Sadly there is no way to prevent a recurrence. Nutrition, surgeon, hospital, overall health, age, lifestyle habits, treatment and initial dx all play a role.

What I can tell you is that cancer can spread anywhere once it hits the nodes. The location of the primary tumor can lead to different locations. For distant metastasis most oral cancer likes to make the hike to the lungs as a next stop - however - I've known people with it in their hips, vertebrae, colon, ribs, and yes... glands etc.. (though that is more local that distant) Its odd - on a local level - tongue cancer tends to move around the tongue, nodes base of tongue floor of mouth salivary glands. Oral mucosa cancer - like cheek - seems to hop around the gums, cheek and into the jaw area so yes the parotid is a possibility simply based on its location in conjunction to the original tumor. The standard treatment for your dad's cancer was removal of the nodes and primary tumor which they did with clear margins... but because cancer moves on a cellular level (microscopic) there is always the potential for it to still exist and NOT be picked up on a scan until it is a fair size.

Radiation and chemo would normally take care of residual cells but that depends on where the radiation was aimed (did they include the parotid in the radiation field? Spare it all together - or simply paint it with a low dose) and if they spared it then can they do rads to the area again after the parotid is removed? If they did hit it with rads can they give him more?

Remember more radiation will cause more issues. Based on the location of the parotid, he could end up with trismus (difficulty opening his mouth) ear troubles, skull/jaw bone issues as well.

If you trust your dr. and you are at a CCC - then do what the dr. says. If you are not at a CCC I would ask for a second opinion, though this treatment you described seems very much on par with what should be done.

At this point I would call it salvage surgery. I would hope they plan to maybe follow it up with something else though since it seems persistent.

To help him I would recommend proper nutrition including high protein and lots of fluids. hugs. and welcome and sorry about the recurrence. Unfortunately is a little like playing craps - very unpredictable.