CT in Radiation Therapy
The modern practice of radiation therapy relies on volumetric images. For the purposes of dosimetric treatment planning, CT is the primary imaging modality since it provides information about target volumes and other normal critical organs. The conventional CT used in radiology equipped with radiation therapy applications modules is referred to as CT- Simulator. Like conventional CT, CT-Simulator provides density information for electron density corrected dose calculations. Below, we discuss the technical components and the needs of the present state-of-the-art radiation therapy CT-Simulator.1
Components of modern CT- Simulator
A: Image Acquisition
(i) Large number of images per study with higher resolution—CT must have large heat anode loading and heat dissipation (i.e. effective cooling mechanism)
(ii) Rapid study acquisition. Multislice CT such as four-slice acquires volume data up to 8 times faster than single slice- useful for IMRT and IGRT treatments
(iii) Multi-slice CT allows thinner slice thickness to be used for scanning hence smaller tumor location- adequate for the diseased volume dose coverage
(iv) Multi slice allows longer volumes to be scanned
B: Treatment volume localization
(i) Three orthogonal in-built laser system
(ii) Stable room (external) lasers and patient marking system for patient positioning with software for the laser coordinates transfer into the treatment planning system. This increases patient setup reproducibility during treatment parameters verifications and treatment delivery
C: Patient positioning and immobilization
(i) Flat tabletop to represent treatment geometry. This increases the patient positioning accuracy
(ii) Flat tabletop should accommodate larger immobilization devices
(iii) CT couch should have a sag less than 2mm
(iv) CT couch weight limit preferably be about 650 lbs to support heavier patients
(v) Bore opening. Depending on the immobilization device used, the patient positions are often in positions that can prevent them from entering the standard bore sizes. E.g. breast patients with arms subtended at close to a 90 degrees angle. Currently a wide bore size of 85 cm is available
(vi) Scanned Field of View (SFOV). Larger scanned field of view allows for full visualization of larger patients. This is important for assessing external dimensions for accurate dose calculations. The larger the SFOV, the better. 60cm SFOV is better than 50cm SFOV
D: Image quality
(i) Spatial and contrast resolution. This allows for smaller objects such as 2mm with varying densities to be visualized
(ii) Cross-field uniformity—allows for heterogeneity dose calculations to avoid under dosage or over dosage
E: Virtual simulation
(i) Radiation beam design/placement.
(iii) Image fusion between different imaging modalities
(iv) Contouring of critical structures and target volumes
(v) Remote access and connectivity with treatment planning computer
F: 4D Radiation Therapy2
(i) Image acquisition over respiratory time scale
(ii) 4D image rendering
(iii) Suitable for abdomen and thoracic regions having mobile organs
(iii) Gating technology for image synchronization during treatment delivery
References
1. American Association of Physicists in Medicine Task Group 66
2. Image-Guided IMRT. Springer, Ed. Bortfield T, Schmidt-Ullrich, De Neve W and Wazer E D
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