OTHER MACHINES

1. OTHER MACHINES Chapter 10 S&S Chapter 7 W/L Part 2

2. Low Energy Machines • Limitations: • Can not reach deep-seated tumors with an adequate dosage of radiation. • Do not spare skin and normal tissues. • Kilovoltage filtration: • Inherent: glass envelop (similar to x-ray tube), insulating oil between tube and housing. • Added: clinical use, thin sheets between target and collimator to decrease weak x-rays that add skin dose (Cu, Al).

3. Central Axis Depth Dose • Central axis depth dose and physical penumbra are related to beam quality. • The central axis depth dose distribution for a specific beam depends on the energy.

4. Isodose Curve • Isodose curve: a line representing various points of similar value in a beam along the central axis and elsewhere • The depth of an isodose curve increases with beam quality (tells depth dose for different energy beams) • The absorbed dose in the medium outside the primary beam (physical penumbra) is greater for low-energy beams than for those of a higher energy. • Limited scatter outside the field for megavoltage beams occurs because of predominantly forward scattering of the beam.

6. Radiation Therapy Equipment • Low-energy machines: Uses x-rays generated at voltages up to 300 kVp • Primary application is in the treatment of superficial lesions. • Kilovoltage units include: • Grenz: 10-15 kVp • Contact: 50 kVp • Superficial: 50-150 kVp • Orthovoltage: 150-500 kVp • Super Voltage: 500-999 kVp use???

7. Grenz rays • 10-15kvp • Grenz rays are almost entirely absorbed in the first 2 μm of skin and have a useful depth dose range of about 0.5 μm. • Treatment of inflammatory disorders (langerhans’ cells), bowen’s disease, patchystage mycosis fungoides, herpes simplex

8. Contact therapy • Superficial skin lesions, treatment unit comes in contact with patient. • Endocavitary treatments for curative intent (rectal). Advantage- preserves sphincter • Low to middle third of the rectum • Confined to bowel wall • Maximum tumor size of 3 x 5 cm • Hemangiomas • 4 treatments of 3000 cGy each, separated by a 2 week interval • SSD 4 cm • 1 μm aluminum filtration

9. Superficial equipment • 50-150kVp • 1-6 μm aluminum filtration • Cones 2-5 cm diameter • Pb cutouts • SSD 15-20 cm • Skin cancer and tumors no deeper than 0.5 cm treated as a result of the rapid falloff of the radiation.

10. Orthovoltage machines • 150-500kVp • HVL 1-4 mm Cu • SSD 50 cm • Skin, mouth, and cervical carcinoma treatment last several minutes • Experience limitation in the treatment of lesions deeper than 2 to 3 cm.

11. Cobalt-60 Machines • Introduced in the 1950’s, being replaced by linacs. • The first practical radiation therapy treatment unit to provide a significant dose below the skin surface and simultaneously spare the skin the harsh effects of earlier methods. • Still used in developing countries: simpler design, cost, little tech support.

12. Cobalt 60 Machines • Constantly emit radiation. • The 60Co source must be shielded in a protective housing (source head). • The source head is a steel shell filled with lead (may be up to 2 ft in diameter). • Uses a counterweight to balance the lead shielding in the head of the machine housing, (similar to the beam stopper on linac): absorbs a significant amount of radiation transmitted through patient.

13. Linear Accelerators vs. Cobalt 60 • Linear accelerators: • Provide better isodose distribution (greater dose to the tumor and less dose to normal tissue) • Higher and faster dose rate • More manageable radiation protection concerns.

14. Application • The 60Co unit is commonly used today to treat cancers of the head and neck area, breast, spine, and extremities. • Areas just below the skin surface. • Ideal in treating lymph nodes. • A 60Co beam can provide an adequate distribution of dose by using parallel opposed fields.

15. Cobalt-60 Production • Cobalt: produced in nuclear reactors by the irradiation of neutrons of the common stable form of 59Co. • The 59Co nucleus absorbs a neutron in the reactor and becomes 60Co.

16. Cobalt 60 • Radioactive 60Co produces a useful therapy beam when it undergoes beta decay. • The nucleus emits a beta particle and then two photons, 1.17 MeV and 1.33 MeV for an effective energy of 1.25 MeV 60Co  60Ni+ + B- + neutrino (v) + gamma rays • Radioactive 60Co emits radiation in the form of high energy gamma rays in an effort to return to its more stable state.

17. 60Co Source • The overall diameter of a 60Co source is 1 to 3 centimeters. • Consists of pellets of radioactive 60Co encased in multiple layers of welded metal to prevent contamination of the environment and to absorb β- particles produced by the decay process. • Or 60Co sources are made with the 60Co fused into a solid cylinder. Advantages: • Smaller source with less penumbra for the same beam intensity • Less hazard of contamination should a source ever become exposed to the environment.

