Why hemicolectomy done for ceacal mass?

Hemicolectomy is done for colonic neoplasm only when ileocecal valve involvement needs to be included in tumour resection (1 of the five indications of hemicolectomy). Here in your query you mentioned ceacal mass for which hemicolectomy isn't the preferred choice rather ileocecectomy should be planned as ileocecal valve is almost in most cases not involved if not then right colectomy would be done (as here we know the involvement is only upto ceacum or uptill hepatic flexure of colon ) depending upon tumour spread , site , and degree of lymphatic spread.

Hope, your doubt has been clear. For reference purpose - you must refer Bailey and Love , Sabiston TextBook of Surgery or Schwartz .