Room: Exhibit Hall | Forum 7
Purpose: Bulk density method (BDM) can solve the problem of lack of electron density information in Magnetic resonance imaging (MRI), and it is one of the common methods of MRI-only planning. In this study, we compared the dosimetric differences among pseudo-CTs (pCTs) generated by different BDMs.
Methods: Thirty-five patients with their CT simulated localization image and MRI simulated localization image were used for this study. All patients were treated with radiotherapy for brain metastases. The average Hounsfield Unit (HU) values of each tissue and organ were analyzed on RayStation 7.0 workstation, which provided the basis for BDM. Four groups of pCT were generated from MRI applying different BDMs, including all tissue set 140HU (pCT1), air cavity set -700HU and other tissue set 160HU (pCT2), air cavity set -700HU and bone set -700HU and other tissue set 0HU (pCT3), different tissue set individual HU values (pCT4). Dose distribution was calculated on four groups of pCT and compared with that of the original plan.
Results: The average HU values of head, bone, air cavity, brainstem, corpus callosum, whole brain tissue, lens, eyeball, optic nerve, parotid gland and skin were 140.2Â±28.4, 731.3Â±77.6, -689.8Â±28.7, 28.5Â±2.3, 12.3Â±1.6, 33.7Â±3.0, 62.0Â±5.5, 23.4Â±3.8, 9.4Â±6.8, -13.3Â±16.8, -47.6Â±19.2, respectively. There was significant difference among these tissues (P<0.05). After recalculating the doses of four groups of pCT, it was found that the difference ranges of the main dose parameters compared with the original plan were 0~3.7% (median 2.1%), 0~2.9% (median 1.4%), 0~2.1% (median 0.7%) and 0~2.2% (median 0.5%), respectively. There was significant difference among the four groups (P<0.05).
Conclusion: The distribution of electron density in brain tissue is relatively simple and uniform, and there is little difference between the pCT4 generated by BDM and the original CT for dose calculation. This pCT4 can basically meet the clinical requirements.