I had to wait for 3 years all clear before my doc would move me to 6 months due to my 2 recurrences. I got so nervous after 4 months and I called and got a cysto, which was also all clear. Now 4 1/2 years out I feel comfortable about the 6 month check-ups.
Congrats on the 18 months, and oh yes continue with the seeds, one just never knows.
All the best,
Jack