Background:The aim of this study is to design and fabricate a thorax phantom with irregularly shaped trapezoidal slots across the left side of the chest wall, allowing for the creation of unwanted air gaps under the bolus. Method: Surface dose (D surf ) measurements were made with Gaf Chromic EBT3 films at air gaps (0.0, 5.0, 10.0 and 15.0 mm) under gel bolus of thickness (5.0 mm & 10.0 mm), for 3DCRT technique (2 and 3 field) with clinical 6 MV photon beam under uniform and non-uniform air gap condition. The obtained values were compared with TPS estimated ones. Results: In the presence of 15.0 mm uniform air gap, the mean estimated and measured D surf values with two and three field techniques decreased by 14.0 % to 15.2% and 14.7% to 17.4% under 5.0 mm and 10.0 mm bolus applications respectively. In presence of non-uniform air gap condition, the effect on D surf was minimal (3 to 3.5%) compared with the uniform air gap condition. Conclusions: Based on the study's findings, it is recommended that when using bolus in clinical radiotherapy applications, special care be taken to avoid unwanted air gaps under the bolus in order to achieve a uniform surface dose across the treatment region, where a customized 3D printed bolus may be a better option.