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Seven of the eight cases of acute leukemia occurred in a series of 553 patients treated with Treosulfan for ovarian cancer in the period from 1970 to 1977 and followed closely for a total of 1159 patient-years up to February 1978.
 ( adverse event: occurred ,  Seven of the eight cases: Subject, eight: Subject.Population, acute leukemia: Effect, Treosulfan: Treatment, Treosulfan: Treatment.Drug, ovarian cancer: Treatment.Disorder) 
Locate the role in the text that participated in the event based on the event type and return it in the event list. 
 Option: Event type: adverse event,potential therapeutic event, Arguments type: Treatment.Disorder,Treatment.Time_elapsed,Treatment.Dosage,Subject.Race,Treatment.Drug,Treatment.Route,Subject.Disorder,Treatment.Freq,Treatment,Effect,Subject.Population,Treatment.Duration,Combination.Drug,Subject,Subject.Age,Subject.Gender. 
Text: {0} 
Answer:
 ( adverse event: occurred ,  Seven of the eight cases: Subject, eight: Subject.Population, acute leukemia: Effect, Treosulfan: Treatment, Treosulfan: Treatment.Drug, ovarian cancer: Treatment.Disorder) 