Dose comparison of megavoltage cone beam (MVCB) and orthogonal pair portal images

Abstract

Megavoltage cone beam (MVCB) is available for image-guided radiation therapy (IGRT) to improve the accuracy of patient setup and target localization. However, development of strategies to efficiently and effectively implement MVCB to replace the orthogonal portal image technique, the current standard practice, remains challenging in the clinical environment. It is useful to compare the difference in absorbed dose between the MVCB and the orthogonal portal image technique. This study analyzed the doses generated from these two imaging techniques for six treatment sites (pelvis, abdomen, lung, head and neck, breast, prostate). The analysis was made by simulating the MVCB technique with an arc beam and a beam-on-time of 9MUs, and the orthogonal portal technique with an anterior-posterior (AP) and lateral (LAT) pair and a beam-on-time of 6MUs. The results were presented as dose per MU (cGy/MU) and absolute dose (cGy). Doses to the isocenter, maximum dose, mean doses to the target and critical organs, two-dimensional (2D) isodose distributions and dose volume histogram (DVH) of each critical organ were investigated. The absolute dose difference at the isocenter is 2.33±0.41 cGy between MVCB and orthogonal portal vs. Major difference between those two techniques appears in critical organs whose locations were away from the tumor. These organs such as the contralateral breast or lung receive higher dose from MVCB images than that from orthogonal portal images. Additionally, higher and larger dose area of MVCB images had been observed in normal tissues due to 200 beam projections delivering from different angles of MVCB simulation, than the orthogonal portal images which only comes from two perpendicular angles. The high dose area from orthogonal portal images is always located inside the tumor, whereas from MVCB images it is most likely outside the tumor. Therefore, the potential higher dose values from MVCB images should be properly analyzed to ensure that it does not exceed the tolerance dose of the critical organ. However, on the other hand, in order to receive good image quality, the higher MU of MVCB images may be necessary. The absorbed dose to the target and critical organs should be calculated and included in treatment plan evaluation. MVCB image provides a great tool for patient treatment position verification. The trade-off is that it may deliver higher doses to the normal tissues than standard orthogonal portal images.