MDCB EXAM 2 questions with verified answers
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✓✓✓ 13.1 Gy
A four-field isocentric isodose plan normalized to 100% at the isocenter
shows the entire target volume encompassed by the 97% isodose line.
The target volume is to receive 65 Gy; however, it is required that the
dose be prescribed at the isocenter. The dose prescription should be
written for: Ans✓✓✓ 67 Gy to the isocenter
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?
18 MV unit 6 MV unit
tray factor = 0.981 tray factor 0.970
output factor = 0.989 cGy/MU output factor = 0.994 cGy/MU
%DD (5 cm) = 96.5% %DD (5 cm) = 86.6% Ans✓✓✓ 225 cGy
A patient is treated to the left chest wall with electron arc therapy. In
order to sharpen the distribution at the medial and lateral field borders,
the medical dosimetrist should: Ans✓✓✓ shape the field on the skin
surface with lead strips
, A simulation is done for a treatment to be given isocentrically, source-
axis distance 80 cm, and with a field size of 5 cm x 12.5 cm. The image
of the field size on the radiograph measures 6 cm x 15 cm. What is the
actual size of an object placed on the patient's surface, 72 cm from the
source, if the image of the object on the radiograph measures 4 cm?
Ans✓✓✓ 3 cm
According to the American Association of Physicists in Medicine (AAPM)
standards (TG40), when periodic linear accelerator quality assurance
procedures are performed, which of the following is the acceptable
tolerance for an optical distance indicator? Ans✓✓✓ ± 2.0 mm
According to the American Association of Physicists in Medicine (AAPM)
standards (TG40), when periodic linear accelerator quality assurance
procedures are performed, which of the following is the acceptable
tolerance for field size indicators? Ans✓✓✓ ± 2.0 mm
According to the National Council on Radiation Protection and
Measurements (NCRP) guidelines (#116), the annual equivalent dose
limit for the hands of an occupational worker in 1 year is: Ans✓✓✓ 500
mSv (50 rem)
After 6.5 years, the activity of a 25 mCi Cesium-137 source will be:
Ans✓✓✓ 21.5 mCi
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✓✓✓ 13.1 Gy
A four-field isocentric isodose plan normalized to 100% at the isocenter
shows the entire target volume encompassed by the 97% isodose line.
The target volume is to receive 65 Gy; however, it is required that the
dose be prescribed at the isocenter. The dose prescription should be
written for: Ans✓✓✓ 67 Gy to the isocenter
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?
18 MV unit 6 MV unit
tray factor = 0.981 tray factor 0.970
output factor = 0.989 cGy/MU output factor = 0.994 cGy/MU
%DD (5 cm) = 96.5% %DD (5 cm) = 86.6% Ans✓✓✓ 225 cGy
A patient is treated to the left chest wall with electron arc therapy. In
order to sharpen the distribution at the medial and lateral field borders,
the medical dosimetrist should: Ans✓✓✓ shape the field on the skin
surface with lead strips
, A simulation is done for a treatment to be given isocentrically, source-
axis distance 80 cm, and with a field size of 5 cm x 12.5 cm. The image
of the field size on the radiograph measures 6 cm x 15 cm. What is the
actual size of an object placed on the patient's surface, 72 cm from the
source, if the image of the object on the radiograph measures 4 cm?
Ans✓✓✓ 3 cm
According to the American Association of Physicists in Medicine (AAPM)
standards (TG40), when periodic linear accelerator quality assurance
procedures are performed, which of the following is the acceptable
tolerance for an optical distance indicator? Ans✓✓✓ ± 2.0 mm
According to the American Association of Physicists in Medicine (AAPM)
standards (TG40), when periodic linear accelerator quality assurance
procedures are performed, which of the following is the acceptable
tolerance for field size indicators? Ans✓✓✓ ± 2.0 mm
According to the National Council on Radiation Protection and
Measurements (NCRP) guidelines (#116), the annual equivalent dose
limit for the hands of an occupational worker in 1 year is: Ans✓✓✓ 500
mSv (50 rem)
After 6.5 years, the activity of a 25 mCi Cesium-137 source will be:
Ans✓✓✓ 21.5 mCi
