As per your medical history, you had suffered from fibromatosis on the left side of the abdomen and you had undergone three surgeries for the same and a course of radiotherapy. Incidentally, you have a maldescended or undescended testis on the left side. (The testis normally develops in the abdomen during the intrauterine period and gradually descends down over time to be present in the scrotum at the time of birth or just after birth).
Undescended testis has nothing to do with fibromatosis - it is a completely different condition.
Undescended or maldescended testis can give rise to a number of complications including torsion and most importantly a tumour of the testis known as Seminoma. Though seminoma can develop in completely descended testis also, it is much more common in undescended testis. Probably the doctor you had talked to in the past meant that only. There is no decrease in fertility with one testis but definitely you should get your undescended testis removed and subject it to pathological examination after removal to know whether you are harbouring any focus of seminoma in it. Even if there is any, there is no need to panic, as seminoma is a tumour, which is completely curable.