CMD Treatment Planning & Dosimetry 5

Question 1

The radiobiological concept of 'tumor control probability' (TCP) is most sensitive to which parameter?

Accepted Answer

Clonogen density within the tumor

Answer

TCP is exponentially sensitive to the number of surviving clonogens; the clonogen density (number per tumor volume) is the dominant parameter determining cure probability.

Question 2

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?

Accepted Answer

120 Gy

Answer

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.

Question 3

Which dose constraint is most commonly cited for the spinal cord to avoid radiation myelopathy?

Accepted Answer

Maximum dose < 54 Gy in conventional fractionation

Answer

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%.

Question 4

When optimizing an IMRT plan, the optimizer increases fluence in a region of the beam. This is primarily done to:

Accepted Answer

Compensate for reduced tissue attenuation in that beam direction

Answer

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.

Question 5

In electron beam therapy, the 'practical range' (Rp) in tissue is clinically important because it represents:

Accepted Answer

The depth beyond which the electron beam has negligible dose contribution

Answer

Rp is the depth at which the electron beam dose approaches zero, defining the effective maximum depth of treatment and protecting deeper structures.

(CMD) Certified Medical Dosimetrist Practice Test

(CMD) Certified Medical Dosimetrist Practice Test

CMD Treatment Planning & Dosimetry 5