Hi- I don't know much about porokeratosis except it is an uncommon skin condition that leads to reddish brown scaly spots. The spots are mostly seem on the arms and legs, but sometimes will show up on other sun-damaged skin. It is due to an abnormal sun sensitivity leading to pre-cancerous skin cells. But it is NOT skin cancer.
Most cases are inherited, but some occur in people whose immune systems are not working well.
While some of the lesions can be pre-cancerous, it is uncommon for it to develop into a true skin cancer. BUT sometimes the spots develop worrisome changes, such as redness, crusting or scaling and need to be biopsied, just to make sure that there are no cancerous changes.
Your doctor gave you the correct advice....... the best thing one can do is avoid the sun by wearing long sleeves and using strong sunscreens, etc. It's a chronic skin condition and as you say, there is really no definitive cure per se, but sometimes creams such as Retin-A, Tazorac and Efudex offer some slight help.
Most dermatologists advise an exam for skin cancer once or twice a year.
So don't be scared.... follow your doctors instructions and advice, be vigilent about getting regular skin check-ups, and you'll be fine.
Use your search engine to find out more info on "porokeratosis" and have your doctor explain it to you again if you still have questions.