- At least 4 weeks since prior radiotherapy or chemotherapy (6 weeks for nitrosoureas), unless there is unequivocal evidence of tumor progression.. However, patients treated with chemotherapeutic agents such as etoposide who would normally be retreated after shorter intervals (eg, 21 days on, 7 days off schedule) may be treated at the usual starting time even if less than 4 weeks from the last prior dose of chemotherapy.