We don't have synonyms for mass effect yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are mars effect, mass defect and haas effect.
(n) :
(medicine) The anatomic and physiological effect that an ectopic mass (such as a tumor, hematoma, or pleural effusion) causes by displacing or pushing against eutopic structures. It may squeeze and thus occlude blood vessels or lymphatic vessels; pinch nerves; disrupt gastrointestinal volume and peristalsis; and so on.
(n) :
(medicine) The anatomic and physiological effect that an ectopic mass (such as a tumor, hematoma, or pleural effusion) causes by displacing or pushing against eutopic structures. It may squeeze and thus occlude blood vessels or lymphatic vessels; pinch nerves; disrupt gastrointestinal volume and peristalsis; and so on.