Lect 01 Teletherapy Machines 1 PDF

M.Sc. MEDICAL PHYSICS Principles of Physics in Radiation Oncology - RAD 6135 Treatment Machines for External Beam Radiotherapy Part 1 Dr. Khalid Ibrahim Hussein  Introduction  Radiotherapy plays a crucial part in treating lots of different...

M.Sc. MEDICAL PHYSICS Principles of Physics in Radiation Oncology - RAD 6135 Treatment Machines for External Beam Radiotherapy Part 1 Dr. Khalid Ibrahim Hussein  Introduction  Radiotherapy plays a crucial part in treating lots of different cancers.  New technology is making it more precise and reducing side effects.  It works by firing x-rays or other intense bundles of energy at cancer cells, damaging their DNA. If a cancer cell can't repair its damaged DNA, it dies. Patient being treated with modern radiation therapy equipment. M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN  Introduction Role of Medical Physicists Any time radiation used in medicine, the MP Optimization of machine setting to improve is in the background to sure that every thing image quality, making an accurate diagnosis safe and no more than the necessary radiation is used. M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN  Introduction Role of Medical Physicists Design and testing of radiation shielding to In radiotherapy the role physicists is to help keeping staff and patients safe. make sure the patient is getting the right amount radiation in a right place and minimize the dose to the OAR. M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN  Introduction  During the first 50 years of radiation medicine the technological progress was aimed mainly towards: ◦ Development of analog imaging techniques. ◦ Optimization of image quality with concurrent minimization of dose. ◦ Ever increasing energies and beam intensities.  During the past two decades most developments in radiation medicine were related to: ◦ Integration of computers in imaging ◦ Development of digital imaging techniques ◦ Incorporation of computers into therapeutic dose delivery with high energy linear accelerators (linacs). M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN  Introduction Standard machines used for modern radiotherapy:  X-ray machine: ◦ Superficial x-ray machine: 50 - 80 kVp ◦ Orthovoltage x-ray machine: 80 - 350 kVp  Cobalt-60 teletherapy machine  Linear accelerator (linac): ◦ Megavoltage x rays: 6 - 25 MV ◦ Electrons: 6 - 30 MeV M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN  Introduction Specialized machines used for modern radiotherapy:  Microtron: megavoltage x rays and electrons  Betatron: megavoltage x rays and electrons  Neutron machines  Proton machines M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN  Aim of Radiotherapy The aim of radiotherapy is to deliver maximum dose and uniform dose to the tumor volume and minimal dose to the normal tissue or organ at risk. The main requirements of Radiation used in radiotherapy: High particle energy for penetration and skin sparing. High particle flux for sufficient dose rate. Not too expensive. Reliable Simple to operate Safe. M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN  Type of treatment Radical intent: aiming for a cure without producing undesirable long-term consequences for patient. for most common cancers- requires high doses. Adjunct therapy: add-on therapy, and adjuvant care, is therapy that is given in addition to the primary or initial therapy to maximize its effectiveness. Palliative intent: disease cannot be cured - aiming to “control” the disease, reduce unpleasant symptoms, improve the quality of life for patient but not “cure” the cancer - lower doses usually given. M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN  Fractionation: A Basic Radiobiologic Principle Fractionation, or dividing the total dose into small daily fractions over several weeks, takes advantage of differential repair abilities of normal and malignant tissues. Prescription of radiotherapy treatment is the responsibility of the radiation oncologist and usually follows agreed guidelines, taking into consideration individual patient factors, such as the expected risk–benefit ratio of treatment, comorbidities and consideration of scheduling of other treatment modalities. Alternative fractionation schedules using fewer larger fractions in a shorter overall time (hypofractionation) have been developed, especially in the UK and Canada, driven initially by resource constraints, but now supported by extensive published clinical data, e.g. for breast and prostate cancer as well as for palliative treatments.  