In addition to when she is becoming tehorah, a married woman is permitted to immerse in a mikveh whenever she is tehorah already. Therefore, she may immerse specifically for spiritual purposes as part of recovery from chemotherapy and radiation, so long as she is not niddah when immersing.
Single women do not generally immerse in the mikveh, even for spiritual purposes. A single woman wishing to immerse in this situation would need to ask a specific halachic question.