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MDCB Concepts Latest Updated 2024/2025 with Complete Solutions
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MDCB Concepts Latest Updated 2024/2025 with Complete Solutions Hounsfield Units - Correct Answer A measure of the beam attenuation capability of a specific structure. Also called pixel values, density numbers or CT numbers. HU = 1000 · [µ(tissue) - µ(water)] ÷ µ(water) bone ∼ 1000 tissue...

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  • September 7, 2024
  • 21
  • 2024/2025
  • Exam (elaborations)
  • Questions & answers

Subjects

  • hounsfield units
  • using inverse square law
  • factors in calculating monitor units
  • sad set up finding dose at 2nd point of calculati

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MDCB Concepts Latest Updated 2024/2025
with Complete Solutions
Hounsfield Units - Correct Answer A measure of the beam attenuation capability of a specific
structure. Also called pixel values, density numbers or CT numbers.

HU = 1000 · [µ(tissue) - µ(water)] ÷ µ(water)

bone ∼ 1000

tissue ∼ 0

air ∼ -1000



Factors in Calculating Monitor Units - Correct Answer output factor

scatter factors (Sc, Sp, Scp)

inverse square factor

depth dose factor (PDD, TMR)

off-axis factor

transmission factor (tray, wedge)



MU = Rx Dose ÷ Final Output



Using Inverse Square Law - Correct Answer Relationship between intensity and the distance from the
source: d₁² x I₁ = d₂² x I₂

ex: Output at 1cm = 160 cGy

Output at 100cm = ?

(1cm)²(160cGy) = (100cm)²I₂

I₂ = 0.016cGy



SAD Set Up: Finding Dose at 2nd point of calculation using TMRs - Correct Answer D1 = D2 x
(TMR1/TMR2) x (SAD2/SAD1)²



Definition of a Wedge Angle - Correct Answer the angle between an isodose curve at specified depth
and a line perpendicular to the CAX at the same specified depth

,Dose Computation Algorithms - Correct Answer Correction-Based: based on measured data in a
phantom and applied correction factors (attenuation, scatter, f/s, shape, geometry, inverse square
law, etc)

Model-Based: based on physical model simluating actual transport of scattered photons and
electrons: ie. Convolution-Superposition and Monte Carlo....

Convolution-Superposition: simulates primary photon track and calculates separately scatter
photons/electrons

Monte Carlo: simulates transport of millions of photons and particles through matter using
fundamental laws of physics (requiring long computation time and power)



Proton Radiation Therapy - Correct Answer - High LET means minimal dose dumping along path until
the velocity decreases enough to dump the rest of the dose in a small area (narrow bragg peak). Brag
peak can be spread out (SOBP) across target volume by adjusting beam energy.

- protons increase cell kill over xrays and has superior dose distribution and critical structure sparing.

- protons produced in cyclotron.

- stopping power is inversely proportional to velocity squared.



TBI Treatment - Correct Answer - used to condition the body for bone marrow transplantations for
diseases like leukemia.

- use photons to achieve ±10% uniformity of dose using spoiler and TBI filters, prescribed to
midplane at the umbilicus.

- 6 fractions BID is per usual

- pneumonitis (lungs) are the dose limiting factor

- dose validations are made using diodes, TLDs



Tissue Compensators - Correct Answer sometimes surface irregularities gives rise to unacceptable
non-uniformity in target or excessive dose in critical structures, requiring the use of tissue
compensators like:

- wedges

- multiple fields

- bolus: to even irregular patient contours

- compensators: preserves skin sparing effect whereas bolus does not

- blocked for some fractions

, Physical Penumbra of MLCs vs Cerrobend vs Jaws - Correct Answer Physical Penumbra of MLCs are
wider than both jaws and cerrobend blocks



CNS Planning: Cranial Field Orientation (Formula, given Spine Field Settings) - Correct Answer
θ(collimator) = arc tan (½ L₁/SSD)

θ(couch) = arc tan (½ L₂/SSD)

where

SSD is with regard to Spine Field

L₁ is length of PA Spine Field

L₂ is length of Cranial Field (sup to inf)



TAR, TPR, TMR, SAR - Correct Answer TAR = tissue air ratio = D(d)/D(air)

TPR = tissue phantom ratio = D(d)/D(ref)

TMR = tissue maximum ratio = D(d)/D(dmax)

SAR = scatter air ratio = TAR(d,r) - TAR(d,0) where r is field size



(4) Personnel Monitoring and how they work - Correct Answer 1. film badges: consists of film
between metal filters. metal filters are used to determine the quality of the radiation absorbed, and
the film is read using optical density proportional to the dose received.

2. TLD's: thermoluminscent dosimeter; uses lithium fluoride (LiF) crystal that stores energy by
trapping electrons, which can be read by heating the crystals to emit light photons.

3. OSL's: optically stimulating luminescence; uses aluminum oxide with carbon that stores energy in
the crystal, measured by shining a green laser at crystal and measure intensity of blue light.

4. pocket ionization chambers: used by charging the device, and looking through the eyepiece to
note amount of charge lost upon irradiation.



Pinnacle Acronyms: ODM, SVD, TPB - Correct Answer ODM: opening density matrix - use for very
small fields or machines with MLCs <5mm width.

TPB: Todd's/Tradiational Pencil Beam - a method of beam optimization

SVD: single value decomposition - a method of beam optimization similar but superior to TPB



V vs eV - Correct Answer V: polyenergetic energy spectrum

eV: monoenergetic energy

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