Growth of Tumors during Chemotherapy

My sister was diagnosed with Stage IIIC Ovarian Cancer in August of 2002. She had a complete hyterectomy and all tumors were removed during the operation. She qualifed to enter clinical trial GOG 182 and is receiving Taxol, Carboplatin and Doxil. After each treatment her CA125 level has dropped about 50 points per treatment (she started at about 260). After her third treatment her doctor ordered a CT scan and it showed two tumors one almost 5 inches. She has just finished her 5th treatment and her reading as of today is 50. What we can't figure out is why tumors are growing and the CA125 is dropping. Could these tumors be non cancerous or other kinds of ovarian cancer? Her doctor is puzzled as well because the original biopsy showed well differianted cells that should respond well to chemo. The current recommendation is for her to complete chemo and then have surgery to remove the two tumors. Any thoughts or information on this puzzling situation would be greatly appreciated by my family.