Abstract： Objective To evaluate the difference in dose distribution calculated using X-ray voxel Monte Carlo(XVMC)algorithm and ArcherQA Monte Carlo(MC)algorithm in preoperative radiotherapy for rectal cancer,thus providing reference for the planning and evaluation of XVMC algorithm in clinical practice.Methods For 10 patients with rectal cancer undergoing preoperative radiotherapy,the XVMC algorithm in MONACO planning system was used to develop the plan.The CT,tissue structure,and field information from the prepared plan were transmitted to Monte Carlo software,and the MC algorithm was used for the secondary dose calculation.The dose differences between XVMC algorithm and MC algorithm in the target area and organs-at-risk dose calculations were analyzed.Results The differences in Dmin and Dmean of the planning target volume(PTV),V40 of the small intestine,V45 and Dmean of the bladder,and Dmeanof the femoral head calculated by XVMC and MC algorithms for isocenter dose were all within 2%.The differences in the Dmax of PTV,spinal cord expansion,and small intestine between two algorithms were 3.43%,3.59%,and 3.62%,respectively;and the difference in the V5040 of PTV was 2.87%.Conclusion XVMC algorithm underestimates the planned maximum dose and overestimates the prescription dose coverage in the target area as compared with MC algorithm,but the dosimetric differences between the two algorithms are within a clinically acceptable range(within 5%).When evaluating the plan,attention should be paid to the dose calculation errors caused by the algorithm.