18. 60Co Activity • 60Co activity may be expressed in curies (Ci), the historical unit of radioactivity • 3.7 x 1010 Becquerel (Bq) • 1 Bq = 1 disintegration per second • May also be defined in rhm units (roentgens per hour at 1 meter) • Most sources have an activity of 750-9000 Ci, typically 3000-9000 Ci used in radiation therapy

19. Half-life • Half-life: the time necessary for a radioactive material to decay to half or 50% of its original intensity. • Requires a correction factor for this decay of about 1% per month in all treatment calculations. • Source must be replaced at about five year intervals. • The half-life of 60Co is 5.26 years.

20. 60Co Shielding • Cerrobend (Lipowitz metal): lower melting point than Pb, cheaper • 50% Bismuth • 26.7% Lead • 13.3% tin • 10% Cadmium (a toxic metal can get into bloodstream) • Density ratio of Cerrobend to Lead: 1.2 cm Cerrobend to 1 cm lead. • 5 HVL is needed to reduce intensity • A thickness of 7.2 cm of Cerrobend needed, 6 cm lead.

21. Penumbra • Penumbra: the area at the edge of the radiation beam at which the dose rate changes rapidly as a function of distance from the beam axis. • Describes the edge of the field having full radiation intensity for the beam compared with the area at which the intensity falls to 0. • The larger the source size, the larger the penumbra P = S(SSD-SDD)/SDD • When depth is given: P = S(SSD + D -SDD)/SDD • Larger field sizes are necessary to cover the same amount of tissue adequately compared to the linac.

22. Penumbra • Geometric penumbra: the place where a lack of sharpness or fuzzy area occurs at the edge of the beam • Occurs at the skin surface and greater depths in tissue. • Transmission penumbra: occurs as the radiation passes through the edge of the primary collimators • Occurs at the edge of the patients shielding blocks mounted or placed below the collimator • Correlates with the size of the collimator opening- larger field sizes have more transmission penumbra

23. Reducing Penumbra • The transmission penumbra can be reduced by using satellite collimators, penumbra trimmers or trimmer bars. • Trimmers are metal bars that attenuate the edge of the beam providing a sharper field edge. • Should be placed no closer than 15 cm from the patients skin to reduced electron contamination (increased skin dose) by metal devices. • Provides enough distance for the secondary electrons produced by the trimmer bars to lose sufficient energy

24. Dose Maximum • Dose maximum (Dmax): when a greater percentage of dose occurs below the skin surface • Dmax is the depth of maximum buildup, in which 100% of the dose is deposited. • For 60Co, Dmax occurs at 0.5 cm below the skin surface. • Electron equilibrium is another term used to describe Dmax. As energy increases, so does the depth of electron equilibrium.

25. Beam On/Beam Off • Turning the beam on requires physically exposing the source either by moving it into position or by removing shields around the source. • Air pressure: the compressor generates air pressure by pushing the source horizontally into position over the collimator opening. • Rotating wheel: the motor rotates a wheel 180 degrees by placing the source over the collimator opening.

26. Warning Lights • Red light: Radiation present- do not enter room • Green Light: time elapsed • Malfunction: both red and green lights still on- means that machine is still in on position after prescribed dose has been delivered. Remove patient.

27. Timer Error • Timer error (TE, or travel time correction, shutter error): is a consequence of the physical motion of the source. Even if its less than 1 second. • The difference between the beam on time setting and the time that the source is in the treatment position, the time it takes to advance and retract the source • Timer error is determined at the time of machine beam calibration and is checked monthly. TE = (R1t1-R2t2)/R1-R2 • The timer setting necessary for a treatment is the time calculated plus the timer error.

28. Quality Assurance • Required be the Nuclear Regulatory Commission (NRC) • A qualified radiation physicist must perform full calibration testing annually. More frequent if: • The source is replaced • A 5% deviation is noticed during a spot check • A major repair requiring the removal or restoration of major components is done.

29. Quality Assurance • Written directive: the prescription, must be clear, unambiguous, and signed by a licensed authorized user. • Recordable event must be reported to the RSC-Radiation Safety Committee: • A delivered dose more than 15% in excess of the weekly prescription • No WD • Lack of daily recording of dose • Misadministration must be reported immediately to the NRC • Treatment of the wrong patient, wrong site • Weekly dose more than 30% in excess of the prescription • The delivery of more than 20% (25%???) total dose

30. Advisory Agencies • Analyze and assess data, and then develop recommendations for dose limits • National Council on Radiation Protection and Measurement (NCRP) and ICRP • Recommendations may be made into law by state or federal government.

31. Regulatory Agencies • License users of radioactive materials and radiation producing equipment; inspection; enforcement of laws. • Nuclear Regulatory Commission (NRC) oversees isotopes produces in nuclear reactors, teletherapy, and brachytherapy.

32. Tests • The integrity of the source must be tested twice a year to ensure the sealed source continues to be totally sealed. • Wipe tests: wipe collimator edges with a filter paper, background radiation reading done, activity is determined (acceptable under 0.005 mCi) • Radiation and light field coincidence • Timer accuracy • Dose rates • & more.

