DOSI Final Review Exam Questions With Correct Answers

Due to the large size of a cobalt source, the penumbra, or unsharp edge of the beam, is ______ than that of a linear accelerator beam. Smaller Larger Equal to - correct answers Larger The ______ dose was the only measure of dose by which the early pioneer could estimate the necessary length of the treatments. Erythema Lung Internal - correct answers Erythema It was recognized early on that accurate ___________ was fundamental to the success in any type of radiation treatment. - correct answers dosimetry TRUE or FALSE The Roentgen was internationally accepted as a unit of measurement for x-rays and gamma in 1928. - correct answers TRUE The RAD as a unit of absorbed dose was recommended by the ____________ in 1953. - correct answers ICRU (International Commission on Radiation Units and Measurements) The secondary collimator of a linear accelerator is largely responsible for? The flatness of the beam. The sharpness of the beam edges. Determining dose rates. - correct answers The sharpness of the beam edges. A collimator: Flattens the beam at a specified depth in tissue. Determines the dose rate. Defines the beam. - correct answers Defines the beam. The penumbra of a linear accelerator beam is ______________ than that of a cobalt 60 machine. Smaller Larger Equal to - correct answers Smaller The half-value thickness (HVT) is The thickness of a given material that, when introduced into the path of the beam, reduces the intensity of the beam to one half its original value. The thickness of a given material that causes the beam to be more penetrating. The thickness of a filter that determines the quantity of the beam. The thickness of a filter that hardens the beam and causes it to be reduced to half of its original value. - correct answers The thickness of a given material that, when introduced into the path of the beam, reduces the intensity of the beam to one half its original value. Linear Accelerators were first developed Following World War II 1950's During the depression - correct answers Following World War II The Inverse Square Law: (Choose all that apply) A. Can be used to find the change in dose rate (output). B. Dose rate is commonly measured at the isocenter. The dose rate at the iso for a 10 x10 cm field is often 1.5 cGy/ MU. C. When the MUs are calculated for a treatment distance greater than the standard, the inverse square law is used to account for the in crease in dose rate at distances beyond the isocenter. D. Describes change in beam intensity caused by divergence. - correct answers A. Can be used to find the change in dose rate (output). D. Describes change in beam intensity caused by divergence. The dose rate is INVERSELY proportional to the square of the distance. Even a small change in distance can have a SMALL/LARGE effect on the dose rate. - correct answers LARGE If the dose rate is 1.0 cGy / MU at 100 cm, then the dose rate is ? cGy/MU at 120 cm. Remember to round correctly. - correct answers 0.69 (100/120)^2= 0.694 --0.69cGy/MU Name three Lookup Tables that Equivalent Squares of Rectangular Fields are used for - correct answers output, output factor, and the tissue absorption factors What is the ES of a field that is 5 cm wide and 14 cm long? - correct answers 7.37 (2)5*14/5+14=7.368 Match which Tissue Absorption Factors are Non Isocentric and which are Isocentric. PDD: isocentric or non-isocentric? TAR: isocentric or non-isocentric? TMR: isocentric or non-isocentric? TPR: isocentric or non-isocentric? - correct answers PDD: NON-isocentric TAR: isocentric TMR: isocentric TPR: isocentric TAR is used to perform calculations for SAD treatments involving ___________energy treatment units. Low Midrange High Electron - correct answers Low TAR is the dose in tissue divided by the ____________. - correct answers TAR= dose in tissue/dose in air TAR increases as the energy and field size ______________ and decrease as the ____________ increases. - correct answers increases and decrease as depth increases TAR and energy and field size are directly proportional TAR and depth are inversely proportional Is TAR dependent or independent of SSD? - correct answers INDEPENDENDT TAR and SSD have nothing to do with eachother Absorbed dose has two DISTINCT components as it is measured along the CR of the beam at the point of calculation: Primary radiation that is....... Scatter radiation from the.... - correct answers Primary radiation that is emitted from the tx unit & scatter radiation from the surrounding irradiated tissue The contribution of ________________ to calc points in the irradiated tissue, along the central ray, and off axis points is very important as the irregular shapes of the treatment fields are considered when