Which of the following is NOT a type of tumor growth?

Metaplastic is the correct answer because it refers to a type of cellular change rather than a tumor growth classification. In the context of pathology, metaplasia describes a reversible transformation of one differentiated cell type to another. This process can occur as an adaptive response to stress or injury, but it does not represent a growth pattern of tumors.

In contrast, benign tumors are characterized by non-invasive growth and generally consider less harmful due to their limited potential for spreading. Malignant tumors, however, are aggressive, invasive, and have the capacity to metastasize, making them more severe in nature. The term "immortal" refers to the ability of certain cells, particularly cancer cells, to divide indefinitely. While this characteristic is relevant to some types of tumors, it is not classified as a type of tumor growth like benign or malignant. Thus, metaplastic accurately stands out as not being a classification of tumor growth.