low grade spindle cell tumor??

my son is 11 years old. in December of 2011 he came to me and showed me a "lump" on his side. it was the size of a golf ball i'd say and i was concerned, so i took him to the ER, from there they rushed him to Riley Children's Hospital in Indianapolis. once there they did an ultrasound and then came back and told us it was a "mass" but they did not know what kind of mass it was and would have to do surgery to remove it and then biopsy it. he had surgery on December 29th 2011. surgery went well they were able to remove it all but had to remove two layers of muscle from my sons abdomen wall because the tumor was in the second layer of muscle. for weeks after the surgery i kept callin to find out the answers to the biopsy. the doctor still had NO clue what it was. she said they had sent it off to 4 different pathologists and NONE of them had ever seen anything like it. finally a final report came back and we were told they still do not know what it is exactly, but gave it a name of "low grade Spindle cell Tumor"

the surgeon explained that they really still don't know much about what it is and want to do studies on it. i looked it up on the internet and have not really gotten answers there either. anyway, we went to the doctor on July 12, 2012 for his 6 month ultrasound and were told he now has a "knot" where the tumor was and that they are not sure if the tumor is back or if it is scar tissue from surgery. i am so confused and cannot find answers at all on the internet about "spindle cell tumors" everything i pull up talks about this cancer in DOGS. can someone please help me? give me some information, something?? we go back in 3 months for another ultrasound to see if the "knot" has grown. i'm just worried and would really like some information!