MDCB Practice Questions and Answers
Verified by Experts
When calibrating a cobalt-60 unit, the electrometer reads 50 for a one minute exposure and 47
for four short exposures of 0.25 minutes each. The net irradiation time for 1.0 minute set is
0.98minutes
Scatter Air Ratio (SAR) the ratio of the scatter dose at a given point in the phantom to a
dose in free space at the same point
A patient is supine for treatment of the thoracic spine from a posterior field 7cm wide, 25cm long
at 100cm SAD, 94cm SSD. This field must be matched at the isocenter to a previously treated
posterior spine field 7cm wide and 15cm long also at 100cm SAD, 94cm SSD. The patient's
anterior posterior separation throughout the treatment region is 23cm. Th gap between the fields
measured on a patient's anterior skin is 3.4cm
Tissue Maximum Ratio (TMR) the ratio of dose at a given point in a phantom to the dose
at the same point at depth of maximum in the phantom
,Tissue Maximum Ratio (TMR) may be derived by Dividing the tissue-air ratio (TAR) by
the backscatter factor (BSF)
Parallel opposing field are used to deliver 50Gy in 25Fx to the isocenter, located at mid-depth of
10cm. The TAR at zero field size at 10cm depth is 0.534 and the average SAR after irregular
field calculation is 0.195. The tray factor is 0.939, air dose rate is 219.2cGy/min and the timer
corretion is +0.02 min. Calculate the timer setting for each field 0.69min
When the equivalent squareof a rectange field is calculated, the result is less accurate when the
field has an elongation ratio of 2
To ensure that high dose rate (HDR) brachytherapy procedures are carried out correctly without
any misadministrations, which of the following can be used for quick and reliable quality
assurance in in vivo dosimetry: TLDs, calibration water phantom, diodes, GM counter, or survey
meter? diodes
The tissue-air ratio (TAR) at the depth of maximum electron buildup (dmax) is dependent
on fieldsize and equal to the backscatter factor
Tissue-air ratio (TAR) is dependent on beam energy, depth, and field size
,Which of the following will change the buildup in a high energy beam: use of large field sizes on
a linear accelerator with resulting electron contamination, placement of the diaphragm or
treatment cone greater than 20cm from the skin surface, placement of bolus material on the skin
suface, use of an electron filter use of large field sizes on a linear accelerator with
resulting electron contamination, placement of the diaphragm or treatment cone greater than
20cm from the skin surface, and plaement of bolus material on the skin surface
Backscatter factor is identical to tissue-air ratio (TAR) at a depth of maximum dose
The tissue-maximum ratio (TMR) is used in palce of the tissue-air ratio (TAR) for megavoltage
beams because, as energy increases the dose in free space becomes more difficult to
measure
A dose of 30Gy in 10Fx to a depth of 7cm from a 6MV single posterior field using an isocentric
technique, 100cm SAD. The patient was incorrectly treated five times at 100cm SSD. The
approximate dose at 7cm after five fractions is 13.5Gy
A patient is to be given a 4600cGy midplane dose in 23 fractions by parallel opposed fields to the
mediastinum. The dose to the spinal cord each fraction is 209cGy. If the spinal cord block is
, added to a posterior field only, the dose to the spinal cord each fration is 94cGy. The spinal cord
dose through the treatment course is limited to 4000cGy. How many fractions must the posterior
cord block be inserted? 7 fractions
A plan is chosen for a pelvic tumor which uses a 300 degree arc on a Cobalt-60 treatment unit at
80cm SAD. 4000cGy is prescribed to the 90% isodose in 20 treatments. What is the timer and
arc setting if the air dose rate is 105 cGy/min and the average TAR is 0.538 normalized to the
isocenter 3.93min and 0.21 rev/min
Adding a wedge filter to the primary beam does not significantly alter the central axis
depth dose of a Co-60 beam, increases the TARs of the x-ray beam, and can alter the depth dose
distribution, especially at large depths for high energy photon beams.
