This seems to come up all the time here and I think a lot of people don't understand the purpose and appropriate applications of a second opinion.
What a second opinion is not: It is not a way to shop for doctors. It is not a way to get another doctor's opinion on your overall diagnosis or treatment. It is not going to change test results their interpretation which is not usually done by your doctor anyway (the are interpreted by pathologist, radiologists, and other physicians with specific expertise in those areas).
What a second opinion is: It is a consultation by a second physician with specific expertise in a specific area that your current physician may not have. For instance, here are second opinions I have had since my cancer DX last year. Every one of them was suggested and arranged by my medical oncologist.
1. Because I had previously had radiation in the same area 18 years ago, we got a second opinion from a radiation oncologist as to the risks vs. benefits of irradiating the same area again.
2. My MO consulted with a breast surgeon who in tern consulted with a plastic surgeon at the very beginning of my treatment because I had inflammatory breast cancer and multiple opinions on the order and extent of chemotherapy and surgery were advisable.
3. Shortly after surgery I developed some saromas (fluid pockets) around the scar lines from my surgery. My RO was concerned that radiation treatments with them being in the line of fire so she requested a second opinion (to confirm that they were seromas and not something more serious) from my surgeon. She also had me evaluatied and treated by an occupational therapist who specialized in lymphyatic message because she felt I might be at risk for lymphodema.
4. My MO wanted me to go back to weight bearing exercise after chemo but I was have some serious knee pain so she wanted a second opinion from an orthopedist. Turns out I developed osteoarthritis in one knee during cancer treatments. He gave me a cortisone shot to control the pain and now I can do weight bearing exercise again.
These are just a few examples of second opinions I got during my own journey, there are certainly many other circumstances where a second opinion might be appropriate. Discuss it with your oncologist. He/she will not be offended. But do keep in mind that getting a second opinion just because you wish for a different outcome is self-defeating. It needs to be considered in a practical medical perspective, not just to stir the pot to see if something different turns up.