does 5ch2.pdf

Linear Accelerators (LINACs) ❑ In modern LINACs the x-ray beam collimation is achieved with three collimation devices: ❑ Primary collimator. ❑ Secondary adjustable beam defining collimator (independent jaws). ❑ Multileaf collimator (MLC). ❑ The electron beam collimation...

Linear Accelerators (LINACs) ❑ In modern LINACs the x-ray beam collimation is achieved with three collimation devices: ❑ Primary collimator. ❑ Secondary adjustable beam defining collimator (independent jaws). ❑ Multileaf collimator (MLC). ❑ The electron beam collimation is achieved with: ❑ Primary collimator. ❑ Secondary collimator. ❑ Electron applicator (cone). ❑ Multileaf collimator (under development). M.Sc. Medical Physics Dr. Khalid I Hussein  Linear Accelerators (LINACs) LINAC Collimation ❑ To activate the electron mode the x- ray target and flattening filter are removed from the electron pencil beam. ❑ Two techniques for producing clinical electron beams from the pencil electron beam: ❑ Pencil beam scattering with a scattering foil (thin foil of lead). ❑ Pencil beam scanning with two computer controlled magnets. M.Sc. Medical Physics Dr. Khalid I Hussein  Linear Accelerators (LINACS) Dose monitoring system ❑ To protect the patient, the standards for dose monitoring systems in clinical linacs are very stringent. ❑ The standards are defined for: ❑ Type of radiation detector. ❑ Display of monitor units. ❑ Methods for beam termination. ❑ Monitoring the dose rate. ❑ Monitoring the beam flatness. ❑ Monitoring beam energy. M.Sc. Medical Physics Dr. Khalid I Hussein  Linear Accelerators (LINACS) ❑ Transmission ionization chambers, permanently embedded in the linac clinical x-ray and electron beams, are the most common dose monitors in linacs. ❑ Transmission ionization chambers consist of two separately sealed ionization chambers with completely independent biasing power supplies and readout electrometers for increased patient safety. M.Sc. Medical Physics Dr. Khalid I Hussein  Linear Accelerators (LINACS) ❑ Most linac transmission ionization chambers are permanently sealed, so that their response is not affected by ambient air temperature and pressure. ❑ The customary position for the transmission ionization chamber is between the flattening filter (for x-ray beams) or scattering foil (for electron beams) and the secondary collimator. M.Sc. Medical Physics Dr. Khalid I Hussein  Linear Accelerators (LINACS) ❑ The primary transmission ionization chamber measures the monitor units (MUs). ❑ Typically, the sensitivity of the primary chamber electrometer is adjusted in such a way that: ❑ 1 MU corresponds to a dose of 1 cGy delivered in a water phantom at the depth of dose maximum on the central beam axis when for a 10×10 cm2 field at a source-surface distance (SSD) of 100 cm. M.Sc. Medical Physics Dr. Khalid I Hussein  Linear Accelerators (LINACS) ❑ Once the operator preset number of MUs has been reached, the primary ionization chamber circuitry: ❑ Shuts the linac down. ❑ Terminates the dose delivery to the patient. ❑ Before a new irradiation can be initiated: ❑ MU display must be reset to zero. ❑ Irradiation is not possible until a new selection of MUs and beam mode has been made. M.Sc. Medical Physics Dr. Khalid I Hussein