Which of the following is not true about Bronchoalveolar carcinoma -

Ref:Textbook of pathology (Harsh mohan) 6th edition,page no.501 Bronchiolo-alveolar carcinoma is characterised by cuboidal to tall columnar and mucus-secreting epithelial cells growing along the existing alveoli and forming numerous papillary structures . Ultrastructurally, these tumour cells resemble Clara cells or less often type II pneumocytes. it is a type of adenocarcinoma.Thre is preservation of alveolar structure and grows along pre-existing anatomical structures.