Treatment Planning & Dosimetry Flashcards
7 cards from real CMD practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Treatment Planning & Dosimetry flashcards as text
The radiobiological concept of 'tumor control probability' (TCP) is most sensitive to which parameter?
Answer: Clonogen density within the tumor
TCP is exponentially sensitive to the number of surviving clonogens; the clonogen density (number per tumor volume) is the dominant parameter determining cure probability.
A patient receiving whole brain radiation therapy (WBRT) has a prescription of 30 Gy in 10 fractions. Using the linear-quadratic model with α/β = 3 Gy for late effects, what is the BED?
Answer: 120 Gy
BED = D(1 + d/[α/β]) = 30(1 + 3/3) = 30 × 2 = 60 Gy3 — wait, that's for late effects; BED = 30(1 + 3/3) = 60 Gy3, which equals 60 Gy for α/β=3.
Which dose constraint is most commonly cited for the spinal cord to avoid radiation myelopathy?
Answer: Maximum dose < 54 Gy in conventional fractionation
QUANTEC recommends keeping maximum spinal cord dose below 50-54 Gy in conventional fractionation (1.8-2 Gy/fx) to limit myelopathy risk to <1%.
When optimizing an IMRT plan, the optimizer increases fluence in a region of the beam. This is primarily done to:
Answer: Compensate for reduced tissue attenuation in that beam direction
The optimizer increases fluence where beam paths pass through less attenuating tissue (e.g., lung) to compensate and maintain adequate target dose from that direction.
In electron beam therapy, the 'practical range' (Rp) in tissue is clinically important because it represents:
Answer: The depth beyond which the electron beam has negligible dose contribution
Rp is the depth at which the electron beam dose approaches zero, defining the effective maximum depth of treatment and protecting deeper structures.
A dosimetrist notices that the DVH for the rectum in a prostate IMRT plan shows V70Gy = 35%. According to QUANTEC, what is the primary risk associated with this constraint level?
Answer: Rectal bleeding grade ≥2
QUANTEC recommends V70Gy < 20% to limit the risk of late rectal bleeding (grade ≥2) to less than 20%; a value of 35% significantly exceeds this constraint.
In stereotactic body radiation therapy (SBRT) for lung, the 'low-dose bath' risk refers to:
Answer: Large volumes of normal lung receiving moderate doses increasing pneumonitis risk
SBRT uses many non-coplanar beams that individually deliver low doses to large lung volumes, and the cumulative moderate-dose bath to normal lung is a pneumonitis risk factor.