But large chunks of it did become part of the USSR, as the article points out very clearly:
Zosia grew up fatherless in Vilnius, which between the wars belonged to Poland and was called Wilno. On 1 September 1939, she was just about to start her first year of medical school when Germany invaded Poland. Sixteen days later, the Soviet Union joined in, and quickly took Wilno, along with most of Poland’s east. A month later, the Soviets gave the city to Lithuania, which had coveted it since the end of the previous war.
As the sibling comment also points out, though it is mistaken in the implication that these lands were inherited by Russia after the fall of the USSR. In fact they went to Ukraine, Belarus, and Lithuania (first as Republics within the USSR, then as independent countries).
The grandmother received her qualification in Warsaw, which is part of modern day Poland and has never been part of the USSR - so her qualification explains nothing about dentistry in the USSR - as per the comment I was responding to.
Warsaw itself was in ruins. A great portion of the country’s doctors had been killed, and there was a desperate need for medical professionals of every kind. The intensity of this demand led to a certain loosening of standards in training. This relaxation was even more pronounced in the sister discipline of dentistry. Instead of going to years of medical school, all Zosia had to do to become a dentist was endure a short practicum and pass a test. The test was a set essay, on the “role of the mouth in the beauty of the face”.