Donna,
This is a very good question.
Per the NIH:
Cases are attributed to prior therapy based on circumstantial evidence, such as the following: 1) presence of typical clonal cytogenetic abnormalities, 2) significant exposure to leukemogenic therapy (e.g., at least one cycle), and 3) sufficient latency from exposure to diagnosis of t-MDS (e.g., at least six months).
A high rate of MDS is found in patients who have had prior treatement that includes total body irradiation, alkalyting agents, or topoisomerase inhibitors.
Loss of material from chromosomes 5 and/or 7 is detectable in up to 70% of t-MDS/AML patients, often with other abnormalities in a complex karyotype. The same pattern of cytogenetic abnormalities occur in de novo MDS, but at much lower frequency.
The article that I picked this information up from is fairly complex, but does a good job of explaining what the rationale for believient that MDS is therapy related or not.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2832708/
I hope this helps.
Dan