Hi Bills, it is good to know that your dad is doing well.
I have never heard of M-cells. Do you mean cells in mucosa layer (epithelial layer) of the bladder?
I recall that you dad was diagnosed with a single TaHG, with 20% high grade cells rate, with clinical size of > 3cm.
It is difficult for a urologist cannot detect tumor which is less than 2 mm. The size of epithelial cell is bout 20 micron or 0.02mm.
A cytopathologist uses x200 - x 400 times microscope, so they can see individual cells and counts how many cells look abnormal.
Cytology is non-invasive and accurate enough in detecting high grade urothelial carcinoma ( that is what your dad was diagnosed initially). We can consider one of main roles of cytology is to detect recurrence during the treatment for high grade bladder cancer.
It is pretty accurate for detecting high grade bladder cancer. I know a well known urologist always do cytology after cystoscopy for high grade patients. The urologist said if it is negative, then it is almost sure that no cancer. If cytology is positive even the cystoscopy sees negative, then the urologist has to look for the reason further. In this sense, your urologist recommendation for cytology is inline with the practice by other urologists. Below are some information about accuracy of cytology. best
Cytology as well as other urine analysis, eg. FISH or CxBladder are not good at detecting low grade. But they are
good at detecting high grade and CIS. In 2013, international cytopathologists/ urologists came up with Paris Reporting System (PRS)
to define the role of cytology and to improve the accuracy for reporting. Below are the categories of the report. Cytologists consider the goal of cytology is to detect high grade urothelial carcinoma because that can be life threatening. The problem in the past was that there were many cases of Atypical which means not sure. According to a single institution study of more than 27,000 cases, Atypical cases reduced from 29% pre PRS to just 6.2% post PRS. After PRS, accuracy of detecting high grade urothelial carcinoma is 90% ( Sensitivity 83%, Selectivity 91%).
For high grade urothelial carcinoma, a cell is considered as abnormal if the size of nucleus occupies 70% of the cell, or the shape / inside of the cell look bad. If numbers of abnormal cells are greater than 5, it is considered as high grade.
The Paris System for reporting Urinary Cytology - YouTube
training.seer.cancer.gov/bladder/anatomy/layers.html 
