Good question. In most commercial flights the cabin altitude is held to no less than 8000 feet. That is about two-thirds the atmospheric pressure of sea level. As a pilot, I can tell you there is trouble if you have air pockets under your skin, a common issue with tracheotomies. Aside from that, the pressure within you equals the pressure outside.
Perhaps your surgeon who performed the initial procedure on your neck which caused the lymphedema would likely know for certain. If it from radiation and results in a stiff or limited neck range, then likely there is nothing to be concerned about. Only my opinion and experience as both a patient and a pilot. My lymphedema was mild.