They can be either @biscandmatt - I believe Treacle's is benign from the sample, but it can become malignant over time. The sample didn't come back cancerous or the vet would insist on surgery. She told me there is around a 30% chance it is or could become malignant.
Treacle is in great health, and every vet I've seen always believes she's younger than she is before they see her records and see that she's 6 - the last one thought she was only about 2, and Katy (the surgeon who took the sample) said something similar
I believe she would stand a good chance of coming through as she's very spritely and in good health, but as she seems to be absolutely fine right now, it seems like too much of a risk. We've decided not to operate at the moment purely for that. She'll be under regular monitoring, with her weight and the size/shape of the lump, and we'll keep her going in for regular check ups and samples to see how she's getting on. She's worth every penny 
Treacle is in great health, and every vet I've seen always believes she's younger than she is before they see her records and see that she's 6 - the last one thought she was only about 2, and Katy (the surgeon who took the sample) said something similar

