Preventative Chemo

My mother was diagnosed with ovarian cancer in March, 2002.  In April she had surgery, including a complete hysterectomy. The cancer was primarily contained in one large mass, although a few lymph nodes were also affected.  (One node was partially imbedded in the pariaorta and only 3/4 of it could be removed.)  This was followed by a chemo regimen (6 treatments) of carboplatin and taxol.  In October, 2002, she had a second-look surgery.  Cancer cells were detected in 2 of 15 lymph and tissue samples tested (of which one was the remainder of the node examined in April).  This was followed by a course of radiation that went 25 treatments.  Since her 1st round of chemo her CA-125 has been under 30 and is currently 11.  Her gynecologic oncologist would like her to go through another round of chemo - this time he's recommending carboplatin and gemzar.  He said this is a preventative course - he has not detected anything, but there could be microscopic cells.  My 2 questions:  Is this treatment  (carbo/gemzar) generally used with ovarian cancer patients?  Is preventative chemo typical?