MDCB questions with verified answers
A 20 cm diameter patient is treated to midplane by using a 100 cm
source-to-axis distance (SAD) technique. If the same patient is then
treated by using a 100 cm source-to-surface distance (SSD) technique,
the monitor units would increase by approximately: Ans✓✓✓
(110/100)^2=1.21= 21%
A dose of 30 Gy in 10 fractions to a depth of 7 cm from a 6 MV single
posterior field using an isocentric technique at 100 cm source-to-axis
distance (SAD). The patient was incorrectly treated five times at 100 cm
source-to-surface distance (SSD). The approximate dose at 7 cm after
five fractions is: Ans✓✓✓ 3gyx(100/107)^2=2.6x5=13.1gy
A patient is inadvertently treated using the 18 MV photon energy
instead of the prescribed 6 MV. The prescribed dose is 200 cGy to 5 cm
depth at 100 cm SSD. If the calculated monitor units (MUs) were
delivered, what is the dose delivered to 5 cm depth? Ans✓✓✓
200/.970x.994x.866=239mu
239=x/.981x.989x.965=224cgy
According to the American Association of Physicists in Medicine (AAPM)
Task Group Report #142, the constancy of dose per monitor unit for
photon beams of a linear accelerator should be checked: Ans✓✓✓
daily.
, After 6.5 years, the activity of a 25 mCi Cesium-137 source will be:
Ans✓✓✓ 25e-^(.693/30)(6.5)=21.5mci
An image on a radiograph measures 12 cm. The source-to-object
distance is 90 cm and the source-to-film distance is 110 cm. The actual
size of the object is: Ans✓✓✓ 12x(90/110)= 9.8cm
An optimized plan consists of three fields weighted 1.0 : 0.5 : 0.5 at the
isocenter. The plan was normalized to the isocenter and the radiation
oncologist prescribed 4500 cGy in 25 fractions to the 95% isodose line.
What daily dose will be delivered to the isocenter? Ans✓✓✓
4500/25=180/.95=189/2=95:47:47
An unsealed chamber used as a secondary dosimeter gives a reading of
101.0 for a 100 monitor unit exposure at a field size of 10 cm2. If the
ambient temperature is 25 degrees C and the atmospheric pressure is
770 mm Hg, the corrected reading per monitor unit is: Ans✓✓✓
(273+25)295 x (760/770)=.997x101=100.697/100=1.007
Based on clinical experience with total body irradiation (TBI) before
bone marrow transplantation and hemibody irradiation, the most
important dose-limiting tissue is the: Ans✓✓✓ Lung
For a 4-field box technique, which of the following will deliver the
lowest dose to the femoral heads? Ans✓✓✓ 100 source-to-surface
distance (SSD), equal dmax dose to all fields
A 20 cm diameter patient is treated to midplane by using a 100 cm
source-to-axis distance (SAD) technique. If the same patient is then
treated by using a 100 cm source-to-surface distance (SSD) technique,
the monitor units would increase by approximately: Ans✓✓✓
(110/100)^2=1.21= 21%
A dose of 30 Gy in 10 fractions to a depth of 7 cm from a 6 MV single
posterior field using an isocentric technique at 100 cm source-to-axis
distance (SAD). The patient was incorrectly treated five times at 100 cm
source-to-surface distance (SSD). The approximate dose at 7 cm after
five fractions is: Ans✓✓✓ 3gyx(100/107)^2=2.6x5=13.1gy
A patient is inadvertently treated using the 18 MV photon energy
instead of the prescribed 6 MV. The prescribed dose is 200 cGy to 5 cm
depth at 100 cm SSD. If the calculated monitor units (MUs) were
delivered, what is the dose delivered to 5 cm depth? Ans✓✓✓
200/.970x.994x.866=239mu
239=x/.981x.989x.965=224cgy
According to the American Association of Physicists in Medicine (AAPM)
Task Group Report #142, the constancy of dose per monitor unit for
photon beams of a linear accelerator should be checked: Ans✓✓✓
daily.
, After 6.5 years, the activity of a 25 mCi Cesium-137 source will be:
Ans✓✓✓ 25e-^(.693/30)(6.5)=21.5mci
An image on a radiograph measures 12 cm. The source-to-object
distance is 90 cm and the source-to-film distance is 110 cm. The actual
size of the object is: Ans✓✓✓ 12x(90/110)= 9.8cm
An optimized plan consists of three fields weighted 1.0 : 0.5 : 0.5 at the
isocenter. The plan was normalized to the isocenter and the radiation
oncologist prescribed 4500 cGy in 25 fractions to the 95% isodose line.
What daily dose will be delivered to the isocenter? Ans✓✓✓
4500/25=180/.95=189/2=95:47:47
An unsealed chamber used as a secondary dosimeter gives a reading of
101.0 for a 100 monitor unit exposure at a field size of 10 cm2. If the
ambient temperature is 25 degrees C and the atmospheric pressure is
770 mm Hg, the corrected reading per monitor unit is: Ans✓✓✓
(273+25)295 x (760/770)=.997x101=100.697/100=1.007
Based on clinical experience with total body irradiation (TBI) before
bone marrow transplantation and hemibody irradiation, the most
important dose-limiting tissue is the: Ans✓✓✓ Lung
For a 4-field box technique, which of the following will deliver the
lowest dose to the femoral heads? Ans✓✓✓ 100 source-to-surface
distance (SSD), equal dmax dose to all fields
