Intensity modulated radiation therapy (IMRT) is a highly accurate technique that is usually implemented in either dynamic or step-and-shoot fashion with many segments each having low monitor units (MUs). The present s...Intensity modulated radiation therapy (IMRT) is a highly accurate technique that is usually implemented in either dynamic or step-and-shoot fashion with many segments each having low monitor units (MUs). The present study evaluated the effects of beam startup characteristics on the dose delivery accuracy for each segment at low MUs for step-and-shoot IMRT with an Elekta Precise accelerator at the highest dose rates. We used a two-dimensional semi-conductor detector for the dose measurements. The field size of each segment was assumed to be 20 ×20 cm2 and each segment was set to deliver 1 - 10 MUs. Our results show a variation in dose delivery accuracy between segments for the same IMRT beam, which can be attributed to the beam startup characteristics. This variability is attributed to the changes in the transient changes in the temperatures of the electron gun filament and the magnetron. That is, the transient increase in the temperature of the filament leads to increasing doses with time and that of the magnetron leads to decreasing doses with time during the first few MUs.展开更多
Cone-beam CT (CBCT) images acquired during radiation treatment can be used to recalculate the dose distribution as well as to confirm the treatment location. However, it is difficult to obtain the electron densities (...Cone-beam CT (CBCT) images acquired during radiation treatment can be used to recalculate the dose distribution as well as to confirm the treatment location. However, it is difficult to obtain the electron densities (EDs) necessary for dose calculation from CBCT images because of the effects of scatter contamination during CBCT image acquisition. This paper presents a mathematical method for converting the pixel values of CBCT images (CBCT values) into Hounsfield units (HUs) of radiation treatment simulation CT (simCT) images for use in radiation treatment planning. CBCT values are converted into HUs by matching the histograms of the CBCT values with the histograms of the HUs for each slice via linear scaling of the CBCT values. For prostate cancer and head-and-neck cancer patients, the EDs obtained from converted CBCT values (mCBCT values) show good agreement with the EDs obtained from HUs, within approximately 3.0%, and the dose calculated on the basis of CBCT images shows good agreement with the dose calculated on the basis of the simCT images, within approximately 2.0%. Because the CBCT values are converted for each slice, this conversion method can account for variation in the CBCT values associated with differences in body size, body shape, and inner tissue structures, as well as in longitudinally displaced positions from the isocenter, unlike conventional methods that use electron density phantoms. This method improves on conventional CBCT-ED conversion and shows considerable potential for improving the accuracy of radiation treatment planning using CBCT images.展开更多
The reproducibility of patient setup is an important issue for head and neck cancers treated with intensity-modulated radiation therapy (IMRT). In this study, an image-guided radiation therapy (IGRT) system has been u...The reproducibility of patient setup is an important issue for head and neck cancers treated with intensity-modulated radiation therapy (IMRT). In this study, an image-guided radiation therapy (IGRT) system has been used to minimize the uncertainty of patient setup while standard thermoplastic masks were used to provide adequate immobilization for the head and neck. However, they do not provide sufficient immobilization of the shoulders, which is an important requirement in comprehensive nodal irradiation. Therefore, we investigated the setup and rotational shifts in head and neck cancer patients undergoing IMRT for which this immobilization device had been used together with an IGRT system. The setup and rotational shifts of patients were analyzed using the ExacTrac X-ray 6D IGRT system. The patients were classified as having head and neck tumors in the upper or lower regions. The upper neck nodes included lymph nodal level II while the lower neck nodes included lymph nodal levels III and IV. Clinical data from 227 treatment sessions of 12 head and neck cancer patients were analyzed. The random translational error in inter-and intra-fraction errors of the anterio-posterior (AP) direction might influence the rotational errors of pitch and roll in the upper region. At the same time, the random translational error in the inter-and intra-fraction errors of the AP direction might influence the rotational error of roll in the lower region. We believe that these random translational errors should be considered during treatment. We found variability in random translational errors for different regions in the anatomy of head and neck cancer patients due to rotational shifts. Depending on the location of the primary lesion or the selected nodal treatment targets, these relative positional variations should be considered when setup and rotational shifts are corrected with IGRT systems before treatment.展开更多
文摘Intensity modulated radiation therapy (IMRT) is a highly accurate technique that is usually implemented in either dynamic or step-and-shoot fashion with many segments each having low monitor units (MUs). The present study evaluated the effects of beam startup characteristics on the dose delivery accuracy for each segment at low MUs for step-and-shoot IMRT with an Elekta Precise accelerator at the highest dose rates. We used a two-dimensional semi-conductor detector for the dose measurements. The field size of each segment was assumed to be 20 ×20 cm2 and each segment was set to deliver 1 - 10 MUs. Our results show a variation in dose delivery accuracy between segments for the same IMRT beam, which can be attributed to the beam startup characteristics. This variability is attributed to the changes in the transient changes in the temperatures of the electron gun filament and the magnetron. That is, the transient increase in the temperature of the filament leads to increasing doses with time and that of the magnetron leads to decreasing doses with time during the first few MUs.
