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 penumbra of a photon beam is primarily determined by which factor?
Answer: Source size (focal spot)
Geometric penumbra is directly related to the physical size of the radiation source; a larger focal spot produces a wider penumbra at any given distance.
A treatment plan for a right-sided breast cancer uses a physical wedge. The thin end of the wedge should be oriented toward which direction to improve dose uniformity?
Answer: Lateral aspect of the breast
For tangential breast fields, the thin end of the wedge is placed laterally (toward the thicker lateral chest wall) so the wedge compensates for the tissue contour variation.
In radiation therapy planning, the Paddick Conformity Index (PCI) is considered superior to the Shaw Conformity Index because:
Answer: PCI penalizes both geographic miss and spillage outside the target
The Paddick CI equals (TV_PIV²)/(TV × PIV), penalizing plans where the prescription isodose is too large (spillage) or doesn't cover the target (miss), unlike Shaw's simpler ratio.
Which statement correctly describes the difference between a physical wedge and an enhanced dynamic wedge (EDW)?
Answer: EDW uses jaw motion during beam delivery instead of a metal attenuator
An EDW creates a wedge-shaped dose distribution by sweeping one jaw across the field during irradiation, eliminating the need for a physical attenuating wedge.
When calculating the dose to a point at extended SSD using the Mayneord F-factor, which relationship is applied?
Answer: Dose varies with the square of SSD ratio, corrected for scatter changes
The Mayneord F-factor applies an inverse square law correction (SSD ratio squared) and an additional scatter correction factor to relate PDD at one SSD to another.
In TOMO therapy (helical tomotherapy), dose delivery is achieved by:
Answer: A fan beam rotating continuously while the couch translates
Helical tomotherapy delivers dose using a fan beam from a slip-ring mounted linac rotating 360° continuously while the patient couch moves through the bore.
For HDR brachytherapy cervical cancer treatment using a ring-and-tandem applicator, Point A is defined as:
Answer: 2 cm lateral and 2 cm superior to the cervical os along the tandem
Point A is defined as 2 cm superior to the mucous membrane of the lateral vaginal fornix and 2 cm lateral to the tandem, representing the paracervical triangle dose point.