Hey 20andinpain, welcome to the forum.
It ***** to have knee pain for so long!
It reminds me of my younger years , when I suffered from knee pain,
because of overuse (over-training to make the soccer team and then not enough recovery in between games).
Consider this reply as a present for the upcoming Holidays! lol!
You do not have any possibility of cancer appearing on your knee.
For cancer diagnosis, you must meet many clinical criteria consistent with cancer symptomology and then only after a referral to an oncologist and a positive biopsy, the diagnosis can be confirmed.
Let me assure you that from what you posted here, at worse you have arthritis, meniscus tear (the C shaped cartilage in front of the knee acting as a shock absorber) , build up of joint fluid b/c of Baker's Cyst ( that could have dissipated on it's own, judging from what the physiotherapist said)
or something similar.
For arthritis-related issues, a Rheumatologist would be the type of doctor to consult and for specific knee issues, it would be an Orthopaedic doctor
or Orthopaedic surgeon ( optimally a knee specialist)
So if pain persists, I would seek a second opinion from a Rheumatologist
or Orthopaedic Doctor/ Knee specialist.
If you provide a more detailed history up to the onset of your knee problems, perhaps I will be able to offer you more relative advice.
Take care and Happy Holidays!