A cancer diagnosis is never good news, but for two of the most common cancers, it should now be a little less terrifying with the announcement that chemotherapy is unnecessary in many cases of breast cancer and certain lung cancers.

Hundreds of thousands of women each year have surgery for breast cancer that hasn't spread to the lymph nodes. As welcome as the lack of spread is, it leaves patients and their doctors with a conundrum: Should they undergo chemotherapy, with the often horrendous side-effects that involves, or is surgery and hormone therapy sufficient? Most of those confronted with this dilemma have been largely flying blind, leading many to take the chemo just in case.

However, new work published in the New England Journal of Medicine and announced at the American Society of Clinical Oncology meeting shows that for some of the most common breast cancer scenarios, doing without chemotherapy doesn't shorten life expectancy.

Most breast cancers have receptors for the hormones estrogen or progesterone. Cancer growth can be disrupted by lowering the levels of these hormones in the body or by blocking the receptors. Nevertheless, hormone-receptor-positive cancers often come back years after surgery, which has encouraged the use of chemotherapy.

A group of 10,273 women with hormone-receptor-positive breast cancers were tested for 21 genes known to influence cancer return rates. Based on their versions of these genes, the women were given scores assessing their recurrence risk from 0 to 100. The 6,711 whose scores fell between 11 and 25 were randomly assigned either to get chemotherapy and hormone suppression after their surgery or hormone treatment alone.

Nine years later, the two groups had similar survival rates. Fewer than 8 percent of either group had breast cancer reoccur. For women aged under 50 with scores of 16-25, there was some benefit in getting chemotherapy, but for those aged over 50 with scores under 25, or those aged under 50 and scores below 16, there was nothing to be gained from going through the draining process.

Tens of thousands of women each year in North America alone fall into the categories where chemotherapy is unnecessary, accounting for 70 percent of those with the most common forms of breast cancer. Not only will they be able to dodge the side-effects of chemotherapy but they'll also feel more confident in their decision. The 21-gene test is already in use, but its applicability for those with mid-range scores has been unclear.

The same conference heard that the drug pembrolizumab, already approved for lung cancer, resulted in longer survival times and milder side-effects for non-small cell lung cancer patients than traditional chemotherapy treatment. However, even with this improved performance, median survival times remain less than two years. Pembrolizumab won fame for saving former President Jimmy Carter from what was expected to be terminal cancer.