Conversion of percentage depth dose to different source surface distances by using the Mayneord
F factor is an approximation because the method does NOT consider Scatter volume
If a testicular seminioma is biopsied throught the scrotum, what changes in the radiatoin fields
should occur The scrotum on the ipsilateral side should be treated, the ipsilateral inguinal
nodes should be treated
Verified by Experts
When calibrating a cobalt-60 unit, the electrometer reads 50 for a one minute exposure and 47
for four short exposures of 0.25 minutes each. The net irradiation time for 1.0 minute set is
0.98minutes
Scatter Air Ratio (SAR) the ratio of the scatter dose at a given point in the phantom to a
dose in free space at the same point
A patient is supine for treatment of the thoracic spine from a posterior field 7cm wide, 25cm long
at 100cm SAD, 94cm SSD. This field must be matched at the isocenter to a previously treated
posterior spine field 7cm wide and 15cm long also at 100cm SAD, 94cm SSD. The patient's
anterior posterior separation throughout the treatment region is 23cm. Th gap between the fields
measured on a patient's anterior skin is 3.4cm
Tissue Maximum Ratio (TMR) the ratio of dose at a given point in a phantom to the dose
at the same point at depth of maximum in the phantom
,Tissue Maximum Ratio (TMR) may be derived by Dividing the tissue-air ratio (TAR) by
the backscatter factor (BSF)
Parallel opposing field are used to deliver 50Gy in 25Fx to the isocenter, located at mid-depth of
10cm. The TAR at zero field size at 10cm depth is 0.534 and the average SAR after irregular
field calculation is 0.195. The tray factor is 0.939, air dose rate is 219.2cGy/min and the timer
corretion is +0.02 min. Calculate the timer setting for each field 0.69min
When the equivalent squareof a rectange field is calculated, the result is less accurate when the
field has an elongation ratio of 2
To ensure that high dose rate (HDR) brachytherapy procedures are carried out correctly without
any misadministrations, which of the following can be used for quick and reliable quality
assurance in in vivo dosimetry: TLDs, calibration water phantom, diodes, GM counter, or survey
meter? diodes
The tissue-air ratio (TAR) at the depth of maximum electron buildup (dmax) is dependent
on fieldsize and equal to the backscatter factor
Tissue-air ratio (TAR) is dependent on beam energy, depth, and field size
,Which of the following will change the buildup in a high energy beam: use of large field sizes on
a linear accelerator with resulting electron contamination, placement of the diaphragm or
treatment cone greater than 20cm from the skin surface, placement of bolus material on the skin
suface, use of an electron filter use of large field sizes on a linear accelerator with
resulting electron contamination, placement of the diaphragm or treatment cone greater than
20cm from the skin surface, and plaement of bolus material on the skin surface
Backscatter factor is identical to tissue-air ratio (TAR) at a depth of maximum dose
The tissue-maximum ratio (TMR) is used in palce of the tissue-air ratio (TAR) for megavoltage
beams because, as energy increases the dose in free space becomes more difficult to
measure
A dose of 30Gy in 10Fx to a depth of 7cm from a 6MV single posterior field using an isocentric
technique, 100cm SAD. The patient was incorrectly treated five times at 100cm SSD. The
approximate dose at 7cm after five fractions is 13.5Gy
A patient is to be given a 4600cGy midplane dose in 23 fractions by parallel opposed fields to the
mediastinum. The dose to the spinal cord each fraction is 209cGy. If the spinal cord block is
, added to a posterior field only, the dose to the spinal cord each fration is 94cGy. The spinal cord
dose through the treatment course is limited to 4000cGy. How many fractions must the posterior
cord block be inserted? 7 fractions
A plan is chosen for a pelvic tumor which uses a 300 degree arc on a Cobalt-60 treatment unit at
80cm SAD. 4000cGy is prescribed to the 90% isodose in 20 treatments. What is the timer and
arc setting if the air dose rate is 105 cGy/min and the average TAR is 0.538 normalized to the
isocenter 3.93min and 0.21 rev/min
Adding a wedge filter to the primary beam does not significantly alter the central axis
depth dose of a Co-60 beam, increases the TARs of the x-ray beam, and can alter the depth dose
distribution, especially at large depths for high energy photon beams.
Conversion of percentage depth dose to different source surface distances by using the Mayneord
F factor is an approximation because the method does NOT consider Scatter volume
If a testicular seminioma is biopsied throught the scrotum, what changes in the radiatoin fields
should occur The scrotum on the ipsilateral side should be treated, the ipsilateral inguinal
nodes should be treated
