WMK:
An Arteriovenous Malformation, or AVM, is an abnormal collection of blood vessels. Normally, oxygenated blood is pumped by arteries to the brain, where it enters a fine network of tiny capillaries. It is in these capillary beds where the blood nourishes the tissues. The deoxygenated blood then passes back to the heart through veins. Arteriovenous malformations are areas that lack the tiny capillaries. The location of the connection between the artery and the vein is called the shunt. The area of tissue is called a nidus of the AVM. An AVM can be thought of as a "Short Circuit" where the blood does not go to the tissues but is pumped through the shunt and back to the heart without ever giving nutrients to the tissues.
What causes most AVMs is not known. People are born with AVMs although they do not appear to be hereditary. AVMs occur about equally in both sexes and in different races. An estimation of 3 million people in the United States are born with vascular malformation, 10% of which are AVMs.
Most patients do not know that they have an AVM. A number of the patients with AVMs have seizures or persistent headaches. An AVM can put additional strain on the blood vessels and the surrounding tissues. For the very young (under the age of twenty) this is usually not a problem. The increased flow of blood caused by the shunt weakens the blood vessels. These weakened blood vessels can rupture. This is known as a hemorrhage or a bleed. If an AVM bleeds, the patient experiences an extremely severe headache. The bleed may cause a stroke and even death. About 4% of people with AVMs experience initial bleeds each year.
AVMs can be seen on outpatient imaging studies such as CT's or MRI's. Angiograms are inpatient procedures needed to image the arteries and veins before any treatment. An angiogram is an x-ray movie of the blood flowing through the blood vessels. It is made by injecting contrast into the arteries going into the head and taking a series of x-rays films.
Treatment options for AVMs include embolization, radiation, and surgery or a combination of these methods. Recent studies have revealed that for most cases, embolization is the safest and most effective procedure. To avoid bleeding, the aneurysm must be eliminated. Each treatment has its advantages and disadvantages.
Embolization is a method of plugging the blood vessels of the AVM. Under X-ray guidance, a catheter is guided from the femoral artery in the leg up into the area to be treated. Once the area is reached, glue or sometimes even a wire coil is placed to block off the area.
Friday, June 2, 2006
Electron Blocks
WMK:
We read an electron block if the size of the opening in centimeters is less than half the energy in MeV. For example, for a 9 MeV block, if the opening is less than 4.5 cm. near the center of the field we must make a physical measurement. If the opening is greater than 4.5 cm. we can make a calculation as if the block was open.
When measuring a block, we make two readings. One reading is where we know the output at and the other is at the treatment distance. For example, we can take an open field measurement at 100 cm. For a 10 x 10 field at 100 cm. the output is 1 cGy/MU at dmax. We record the electrometer reading. This gives us the reading to deliver 1 cGy/MU. Next we take a reading for the blocked field at the treatment distance. This reading divided by the open field known reading will give us the cGy/MU delivered under the treatment conditions.
We use this information to calculate the monitor units necessary to deliver the prescribed dose.
HRE adds these comments:
We're making a few assumptions here. One is that fractional depth dose doesn't change between your open measurement and your treatment set up measurement. That is, the distance of dmax remains the same for the two measurements. Another assumption is that the electrometer reading is linearly proportional to dose.
DRR adds this:
Note that for purposes of the calculation, we use the electron cone factor from the table at 100 SSD if we measured the open field at this distance. A common mistake when a treatment is at 110 SSD and the block is read would be to use the electron cone factor at 110 SSD. This would be wrong.
We read an electron block if the size of the opening in centimeters is less than half the energy in MeV. For example, for a 9 MeV block, if the opening is less than 4.5 cm. near the center of the field we must make a physical measurement. If the opening is greater than 4.5 cm. we can make a calculation as if the block was open.
When measuring a block, we make two readings. One reading is where we know the output at and the other is at the treatment distance. For example, we can take an open field measurement at 100 cm. For a 10 x 10 field at 100 cm. the output is 1 cGy/MU at dmax. We record the electrometer reading. This gives us the reading to deliver 1 cGy/MU. Next we take a reading for the blocked field at the treatment distance. This reading divided by the open field known reading will give us the cGy/MU delivered under the treatment conditions.
We use this information to calculate the monitor units necessary to deliver the prescribed dose.
HRE adds these comments:
We're making a few assumptions here. One is that fractional depth dose doesn't change between your open measurement and your treatment set up measurement. That is, the distance of dmax remains the same for the two measurements. Another assumption is that the electrometer reading is linearly proportional to dose.
DRR adds this:
Note that for purposes of the calculation, we use the electron cone factor from the table at 100 SSD if we measured the open field at this distance. A common mistake when a treatment is at 110 SSD and the block is read would be to use the electron cone factor at 110 SSD. This would be wrong.
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