Class activity (Group 1 presentation)  What is the purpose of fractionation in radiation treatment for cancer patients?  The Five R’s of Radiobiology.  Survival curve and linear quadratic model.  Biological Effective Dose (BED).  Isotope Machines-Radium  The first isotope used for external beam unit was radium  The source contained 4 to 10 grams of Radium.  The cost of Radium was high.  Large source size due to low specific activity of Radium.  Self-attenuating of radiation by lower layers of Radium.  Large source size means larger penumbra. M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN  Cobalt-60 teletherapy machine  The invention of the cobalt-60 teletherapy machine by Harald E. Johns in Canada in the early 1950s provided a tremendous boost in the quest for higher photon energies and placed the cobalt unit at the forefront of radiotherapy for a number of years.  Most modern cobalt therapy machines are arranged on a gantry so that the source may rotate about a horizontal axis referred to as the machine isocentre axis.  The source-axis distance (SAD) is either 80 cm or 100 cm. M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN Cobalt-60 teletherapy machine  For use in external beam radiotherapy, gamma rays are obtained from specially designed and built sources that contain a suitable, artificially produced radionuclide. 59Co(n,𝛾)60Co 60Co 60Ni + 𝛽(0.32 MeV) + 𝛾 (1.17 and 1.33 MeV)  The parent source material undergoes beta minus decay resulting in excited daughter nuclei that attain ground state through emission of gamma rays (gamma decay). M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN  Cobalt-60 teletherapy machine Cobalt-60 isocentric teletherapy machine built in the 1970s and 1980s by Atomic Energy of Canada, Ltd. Source-axis distance = 80 cm M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN  Cobalt-60 teletherapy machine  The important characteristics of radionuclides useful for external beam radiotherapy are: ◦ High gamma ray energy (of the order of 1 MeV). ◦ High specific activity (of the order of 100 Ci/g). ◦ Relatively long half life (of the order of several years). ◦ Large specific air kerma rate constant.  Of over 3000 radionuclides known only 3 meet the required characteristics and essentially only cobalt-60 is currently used for external beam radiotherapy. M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN Cobalt-60 teletherapy machine Specific activity a is defined as the activity A per mass m of a radionuclide) is linearly proportional to the decay constant  and inversely proportional to the half-life t1/2 A N ln2 NA a   m m t1/2 A Specific activity ◦ Radium-226: a = 0.988 Ci/g (original definition: 1 Ci/g) ◦ Cobalt-60: a = 1130 Ci/g (carrier free); 300 Ci/g (in practice) ◦ Cesium-137: a = 80 Ci/g M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN Cobalt-60 teletherapy machine Air kerma rate in air (K air )air is proportional to the specific air kerma rate constant  AKR and inversely proportional to d2, the distance between the source and the point of interest A  AKR (K air )air  d2 Specific air kerma rate constant  AKR in [  Gy  m2 / (GBq  h)] ◦ Cobalt-60:  AKR  309 Gy  m2 / (GBq  h) ◦ Cesium-137:  AKR  78  Gy  m 2 / (GBq  h) M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN Cobalt-60 teletherapy machine The important characteristics of radionuclides useful for external beam radiotherapy are: M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN  Cobalt-60 teletherapy machine  Treatment machines used for external beam radiotherapy with gamma ray sources are called teletherapy machines. They are most often mounted isocentrically with SAD of 80 cm or 100 cm.  The main components of a teletherapy machine are:  Radioactive source  Source housing, including beam collimator and source movement mechanism.  Gantry and stand.  Patient support assembly.  Machine control console. M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN Cobalt-60 teletherapy machine M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN  Cobalt-60 teletherapy machine  To facilitate interchange of sources from one teletherapy machine to another and from one radionuclide production facility to another, standard source capsules have been developed.  Teletherapy sources are cylinders with height of 2.5 cm and diameter of 1, 1.5, or 2 cm.  The smaller is the source diameter, the smaller is the physical beam penumbra and the more expensive is the source.  Often a diameter of 1.5 cm is chosen as a compromise between the cost and penumbra. M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN Cobalt-60 teletherapy machine  Typical source activity: of the order of 5 000 – 10 000 Ci (185 – 370 TBq).  