文摘Cone-beam CT (CBCT) images acquired during radiation treatment can be used to recalculate the dose distribution as well as to confirm the treatment location. However, it is difficult to obtain the electron densities (EDs) necessary for dose calculation from CBCT images because of the effects of scatter contamination during CBCT image acquisition. This paper presents a mathematical method for converting the pixel values of CBCT images (CBCT values) into Hounsfield units (HUs) of radiation treatment simulation CT (simCT) images for use in radiation treatment planning. CBCT values are converted into HUs by matching the histograms of the CBCT values with the histograms of the HUs for each slice via linear scaling of the CBCT values. For prostate cancer and head-and-neck cancer patients, the EDs obtained from converted CBCT values (mCBCT values) show good agreement with the EDs obtained from HUs, within approximately 3.0%, and the dose calculated on the basis of CBCT images shows good agreement with the dose calculated on the basis of the simCT images, within approximately 2.0%. Because the CBCT values are converted for each slice, this conversion method can account for variation in the CBCT values associated with differences in body size, body shape, and inner tissue structures, as well as in longitudinally displaced positions from the isocenter, unlike conventional methods that use electron density phantoms. This method improves on conventional CBCT-ED conversion and shows considerable potential for improving the accuracy of radiation treatment planning using CBCT images.
文摘The reproducibility of patient setup is an important issue for head and neck cancers treated with intensity-modulated radiation therapy (IMRT). In this study, an image-guided radiation therapy (IGRT) system has been used to minimize the uncertainty of patient setup while standard thermoplastic masks were used to provide adequate immobilization for the head and neck. However, they do not provide sufficient immobilization of the shoulders, which is an important requirement in comprehensive nodal irradiation. Therefore, we investigated the setup and rotational shifts in head and neck cancer patients undergoing IMRT for which this immobilization device had been used together with an IGRT system. The setup and rotational shifts of patients were analyzed using the ExacTrac X-ray 6D IGRT system. The patients were classified as having head and neck tumors in the upper or lower regions. The upper neck nodes included lymph nodal level II while the lower neck nodes included lymph nodal levels III and IV. Clinical data from 227 treatment sessions of 12 head and neck cancer patients were analyzed. The random translational error in inter-and intra-fraction errors of the anterio-posterior (AP) direction might influence the rotational errors of pitch and roll in the upper region. At the same time, the random translational error in the inter-and intra-fraction errors of the AP direction might influence the rotational error of roll in the lower region. We believe that these random translational errors should be considered during treatment. We found variability in random translational errors for different regions in the anatomy of head and neck cancer patients due to rotational shifts. Depending on the location of the primary lesion or the selected nodal treatment targets, these relative positional variations should be considered when setup and rotational shifts are corrected with IGRT systems before treatment.
