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Flashcards in QA, Radiation Protection, Treatment & Delivery Deck (27)
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1

What are the 3 categories of dose computational algorithms?

Correction based Model based Direct Monte Carlo

2

Correction-based dose computational algorithms are based primarily on measured data in a water phantom, and may correct for contour irregularity, scatter/volume, and tissue heterogeneity. In which situations is the accuracy for correction-based algorithms limited?

Lung & tissue interfaces - situations where electronic equilibrium is not fully established

3

What is a convolution-superimposition computational algorithm?

separately considers transport of primary photons and scatter photon and electrons emerging from primary photon interaction

4

What is a direct Monte Carlo computational algorithm ?

simulates transport of millions of photons and particles through matter using fundamental physics to determine probability distributions of individual interactions of photons and particles -most accurate treatment planning algorithm, use is limited by computational time

5

What defines intensity modulated RT?

nonuniform fluence delivered from any given position of the treatment beam to optimize composite dose distribution

6

Which two computational methods of inverse planning can be used to generate IMRT plans?

analytic - desired dose distribution is inverted using a back projection algorithm iterative - beamlet weights are iteratively adjusted to maximize the value of a cost function

7

In dynamic MLC (sliding window) IMRT, which leaf should provide the intensity modulation if the intensity profile is positive (increasing fluence)? Negative?

Trailing leaf - increasing fluence/positive intensity profile gradient (leading leaf should move at max speed) Leading leaf - decreasing fluence (trailing leaf should move at max speed

8

What distinguishes VMAT from IMAT? Which is more efficient?

VMAT = *variation in dose rate* while gantry rotates and MLC leaves move IMAT = rotational IMRT, no variation in dose rate -- inefficient and often requires several arcs

9

What distinguishes SRS from SRT?

SRS = single-fraction both refer to treatment of intracranial lesions, traditionally using a stereotactic apparatus

10

What dose rate/min defines HDR brachytherapy?

20cGy/min or higher dose rate

11

What is the dose rate/min of LDR brachytherapy?

0.5-2cGy/min

12

What is the preferred radioisotope for HDR brachytherapy?

192-Ir (iridium-192) - higher specific activity, lower photon energy disadvantage = short half-life

13

Which agency regulates the limits of radiation exposure to the public? To radiation workers?

NRC - Nuclear regulatory commission (both)

14

Which agency regulates control of radioactive materials

NRC - Nuclear regulatory commission (both)

15

Which agency regulates operation of radiation-producing machines? Manufacture of these machines?

States regulate operation FDA regulates manufacture

16

What are the three factors to consider in designing vault shielding (W, U, T)?

W - Workload: absorbed dose @ isocenter per week U - Use factor: fraction of time primary beam is directed at barrier in question T - Occupancy factor: fraction of time an individual is expected to be in the area while beam is on

17

What is the definition of workload (W) in regards to vault shielding?

Absorbed dose at isocenter per week (Gy/week)

18

What is the definition of use factor (U) in regards to vault shielding?

Fraction of time primary beam is directed at barrier in question

19

What is the definition of occupancy factor (T) in regards to vault shielding?

T - Occupancy factor: fraction of time an individual is expected to be in the area while beam is on

20

What do the NCRP/ICRP regulate?

trick question - they make recommendations only!

21

What does the IAEA regulate?

trick question - they make recommendations only!

22

What is exposure? How does this differ from absorbed dose? What are their units?

Exposure - quantity of electric charge liberated per unit mass of AIR (C/kg) Absorbed dose - quantity of energy absorbed per unit mass in material (ie tissue) (Gy)

23

What is dose equivalent? What are the units?

Reflects the biological effect on tissue for a given absorbed dose deposited by a given type of radiation (Sv)

24

What is effective dose equivalent?

Reflects organ sensitivity for given dose equivalent (Sv)

25

What is the formula for cumulative (lifetime) dose limit recommended by the NCRP?

10mSv x age (yrs) ie 60yr old = 600mSv lifetime dose

26

What is the D2%? D98%?

D2% = near-maximum dose; minimum dose delivered to hottest 2%

D98% = near-minimum dose, dose received by 98% of PTV

27