Good morning kmlnj!
My personal experience with radiation for thyroid cancer was radioactive iodine therapy.
This treatment is used for local thyroid cancers with clear margins. It is considered brachytherapy (internal) radiation and is a pill I swallowed and is designed to be picked up by any residual thyroid cells that may be left in my body and destroy them.
What you describe is external radiation therapy and is treatment of choice for lesions like you described "small cancer was left on my trachea".
I worked for years in radiation oncology as a therapist and dosimetrist and can tell you, 6-8 weeks is common for external radiation. Typically, they will utilize Intensity Modulated Radiation Therapy (IMRT) to pinpoint the previous tumor bed, local/regional lymph nodes and any area that may have remaining cancer cells. The beams can be either fixed (machine stays in one spot during radiation delivery, then moved to the next angle) or the linac can rotate around the patient during the radiation delivery. The Radiation Oncologist, Medical Dosimetrist and Radiation Physicist create a treatment plan utilizing your simulation CT, MRI and possibly PETCT to view and pinpoint the area they want to treat and the location in comparison to nearby critical structures (the area they want to limit dose to). They then use a special computer to design the radiation beams centered on the target and avoiding/blocking the normal structures. IMRT involves multiple mini beams with lead leaves continuously moving and changing shape as needed to limit dose to normal tissues.
All tissues receive some radiation, but the planning allows the radiation team to monitor and track dose to normal tissues to avoid overdosing while delivering an adequate dose of radiation to the target (cancer bed). Unfortunately, sometimes the normal tissues do react to the radiation either as a sore throat, difficulty swallowing or as you described a burn. Typically, these symptoms are temporary and will subside a few weeks after you complete radiation. In the meantime, your radiation oncologist/nurse should be able to prescribe for you medications/treatments that can help relieve the discomfort you are feeling.
As Colleen mentioned, the length of radiation treatment does vary patient to patient and I would certainly ask the Radiation Oncologist "why" he/she changed the treatment course.
Hope this helps....hang in there. I understand the treatment can be difficult, but there are people out there who can help you. Best regards, Gwen Lickteig