The Henry George Liddell, Robert Scott, A Greek-English Lexicon defines ἐντὸς as only having the meaning “within” or “inside.” The word as a preposition or as an adverb still retains this specific meaning of something inside something. The George Autenrieth, A Homeric Dictionary defines ἐντὸς as meaning “within,” as well.
It's interesting that the same modern translations (NAS, INT, NIV) translate Matthew 23:26 , “Blind Pharisee! First clean the inside of the cup and dish, and then the outside also will be clean.” The translators evidently do recognize that ἐντὸς does in fact mean “inside.” In both Luke 17:21 and Matthew 23:26 ἐντὸς is being used as a preposition.
The real translation (if meaning amongst) would use: η βασιλεία του Θεού είναι ανάμεσά σας and not entos.
Word construction with roots from entos, always mean "within," such as Entosternum Noun. (Greek, Entos = Within + sternon = chest. PL, Entosterna.) Internal Processes from Sternum. Sync furca. See sternum. Entothorax Noun. (Greek, Entos = Within + Thorax = Breastplate. PL Ehthothoraces;) Apodemes or precess extrending inward from sternal sclerites. See apophysis; furca.