Purpose To investigate the effect on surface dose, as a function of different field sizes and distances from the solid water phantom to transmission detector (D sd ), of using the monolithic silicon detector MP512T in transmission mode. Methods The influence of operating the MP512T in transmission mode on the surface dose of a phantom for SSD 100cm was evaluated by using a Markus IC. The MP512T was fixed to an adjustable stand holder and was positioned at different D sd , ranging from 0.3 to 24 cm. For each D sd , measurements were carried out for irradiation field sizes of 5 × 5cm 2 , 8 × 8 cm 2 and 10 × 10 cm 2 . Measurements were obtained under two different operational setups, (i) with the MP512T face-up and (ii) with the MP512T face-down. In addition, the transmission factors for the MP512T and the printed circuit board were only evaluated using a Farmer IC. Results For all D sd and all field sizes, the MP512T led to the surface dose increasing by less than 25% when in the beam. For D sd > 18 cm the surface dose increase is less than 5%, and negligible for field size 5 × 5 cm 2 . The difference in the surface dose perturbation for the MP512T operating face up or operating face down is negligible ( < 2%) for all field sizes. The transmission factor of the MP512T ranged from 1.020 to 0.9950 for all measured D sd and field sizes. Conclusion The study demonstrated that positioning the MP512T in air between the Linac head and the phantom produced negligible perturbation of the surface dose for D sd > 18 cm, and was completely transparent for 6 MV photon beams.