Much of the improvement in breast cancer survival in recent years is because the average tumor is smaller, not just because treatments are so much better, a new study has found.

The study didn’t look at why tumors are smaller on average. Doctors often cite an emphasis on regular mammograms for an increase in early detection of breast cancers, which can lead to earlier treatment when the tumors are smaller.

Examining 25 years of cancer records nationwide, researchers found that smaller tumor size accounted for 61 percent of the improvement in survival when cancer had not spread beyond the breast, and 28 percent when it had spread just a little.

For women 65 and older with early-stage tumors – the most common scenario – the shift in size accounted for virtually all of the improvement in survival.

“We don’t in any way want to diminish the benefits we’ve seen from advances in treatment because they’ve been enormous,” said lead researcher Elena Elkin. “But not all of the improvement in survival is due to treatment when important characteristics like size have also changed over time.”

The study is being published today online by the society’s journal Cancer and will be in its Sept. 15 print edition.

It was conducted by doctors at Memorial Sloan-Kettering Cancer Center in New York and used a federal government’s database that includes nine cancer registries covering 10 percent of the U.S. population. More than 265,000 breast tumors were analyzed.