33. Leakage Limits • Leakage in the off position: • Cannot exceed 2 mrem/hr at 1 meter • Maximum of 10 mrem /hr at 1 meter at any measurable location at any time • Leakage in the on position: • Cannot exceed 0.1% of the useful beam at 1 meter from the source.

35. Betatrons • Developed by Kerst in 1941 • Can provide x-ray and electron therapy beams from less than 6 to more than 40 MeV. • Applied to industrial radiography- used in World War II • Operation is based on the principle that an electron in a changing magnetic field experiences acceleration in a circular orbit.

36. Betatrons • Accelerating tube shaped like a hollow doughnut and placed between the poles of an AC magnet. • A pulse of electrons is introduced at the moment the AC cycle begins • As the magnetic field rises, the electrons accelerate continuously around the tube. • Energy depends on the number of revolutions and the speed. • By the end of the first quarter cycle, electrons are made to spiral out of orbit, then strike a target to produce x-rays or a scattering foil to produce a broad stream of electrons. • By choosing the timing of electron injection and extraction, continuous control of electron energy is possible

37. Betatrons • Most often used for electron therapy • Can produce x-ray beams with energies over 40MeV, though cannot compare with dose rates of linacs. • Used Lucite cones to treat gynecologic, bladder, and prostate carcinomas. • Treatment times 3-5 min. • Disadvantage: Machine is noisy, bulky- isocentric design impossible, non-isocentric, slower dose rates • Makes small field sizes and precise patient setup cumbersome.

39. Van de Graff Generator • Developed by R.J. Van de Graff while working at the MIT • First electrostatic Linear accelerator • Capable of accelerating either positive or negative ions. • Disadvantages: • Size • Warm-up took as long as 1 hour. • Used a front pointer device to measure the distance to the patient because no ODI was available.

40. Van de Graff Generator • An insulating belt transports electric charge to a collector screen within a metal dome • The accumulation of charge produces the high voltage used to accelerate charged particles • The maximum possible x-ray energy in MeV is directly proportional to the voltage on the dome and on the total length of the machine- limits the voltage of clinical machines to a few MV • Smaller source size  smaller penumbra

41. Van de Graff Generator • Operate at 200cGy/minute. • Provide a standard SSD of 100 cm and can approximate much greater treatment distances- useful in the treatment of extended fields. • Used to treat seminoma, whole brain, and mantle field (used to treat lymph nodes in the neck and thorax for Hodgkin’s disease).

43. Cyclotrons • Developed by E.O. Lawrence in 1928. • Accelerate heavy charged particles (protons, deuterons, positive nuclei, etc) • Use large magnets to confine the charged particles to a circular or spiral path. • Particles are accelerated by an oscillating electric field. • Medical uses of the cyclotron: • Production of radionuclides applied in nuclear medicine. • The use of neutrons and protons in radiation therapy.

44. Dees • Metal half-disks with a hollow evacuated center through which the particles can travel • Two dees of opposite polarity are separated by an alternating voltage produced by a high frequency generator, pull the particles back and forth in a spiral pattern. • Charged particles enter the center of one of the dees with a moderate velocity and are bent in a circular path by the magnet. • When the particles enter the gap between the dees, they experience the high frequency, high voltage electric field and are accelerated, reenter the opposite dee, only magnetic force within the dee- no velocity increase. • Final energy is equal to the sum of the energy gained at each gap crossing.

47. Theory of Relativity • Acceleration causes particles to gain mass. • The increase in weight causes to particles to slow down and be out of sync with the frequency of the alternate potential applied to the dee. • Electrons cannot accelerate in a dee.

48. PET • Cyclotrons are used for the production of radionuclides used in PET. • PET: scanning technique that involves the systemic administration of a radiopharmaceutical agent labeled with a positron emitting radionuclide. • PET scanners are used in nuclear medicine to measure important physiologic and biomedical processes such as blood flow, oxygen, glucose and metabolism of free fatty acids, etc.

49. PET • PET scanners use radiation emitted from within the patient to produce images. • Anatomy can be evaluated in microscopic detail- patients receive a small amount of an agent that closely resembles a substance naturally found in the body. • PET imaging involves positrons emitted during the breakdown of the nuclei of certain radioisotopes. • Pure energy, released as gamma rays, is a result of the collision and subsequent annihilation of matter and antimatter. • Radiation from the positron emitting isotope is detected be the PET scanner and displays in microscopic detail the chemical processes occurring.

50. Neutrons • To produce neutron beams, deuterons are accelerated to high energies and then forced to strike a beryllium target, producing neutrons via nuclear reactions. • Use Teflon flattening filters, polyethylene filters. • Neutrons: effective in penetrating nuclei and producing reactions by a process called stripping. • Neutron studies abandoned due to the high concentration of hydrogen in the fatty tissue (lack of information about its RBE).