The National Council on Radiation Protection and Measurements (NCRP) Report No. 151 is an essential document for bunker design commonly applied for radiotherapy treatment rooms. This document is used as a reference by several countries, including Brazil. The objective of this study is to evaluate the shielding dimensioning methodology recommended by NCRP 151, and compare it with the one adopted by the Brazilian regulatory authority. Radiotherapy rooms and respective doors were designed to use linear accelerators operating at 6, 10, 15, and 18 MeV under two different ways: (a) applying exclusively the methodology recommended by the NCRP 151, and (b) taking into consideration the complementary recommendations from the Brazilian authorities. The results suggest that designers in Brazil can count on at least 4 and 11% safety margin for dimensioning primary barriers in controlled and free areas respectively. Also 8% for secondary barriers in controlled areas, 9.7% for secondary barriers adjacent to the primary belt of free areas, and 6.6% for the lead of the doors.