Can a benign goiter be converted to a malignant tumor?

Not usually: Usually a thyroid goiter (enlargement) does not convert to malignancy or cancer. If there is a nodule found by ultrasound imaging or by physical exam, depending on the characteristics it should be evaluated. There is no 100% way of knowing what will turn in to cancer.

It can - but low: Although it is assumed that multinodular goiter is unlikely to contain cancer, a few study has suggested that the risk of cancer was similar in a patient with a single or multiple nodules, but the risk of malignancy was higher in a solitary nodule than in a non-solitary nodule. Thus, a dominant nodule in a multinodular goiter should be evaluated as if it were a single nodule.- the risk is about 5%.

Goiter: Goiter is enlargement of the thyroid due to inflammation usually. Nodules or growths within a thyroid gland can be inflammatory but over time turn cancerous. Close monitoring with ultrasound is strongly recommended.