Typical dose rates at 80 cm from source: of the order of 100 – 200 cGy/min  Teletherapy source is usually replaced within one half-life after it is installed. Financial considerations often result in longer source usage. M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN  Cobalt-60 teletherapy machine Teletherapy radionuclides: cobalt-60 and cesium-137  Both decay through beta minus decay  Half-life of cobalt-60 is 5.26 y; of cesium-137 is 30 y  The beta particles (electrons) are absorbed in the source capsule. M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN  Cobalt-60 teletherapy machine  The source head consists of:  Steel shell with lead for shielding purposes  Mechanism for bringing the source in front of the collimator opening to produce the clinical gamma ray beam.  Currently, two methods are used for moving the tele-therapy source from the BEAM-OFF into the BEAM-ON position and back:  Source on a sliding drawer  Source on a rotating cylinder M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN Cobalt-60 teletherapy machine Methods for moving the teletherapy source from the BEAM-OFF into the BEAM-ON position and back: Source on a sliding drawer Source on a rotating cylinder M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN Cobalt-60 teletherapy machine Teletherapy source housing Both methods (source-on-drawer and source-on-cylinder) incorporate a safety feature in which the beam is terminated automatically in the event of power failure or emergency. When the source is in the BEAM-OFF position, a light source appears in the BEAM-ON position above the collimator opening, allowing an optical visualization of the radiation field, as defined by the machine collimator. M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN  Cobalt-60 teletherapy machine Teletherapy source housing  Some radiation (leakage radiation) will escape from the teletherapy machine even when the source is in the BEAM-OFF position.  Head leakage typically amounts to less than 1 mR/h (0.01 mSv/h) at 1 m from the source.  International regulations require that the average leakage of a teletherapy machine head be less than 2 mR/h (0.02 mSv/h). M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN  Cobalt-60 teletherapy machine Dose delivery with teletherapy machines  The prescribed dose is delivered to the patient with the help of two treatment timers: primary and secondary. The primary timer actually controls the treatment time and turns the beam off upon reaching the prescribed beam-on time. The secondary timer serves as a backup timer in case of the primary timer’s failure to turn the beam off.  The set treatment time should incorporate the shutter correction time to account for the travel time of the source from the BEAM-OFF to the BEAM-ON position at the start of the irradiation and for the reverse travel at the end of irradiation. M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN  Cobalt-60 teletherapy machine Collimator and penumbra  Collimators of teletherapy machines provide square and rectangular radiation fields typically ranging from 5×5 to 35×35 cm2 at 80 cm from the source.  Penumbra is the region, at the edge of radiation beam, over which the dose rate changes rapidly as function of distance from the beam axis.  TYPES OF penumbra:  Transmission Penumbra  Geometric Penumbra  Physical Penumbra M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN Cobalt-60 teletherapy machine The Transmission Penumbra resulting from the inner surface of the blocks, if it is parallel to the central axis of the beam. It is due the transmission through the edge of the collimator block. The transmission Penumbra increase with increase of SSD and the collimator opening. It can be reduce by shaping the collimator M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN Cobalt-60 teletherapy machine The geometric penumbra resulting from the finite source diameter, may be minimized by using: ◦ Small source diameter ◦ Penumbra trimmers as close as possible to the patient’s skin (z = 0) P(z) (SSD  z  SDD)  s SDD M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN  Physical Penumbra  Physical penumbra is the later distance between specified isodose curves at a specific depth (90% and 20% at Dmax). Takes the scattered radiation into account in addition to the transmission and geometrical penumbra. Electron Contaminations  The blocks should not be placed closer than 10 to 20cm form the patient because of the excessive electron contamination produced by the block. M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN