BASIC PRINCIPLES OF RADIATION THERAPY 
What is radiation?
Radiation is a form of energy. There are two basic types: particulate radiation and electromagnetic radiation. Particulate radiation is caused by the movement of subatomic particles and thus has a mass. This is contrary to electromagnetic radiation which is pure energy with no mass and is transmitted by massless photons.
What are the subatomic particles used as therapeutic radiation?
There are three particles that radiation oncologists utilized for therapeutic ends:
• Electrons: these are the standard particles delivered in the vast majority of therapeutic radiation.
• Protons: the opposite charge of electrons these have a unique dose distribution.
• Neutrons: these particles have a high mass and therefore have 20 to 100 times the energy of electrons.
What is ionizing radiation opposed to nonionizing?
Radiation is called ionizing when it contains enough energy so that during interaction with an atom it can remove tightly bound electrons from the orbit of an atom. This causes the atom to become charged or ionized. Ionizing radiation damages DNA. Nonionizing radiation does not, for example, radio, infrared, and microwave.
What are gamma rays opposed to x-rays?
Both gamma and x-rays are a form of electromagnetic radiation, that is, weightless packets of energy. They are similar to visible light, but with much more energy. X-rays are artificially produced by a roentgen tube, whereas gamma rays originate from an atomic nucleus with subsequent defined characteristics.
What are free radicals?Atoms or molecules with an unpaired electron which makes them highly chemically damaging.
What is radiosensitivity?
The relative susceptibility of cells, tissues, or organs to the action of radiation. The most actively dividing cells or those not fully mature are most at risk from radiation.
Which tissues possess high radiosensitivity and which contain low?
High Radiosensitivity
Lymphoid organs, bone marrow, blood, testes, ovaries, intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea, oral cavity, esophagus, rectum, bladder, vagina, uterine cervix, ureters)
Moderate Radiosensitivity
Optic lens, stomach, growing cartilage, fine vasculature, growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones, salivary glands, respiratory organs, kidneys, liver, pancreas, thyroid, adrenal and pituitary glands
Low Radiosensitivity
Muscle, brain, spinal cord
What is absorbed dose?
The amount of energy that ionizing radiation imparts to a given mass is called absorbed dose. The SI unit is the gray (Gy). One gray is defined as a dose of 1 joule per kilogram.
What is dose equivalent?
The dose equivalent relates the absorbed dose to the biological effect of the absorbed dose. It takes into account the biological effectiveness of radiation which is dependent on radiation type and energy. A sievert (Sv) is the unit used for dose equivalent. Radiation doses in medical imaging are expressed as millisieverts (mSv).
What is the average natural background radiation?
On average 3 mSv (milli-sievert) per year for US citizens.
How can radiation cause cancer?
Ionization of water molecules can create radicals that may damage DNA. Although most damage is repaired, misrepair can occur leading to mutations, chromosome translocations, or gene fusions. It is estimated that 2% of the cancers in the United States is related to computed tomography (CT).
Is all ionizing radiation stochastic?
Yes. It can occur at any level of radiation exposure. With increasing dose, the risk increases as well.
What is the usual interval between radiation exposure and a possible cancer?
The period between exposure and cancer diagnosis is at least 5 years, but in most cases 10 to 20 years or longer.
Which doses are associated with radiation-induced cancer?
There is clear evidence that doses above 100 mSv are stochastic. However, in normal medical imaging these levels are rarely reached. Only in cases of multiple high-dose examinations (e.g., CT or complex interventional radiology) in a short time this can be reached. The risk at doses between 10 and 100 mSv is controversial, but this is more frequently seen in normal medical practice, for instance a single multiphasic CT.
What is the influence of the cell cycle phase on radiation damage?
The phase in which cells are most susceptible to damage is M phase as this is where chromosomes condense and therefore result in a higher probability of double-strand breaks.
The phase in which cells are most resistant is S phase where enzymes and proteins responsible for replication and repair overexpressed thereby conferring more protective capability of the cell to recover from DNA damage.
How are cancer cells inherently more susceptible to radiation than normal cells?
Radiation therapy capitalizes on two essential differences in cancer cell biology: (1) rapid growth and (2) DNA repair ability. The comparatively rapid growth of cancer cells from more frequent cell division means that they spend more time in the most radiation susceptible of the cell cycle (M phase). Cancer mutations are often DNA repair mutations and thus cancer cell populations are less able to prevent the accumulation of lethal mutations.
What radiation dose is given by medical imaging?
Computed tomography
Sinuses 0.6 mSv
Head 2.0
Chest 7.0
Chest (pulmonary embolism) 10.0
Abdomen and pelvis 10.0
Multiphase abdomen and pelvis 31.0
Radiography
Extremity 0.001
Chest 0.1
Lumbar spine 0.7
Abdomen 1.2
Mammography 0.7
Bone densitometry (DEXA) 0.001
Nuclear medicine
Lung ventilation/perfusion 2.0
Bone scan 4.2
Cardiac perfusion (sestamibi) 12.5
Fluoroscopy
Barium swallow 1.5
Coronary angiography 5 to 15
How can radiation dose be reduced?
One can decide not to perform the study. Alternatively, alternatives without ionizing radiation can be sought or less radiation can be used (e.g., low-dose CT in case of suspected urinary stone disease).
Which patients are most at risk and why is that?
Younger patients and pregnant patients are at increased risk due to high radiosensitivity. Furthermore, patients with a high BMI and those undergoing multiphasic CT are at higher risk as a result of higher radiation dose.
What is the ALARA principle?ALARA is an acronym for “As Low As Reasonably Achievable,” which refers to the radiation safety principle of keeping radiation doses and releases of radioactive materials to the environment as low as can be achieved, based on technologic and economic considerations.
