in my case it's osteosarcoma in my right iliac bone. All tests until now showed no other tumors.

He said that a patient with plasmacytoma usually presents with symptoms like weight loss, severe pain, altered blood tests, a rapid tumor growth and it usually manifests in older people (+65 and I'm 37). Since I don't have any of these symptoms and an x-ray from 5 months ago shows that the lesion didn't grow, he said that it's probably osteosarcoma, but he will perform another biopsy (if the samples from the clinic are damaged or in small quantity).

Does anyone here know if this makes any sense? Is it that common to be wrong in a biopsy when you ARE looking for osteosarcoma (that was too the prognostic of the doctors in the clinic) and diagnose plasmacytoma?

Thank you all,
Alexandre