The most common complication that requires alteration of planned chemotherapy regimens is which of the following?

c. Hematologic suppression

In chemotherapy regimens, it is important not only to deliver adequate doses with each treatment cycle but also to deliver them in a timely fashion. The interval between each treatment cycle is determined by the toxic effects experienced by normal tissues and the amount of time required for resolution of the effects. The scheduling of treatment courses is crucial. If a course follows too closely upon the preceding one, additive toxicities are noted, much to the detriment of the patient, and often the ability to deliver further therapy is at least temporarily compromised. For most agents, the dose-limiting toxicity is myelosuppression, usually leukopenia and/or thrombocytopenia. Nadir blood counts are reached approximately 14 days after the initiation of each cycle and begin to improve 3 to 5 days later, often with complete resolution by day 28. The resiliency of the bone marrow reserve is dependent upon prior chemotherapy and radiotherapy. Agents such as busulfan, mitomycin C, procarbazine, and the nitrosoureas display delayed myelosuppression, often up to four weeks after the initiation of therapy and lasting several weeks. Therapy with these drugs may be delivered only every 6–8 weeks; in addition, therapy with these agents may lead to chronic cumulative myelosuppression and, in some cases, marrow failure.