Thank you for your question and welcome to ADHDCentral.com.
There are many instances when doctors prescribe medication for other than their approved use. This is considered "off-label" use. Sometimes, medications are not originally intended to treat a specific disorder or illness, but it is found to be effective when used off-label. For example, Provigil is not approved by the FDA to treat ADHD but has been found to be effective and so may be prescribed off-label. It seems this is what your doctor is doing with the Seroquel.
If you have concerns, talk with your doctor or pharmacist about what side effects you should look for and which side effects would necessitate you contacting your doctor.
Eileen