Which cancer is least likely to cause brain metastases?

Thyroid cancer is indeed the type of cancer least likely to cause brain metastases among the options presented. While most cancers can metastasize to various organs, lung and breast cancers have a well-established propensity for metastasizing to the brain due to their aggressive nature and vascularization, particularly lung cancer, which is a leading cause of brain metastases.

Colorectal cancer can also spread to the brain, but it is far less common than lung or breast cancer metastasizing to this organ. Thyroid cancer, especially when well-differentiated types such as papillary or follicular thyroid cancer, is known for its generally favorable prognosis and lower likelihood of spreading to distant sites compared to the other cancers listed. Even when thyroid cancer does metastasize, it typically spreads to the lungs and bones rather than the brain.

Thus, the characteristics of thyroid cancer lead to it being the least associated with brain metastases, making it the correct choice in this context.