Thyroid cancer come back after treatment. Recurrent or persistent disease

Papillary thyroid cancer is the most common type of thyroid cancer. It accounts 85 % of cases of thyroid cancer.
Generally papillary thyroid cancer has good prognosis, 10 year survival is around 93 % of cases. But even for this cancer, recurrent or persistent disease is not uncommon. up to 20% of patients developing recurrent disease at some point during their lifetime
Although, tumor-specific mortality is infrequent.
According to studies most cases after initial treatment, thyroidectomy and or radioiodine therapy, cancer return is the result of persistence disease and very rare cases of true recurrence.
Persistence disease at this case means, it was not treated completely and or effectively. And there was residual tissue after surgery, or radioiodine ablation was not effective enough.
We can defined persistent disease if:
Positive Tg level, an abnormal ultrasound, or persisting elevated Tg antibodies
True recurrent disease can be defined, when thyroglobulin was undetectable. Also no elevated antibodies to thyroglobulin.
And on ultrasound no abnormal tissue in thyroid bed, no abnormal or suspicious lymph nodes in the central or lateral neck, and no any growth of mass or lymph nodes.
Over half of the patients required re-operation within the first two years. It means many cases of persistence disease is caused by incomplete initial treatment.
It strongly suggests, that improvements in the pre-operative assessment and adequacy of initial surgery need to improve. At this case, many reoperations can be avoided.

Another problem related to thyroid cancer is second cancer.
Some people with thyroid cancer have increased risk or second , different cancer, which is not thyroid cance, specifically:
Breast cancer (in women), Prostate cancer, Kidney cancer, Adrenal cancer. Adrenal cancer risk is especially high in people who had the medullary type of thyroid cancer.
Patients treated with radioactive iodine also have an increased risk of acute lymphocytic leukemia (ALL), stomach cancer, and salivary gland cancer.

By Hellerhoff - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=8337552