Watchful waiting of a 1.8cm ovarian dermoid via ultrasound. ct scan showed fat. if it hasn't grown at all in 1.5 years, is it likely to be cancerous?

No, it: is not likely to be cancerous, if CT identifies the characteristic densities(esp. fat) it is 98-99% diagnostic for benign dermoid. For small lesions like yours, many advocate annual monitoring for stability. If there is a significant size increase, it becomes more prone to rupture or ovarian torsion, so surgical excision is usually recommended for larger lesions.