Very rare - Congenital thoracic custodia

TW: possible cyst on anatomy scan. At 10 weeks + 5 days I had an ultrasound and, while the fetus is otherwise developing well (33mm, normal bpm, etc), a large black spot appeared near what would appear to be the fetus' stomach/rump. The size and location of the spot was such that my Dr. contacted a high-risk pregnancy specialist in the same building with more accurate imaging abilities and the conclusion both my Dr. and the specialist reached was that there is a cyst in the thoracic cavity adjacent to the fetus' heart and lungs. Both noted it was likely serious and to return in two weeks for another ultrasound. My Dr. mentioned tough decisions being a possibility. Google has not revealed much about cysts in this location at this stage and would appreciate any insight. The doctor said it can be called a congenital thoracic custodia.