I have not come across anything of that kind.
But I will give you my thoughts.
If it is a precious pregnancy
then you and the doctor have to take a risk and go ahead.
First trimester is better if one thinks in terms of haemodynamic compromise. But birth defects are common if exposed in the first trimester.
If the thymic mass is benign looking and your symptoms are tolerable, carry the pregnancy and get operated after your labour.
But I telll you there are many safe medications available for the foetus.