planning. - correct answers scatter The intensity of the beam varies across the field. This variation is independent/dependent on depth and distance from the central axis. - correct answers dependant ___________________ is the Ratio of dose rate with scattering medium to dose rate in air at Dmax. - correct answers backscatter factor Backscatter factor is high/low for lower energies? Backscatter factor is high/low for high energies (8-10MV)? - correct answers BSF is HIGH for low energies BSF is LOW for higher energies Think of a baseball being thrown really fast- the ball rolls round and scatters if you throw it low and if you throw it high and really fast then it doesn't scatter or roll around Backscatter is defined as the _____________ at Dmax. - correct answers TAR TAR is dependent on what? and not on what? - correct answers TAR is dependent on energy and field size, but not on SSD Define Tissue phantom ratio? - correct answers Absorbed dose at a given depth in a phantom to absorbed dose at the same point at a reference depth in a phantom Name three devices placed in the path of the beam, not on the patient's skin, that will require a transmission factor when performing the dose calculation. - correct answers blocking tray, wedges, and compensators Given dose is the dose at _____________. - correct answers Dmax Dose Rate is also the: Output of the machine PDD Backup time - correct answers Output of the machine The wedge angle is defined as the angle the ________%_____ isodose curves makes with a line parallel to the surface. - correct answers 50% (50% is for lower energies and 80% for higher energies) What happens to depth dose as energy increases? - correct answers increases What happens to depth dose as distance increases? - correct answers decreases What happens to depth dose as area increases? - correct answers increases What happens to depth dose as depth increases? - correct answers decreases What happens to PDD when distance increases? - correct answers increases What happens to PDD when energy increases? - correct answers increases What happens to PDD when depth increases? - correct answers decreases What happens to PDD when area increases? - correct answers increases What happens to the output when energy increases? - correct answers increases What happens to the output when area increases? - correct answers increases What happens to the output when distance increases? - correct answers decreases What happens to the backscatter when depth increases? - correct answers increases What happens to the backscatter when energy increases? - correct answers decreases (think of the baseball, with tons of energy there's no scatter) What happens to the backscatter when distance increases? - correct answers NO EFFECT What happens to the backscatter when area increases? - correct answers increases What is the Definition of Dose - correct answers Absorbed dose is measured at a specific point in a medium and refers to the energy deposited at that point Define MU's - correct answers treatment time for the beam/ length of time a unit is physically activated to deliver a prescribed dose Define Dmax - correct answers Depth at which electronic equilibrium occurs for photon beams/ point at which the maximum absorbed dose occurs for single-field photon beams and is dependent on the energy of the beams TRUE or FALSE Skin Sparing occurs 0.5cm to midplane within the patient. - correct answers FALSE Compare Dmax dose to patients with a large separation compared to patients with a small separation. Is is lesser at the point of Dmax It is greater at the point of Dmax - correct answers It is greater at the point of Dmax If the Dose Rate is 1.0cGy/MU at 100cm SSD what would the dose rate be at 120 cm SSD? 0.66cGy/MU 0.71 cGy/MU 0.69 cGy/MU - correct answers 0.69 cGy/MU If the Dose Rate is 1.0cGy/MU what would the dose rate be at 85cm SSD? 1.38 cGy/MU 1.36 cGy/MU 1.40 cGy/MU - correct answers 1.38 cGy/MU Determine the PDD if the dose at Dmax is 190cGy and the Depth Dose is 125cGy? 68% 66% 72% - correct answers 66% PDD=abdorbed dose/dose at dmax 125/190=0.657 --0.66X100= 66% What will be the dose at Dmax if the PDD is 54% and the depth dose at 12cm is 190cGy? 352cGy 356cGy 350cGy - correct answers 352cGy PDD=190/x (x).54=190 divide both sides x=190/.54 --351.8 If the distance from the source of radiation increases, then intensity of the beam ______________. - correct answers decreases When the distance from the source to the point of measurement increases, then the dose rate decreases due to the ___________________. - correct answers inverse square law What is the amount of radiation exposure produced by treatment machine or source as specified at a reference field size at specified reference distance? Dmax Scatt