Influence Of CT Contrast Agent On Dose Distributions Of Radiation Treatment Planning

Radiation treatment planning was a plan which was designed according to the position and shape of tumors. The prescribed dose of tumors and the restriction of OARs (Organ at Risk) would be defined by doctors. Proper fields and weights would be designed in the TPS (Treatment Planning System), and the plans were optimized until doctors were satisfied. While reconstructed with the CT numbers related the electron density, CT images were the foundation of the radiation treatment planning, so the dose distributions on the non-enhanced CT images were matched to the actual doses in patients. However, it was more accurate for tumors and lymph nodes delineationin the contrast-enhanced CT than that in the non contrast-enhanced datasets. As a result, it should be meaningful for the radiation therapy to investigate the influence of the contrast agent on the dose distributions.According this study, the differences of the dose distributions due to the contrast agent were analyzed for tumors at various anatomical regions in different types of radiation therapy, such as conventional radiation therapy, three-dimensional conformal radiation therapy and intensity modulated radiation therapy (IMRT). Two schemes were mainly adopted to analyze the influence of CT contrast agent on the dose distributions:1. the plans were separately designed in the enhanced and non-enhanced CT images with the same method. The results were calculated and compared with each other to analyze the difference of the dose distributions.2. The plan doses in the enhanced CT were recalculated in the non-enhanced CT, and then compared with the original plan. The results were analyzed to show the differences between the dose distribution in the enhanced CT and the one in the patient actually.64 cancer patients with pathologically confirmed were enrolled in this study, including 16 cases in brain,13 cases in nasopharyngeal,15 cases in thoracic and 20 cases in abdominal. Two sets of CT were taken in the same position for each patient before and after intravenous contrast agent injection. Target volumes and organs at risk (OAR) were contoured on the enhanced CT, and then copied to the non-enhanced CT after the fusion of the non-enhanced and enhanced CTs. There were two comparison schemes in the plans of conventional radiation therapy and three-dimensional conformal radiation therapy:1.The plans were separately designed in the enhanced and non-enhanced CT images, including the same of the beams, weights and normalized points, etc. The two results were calculated and analyzed through the monitor unit (MU), the doses of random sampling points in the target and normal tissue, D2%, D98%, Dmean, D50%, HI and the dose distributions.2. The plan doses in the enhanced CT were recalculated in the non-enhanced CT, and then compared with the original plan in the same indexes. Similarly, the two schemes in the plans of IMRT were created:1.the IMRT plans were separately designed in the two CT sets with the same optimization for the target volumes and OARs, then the dose distributions were compared in the same indexes.2. The plan doses in the enhanced CT were recalculated in the non-enhanced CT, and then compared with the original plan in the same indexes.The radiation therapy plans were designed according to the two schemes above. Comparing these plans of the various radiation therapy (conventional radiation therapy, three-dimensional conformal radiation therapy and intensity modulated radiation therapy), the influence of the contrast agent was analyzed in plans of tumors at various anatomical regions as follows.Firstly, in the conventional radiation therapy, the influence of the contrast agent was small in the radiation therapy for brain, head and neck cancer. The difference of MU between the plans of the CTs with and without contrast agent was about 0.2%-0.4%, with some differences of doses in the field edge due to the image fusion errors. Comparing with the dose distribution in enhanced CT, the difference was less than 1% for the dose distribution recalculated in the non-enhanced CT. Therefore, the effect of the contrast agent on dose calculations of various plans for brain, head and neck was neglectable. The influence of contrast agent could caused some differences of the dose calculation for thoracic radiation therapy. The maximum difference of MU between the plans of the CTs with and without contrast agent was about 4%. The difference of the dose distribution mainly appearing in the heart and mediastinum was about 2%-10%. Comparing with the dose distribution in enhanced CT, the difference was about 2%-10% for the dose distribution recalculated in the non-enhanced CT, appearing in the target and the fields though the heart and mediastinum. In conclusion, the influence of contrast agent had a large impact for the dose calculation on thoracic radiation therapy, and the non-enhanced CT were recommended in the radiation therapy of thoracic. The influence of the contrast agent was small in the radiation therapy for abdomen cancer. The difference of MU between the plans of the CTs with and without contrast agent was about 0.53%, with some differences of doses due to the motion of the small and large bowel. Comparing with the dose distribution in enhanced CT, the difference was about 2%-6% for the dose distribution recalculated in the non-enhanced CT, but it was less than 1% result from the raise of the CT number. Therefore, it could be acceptable to use enhanced CT for the abdomen plans clinically, and the difference of dose distribution due to the raise of CT numbers was smaller than that caused by the motion of small bowel.Secondly, in the three-dimensional conformal radiation therapy, the influence of the contrast agent was small in the radiation therapy for brain cancer. The difference of MU between the plans of the CTs with and without contrast agent was about 0.16%, with some differences of doses in the field edge due to the image fusion errors. Comparing with the dose distribution in enhanced CT, the difference was less than 1% for the dose distribution recalculated in the non-enhanced CT. Therefore, the effect of the contrast agent on dose calculations of various plans for brain was neglectable. The influence of contrast agent could caused some differences of the dose calculation for thoracic radiation therapy. The maximum difference of MU between the plans of the CTs with and without contrast agent was about 5.5%. The difference of the dose distribution was about 2%-11%, mainly appearing in the heart, mediastinum, and the fields of the larger SSD. Comparing with the dose distribution in enhanced CT, the difference was about 2%-8% for the dose distribution recalculated in the non-enhanced CT, appearing in the target and the fields the fields of the larger SSD. In conclusion, the influence of contrast agent had a large impact for the dose calculation on thoracic radiation therapy, and the non-enhanced CT were recommended in the radiation therapy of thoracic. The influence of the contrast agent was small in the radiation therapy for abdomen cancer. The difference of MU between the plans of the CTs with and without contrast agent was about 0.5%, with some differences of doses due to the motion of the small and large bowel. Comparing with the dose distribution in enhanced CT, the difference was about 2%-7% for the dose distribution recalculated in the non-enhanced CT, but it was less than 1% result from the raise of the CT number. Therefore, it could be acceptable to use enhanced CT for the abdomen plans clinically.Thirdly, in IMRT of the brain, head and neck cancer, the optimization could cause some differences of the dose distribution in enhanced CT and non-enhanced CT, but the difference was less than 1% between the recalculation plan in the non-enhanced CT and the plan in the enhanced CT. So the effect of the contrast agent on dose calculations of various plans for brain, head and neck was neglectable, therefore it could be acceptable to use enhanced CT for the brain, head and neck plans clinically. The influence of contrast agent could caused some differences of the dose calculation for thoracic radiation therapy. Comparing with the dose distribution in enhanced CT, the difference was about 2%-6% for the dose distribution recalculated in the non-enhanced CT. Therefore, the non-enhanced CT were recommended in the radiation therapy of thoracic clinically. The influence of the contrast agent was small in the radiation therapy for abdomen cancer. The difference of the dose distribution was less than 1% between the recalculation plan in the non-enhanced CT and the plan in the enhanced CT. So it could be acceptable to use enhanced CT for the abdomen plans clinically.The influence of CT contrast agent on the dose distributions of radiation treatment planning had many factors affecting the dose distributions, such as the differences of the treatment planning system, the differences of hardwares (CT, medical linear accelerator), the injection rates and concentrations of contrast agent. Although the sizes and distributions of the dose differences were analyzed in this paper, but it was still more one-sided. Therefore, more clinical datas and more detailed studies were needed in the further work.