As much as we women like to pretend, there is always a certain 'time of the month' when we are more emotionally sensitive. According to a study in Cancer Research by Ann F. Chambers and colleagues, emotions aren't the only thing affected by the menstrual cycle — it also affects the metastatic ability of cancers cells.

They injected hormone-independent B16F10 melanoma cells into the tail vein of mice, predicting that most cells would lodge in the lungs and the rest would escape and disseminate. Mice were injected at two different phases of estrous — during proestrus, when oestrogen levels are high, or during metestrus, when progesterone levels are high. After 24 days, as expected, metastatic burden was observed in both groups of mice with no significant difference between the number and size of lung metastases. But, when they examined extrapulmonary metastases they found astonishing differences. After 7 days, 16.7% of mice injected during metestrus had ovarian micrometastases and this rose to 31.6% with prominent ovarian metastases after 24 days. However, metastases were absent in mice injected during proestrus. The incidence of non-ovarian extrapulmonary metastases was not statistically significant between the two groups. So, it seems that the fluctuating hormonal environment at different stages of estrous can have different effects on circulating tumour cells.

As B16F10 cells are hormone-independent, these effects seem to be caused by hormone sensitivity of the host. But why is the timing of tumour cell entry into the bloodstream crucial to the formation of metastases? Blood flow to the ovaries increases during metestrus and decreases during proestrus, indicating differential delivery of the cancer cells to the ovaries. Alternatively, differential support of growth in ovaries at different stages of estrous might occur because of hormone-induced gene expression. So, both theories require thorough investigation.

It is clear that this work might have serious implications for patients with breast cancer, as timing of surgery could have a marked effect on metastasis and, ultimately, survival. Previous clinical studies have been mixed and controversial. This model system should help to determine whether changes in clinical practice are necessary.