Aim: The aim of this study is to rule out the difference in positional setup errors when using immobilization device such as thermoplastic mould for free breathing and vacloc with ABC gating in ca breast patient treating with 3DCRT technique. Objective: The study is to compare the patient setup uncertainties using two different immobilization device, one with standard thermoplastic mould and other with vacuum cushion with active breath control gating in our clinical setup. Methodology: fourteen patients with ca breast are taken for the study. seven patients are undergone immobilization using standard thermoplastic moulding and remaining seven patients undergone, vacuum cushion with active breath control DIBH (deep inspiration breath hold) gating. CT was taken and 3dcrt plan were done according to the standard protocol. Prior to the treatment CBCT was taken, matching was done in XVI with the reference images, to rule out any positional shifts. Positional Shifts on all 3 axes such x, y, z is noted with rotational shift. Comparison were done on systematic error (?), random error (?), mean displacement vector (R), mean setup error (M). Result: immobilization device used by vacloc with DIBH ABC gating shows very less shifts in all direction, especially longitudinal direction compared with mould one, which we are using routinely in our department.