February 7, 2013

Today, we will be discussing particulate radiation and electromagnetic radiation.  We will also talk about the radiation damage they can do.

• Early radiation

1.      1890s:  discoveries by Roentgen and Curie led to the clinical use of both roentgen  rays and gamma rays to treat diseases.

2.      Early 1900s:  accumulated clinical evidence showing effects of ionizing radiations on malignant neoplasms (as well as injury to normal tissue).

• Electromagnetic radiation

1.      Consist of both x-rays and gamma rays.

2.      X-rays are produced in a device that accelerates electrons to a high energy and then stops stops them in an appropriate target of tungsten or copper.

3.      Gamma rays are emitted from the nucleus of a radioactive isotope.

4.      When the unstable nucleus decasys, the excess energy is emitted as a monochromatic gamma ray.

5.      Individual x-rays or gamma rays differ only in origin, not physical or biological properties.

• Particulate radiation

1.      Includes electrons, protons, neutrons, and alpha particles.

2.      Protons and neutrons are 2,000 times heavier than electrons.

3.      Alpha particles are 4 times heavier than neutrons; they have no clinical relevance.

• Compton effect

1.      An energetic short-wavelength photon interacts with an orbital electron.  The now recoiled electron ejects from orbit in one directions, while the now reduced-energy photon goes in another. This releases a “scattered” photon which may start the process all over again and release more enrgy.

• High-LET

1.      Protons, neutrons, and alpha particles interact directly with the molecule’s nucleus, not orbitingn electrons.  These energy deposits are close together and in a “track.”  This can break a DNA double-helix if it hits a DNA molecule.

• Low-LET

1.      Photons and electrons produce sparse and widely separated deposits of energy which do not break through DNA molecules.

• Teletherapy

1.      Delivery of ionizing radiation where source is some distance from the patient.

2.      Advantage of long distance: dose is relatively uniform across a given volume and allows for dose-shaping devices to be used.

• Brachytherapy

1.      Delivery of ionizing radiation place directly into the tumor site.

2.      Advantage of inverse square law:  dose gradient is steep so high levels near tumor and lower levels further away in normal tissue.

• Injury to DNA

1.      Generally believed to be the primary mechanism by which radiation kills cells.

2.      Direct ionization accounts for 1/3 of cell injury, with the majority   arising from the indirect mechanism of hydroxyl (free) radicals.

• Dosing

1.      Radiation dose is recorded as absorbed energy per unit mass.  One gray (Gy) is one J/kg, which is 100 rads.

• Hydroxyl radicals

1.      Have a lifetime of a few microseconds and are only capable of damaging DNA within a radius of 10nm.

2.      Lifetime , and effectiveness, can be prolonged by the presence of oxygen or nitroimidazoles; conversely sulfhydryl molecules reduce their biologic effectiveness.

• Clinical considerations of delayed cell death

1.      Slowly proliferating normal tissues and tumors have delayed response while rapidly growing tissues and tumors will have a rapid response.

2.      Difficulty in interpreting biopsy specimens taken weeks or months after radiation has been completed.

3.      The death of the cell will only occur after it has failed to reproduce and divide and we don’t have a means of know which are damaged and destined to die from division and which are still undamaged.

Reflections:

As a radiation therapist, I will be using both photon radiation and also particulate radiation almost everyday.  Whether the particulate radiation come from an HDR treatment source, or from accelerated electrons, positrons, or protons I will be using them to treat cancer.   My patients might ask me, “What is radiation doing to my body at the cellular level?”  I would be able to answer that the photons and electrons are interacting with water molecules causing the formation of hydroxyl radicals that have a few microseconds to wreak havoc on nearby DNA data.  When this DNA data is damaged there is a chance that it will not get repaired before cell division causing the daughter cells to both die when mitosis fails to occur correctly.  If that patient is using proton therapy or an HDR treatment that uses alpha particles I would be able to tell them that the effects are directly due to the particles interacting with the molecules of DNA.