RADIATION ONCOLOGY 
How is radiation delivered?
Delivery of radiation can be divided into two major routes: external (beam) and internal (brachytherapy).
What is external beam radiation?
The therapeutic radiation is delivered from a source external to the patient. The most common method of delivery is via a linear accelerator (LINAC) which accelerates charged subatomic particles (i.e., electrons) via an accelerator tube and beam transport system to deliver to target areas of a patient’s anatomy.
What is particle therapy?
The various particles in particle radiation have all been used to therapeutic effect. Electrons are most commonly used and tend to deposit their energy more superficially in the body. Thus they are commonly used for skin cancers, breast cancer, oral cancers, etc.
Protons have a unique dose distribution as they travel through, deposit their peak energy deeper in tissue and have a steeper drop off afterward theoretically limiting the absorbed radiation to superficial and surrounding structures. They are ideal for deep lesions like skull base tumors such as chordomas or pediatric craniospinal tumors such as medulloblastomas. However, protons require a cyclotron to be delivered and these devices are not only expensive to construct and maintain but also require substantial space.
Neutrons deliver a much higher degree of radiation than either electrons or protons and are therefore more effective at treating traditionally radioresistant tumors such as sarcomas. However, the collateral damage to surrounding tissues is more severe given the higher amount of radiation delivered.
What is meant by fractionation?
Fractionation is the distribution of delivered therapeutic radiation over time. Types of fractionation are:
• Hypofractionation: smaller total dose with larger doses per fraction
• Hyperfractionation: smaller doses per fraction over the same time period
• Accelerated fractionation: same dose delivered over shorter period of time
What is the rationale behind fractionation?
Fractionation of radiation dosing is both to better target cancer cells and spare normal tissue. In between fractions, cancer cells are allowed to re-oxygenate thereby leading to more free radical when radiation is administered. In addition, in between fractions cancer cells are allowed to progress through the cell cycle transitioning into the more susceptible M phase. Normal tissue is allowed to progress through cellular repair in between fractions.
What is meant by boost?
A boost is an additional dosage of external beam radiation delivered to specific treatment targets (i.e., mastectomy incision, lumpectomy cavity).
What is meant by bolus?Usually in reference to electron beam therapy, a bolus is a tissue density mimicking material placed on the surface of a patient’s targeted anatomy such that the peak amount of radiation absorbed ends up being more superficial than it would be had not the bolus material been there. It is used commonly in breast irradiation to deliver a focused dose more superficially.
RADIATION, WOUND HEALING, AND RECONSTRUCTIVE SURGERY 
How does radiation alter wound healing?
Radiation affects wound healing in 4 ways:
1. Hypovascularity: There is a progressive narrowing of vessels and thus a decrease in tissue perfusion with concomitant decrease in oxygen delivery and cellular actors (neutrophils, monocytes, etc.).
2. Hypoxia: Decreased oxygen has several consequences. Oxygen is required for the oxidative burst used by neutrophils to kill bacteria thus hypoxia results in diminished capacity against battling infection. Oxygen is also needed for the hydroxylation of lysine and proline as well as collagen cross-linking therefore hypoxia results in diminished collagen formation. Angiogenesis requires an oxygen gradient which is altered hypoxia and thus vessel formation is blunted.
3. Fibroblast and epithelial cell effects: Radiation depletes fibroblasts and results in the disorganized collagen deposition from remaining fibroblasts thereby leading to decreased wound breaking strength. Decreased epithelial cell mitotic activity results in atrophic and thinned epithelium.
4. Alterations in cellular signaling: TGF-B, VEGF, TNF-a, IFN-g, proinflammatory cytokines (IL-1, IL-8) are all overexpressed leading to extra cellular matrix accumulation with resultant fibrosis. Reduced levels of nitric oxide (NO) lead to less collagen deposition. Finally dysregulation of metallomatrix protein 1 (MMP-1) results in disorganized collagen deposition.
What is the effect of radiation upon free flap anastomoses?
While there is arterial tunica media fibrosis and subendothelial smooth muscle proliferation there is no change in patency. In addition, arterial patency is maintained whether radiation is received 2 weeks prior, the day of, or 2 weeks after surgery. As discussed later in this chapter, papers evaluating clinical outcomes seem to reflect the basic science data.
What is the effect of a free flap upon a previously irradiated wound bed?
Neovascularization occurs at the recipient-free flap interface but is delayed. However, this process decreases with increasing dose size and decreasing time from radiation. There appears to be no decrease in the tensile strength nor collagen content at the recipient-free flap interface and the tissue repair at the interface seems to approach that or nonradiated surgical incisions.
HEAD AND NECK CANCER AND RADIATION 
Why is radiation frequently used as the primary treatment for laryngeal and hypopharyngeal carcinoma as opposed to surgery?
The Department of Veterans Affairs Laryngeal Cancer Study Group NEJM 1991 was the landmark study promoting radiation alone as the primary treatment in an attempt to preserve the larynx. Primary chemoradiation with laryngeal preservation rate was 64% of surviving patients. Cure rates with radiation only were equivalent to total laryngectomy with postop radiation (68% both).
What is the incidence of pharyngocutaneous fistula?
3% to 65% of all laryngectomies.
What are the risk factors for pharyngocutaneous fistula formation?
Several factors have been associated with the development of pharyngocutaneous fistula including:
• Hgb <12.5
• Neck dissection
• Prior radiation and chemoradiation
What is the incidence of salvage laryngectomy?
Salvage laryngectomy is performed after failure of radiation to cure and occurs at a frequency of 31% to 36% after radiation but is lower at 16% to 28% after chemoradiation.
Related posts:
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
