QA, Radiation Protection, Treatment & Delivery

Question 1

What are the 3 categories of dose computational algorithms?

Answer 1

Correction based Model based Direct Monte Carlo

Question 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?

Answer 2

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

Question 3

What is a convolution-superimposition computational algorithm?

Answer 3

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

Question 4

What is a direct Monte Carlo computational algorithm ?

Answer 4

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

Question 5

What defines intensity modulated RT?

Answer 5

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

Question 6

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

Answer 6

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

Question 7

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

Answer 7

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

Question 8

What distinguishes VMAT from IMAT? Which is more efficient?

Answer 8

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

Question 9

What distinguishes SRS from SRT?

Answer 9

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

Question 10

What dose rate/min defines HDR brachytherapy?

Answer 10

20cGy/min or higher dose rate

Question 11

What is the dose rate/min of LDR brachytherapy?

Answer 11

0.5-2cGy/min

Question 12

What is the preferred radioisotope for HDR brachytherapy?

Answer 12

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

Question 13

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

Answer 13

NRC - Nuclear regulatory commission (both)

Question 14

Which agency regulates control of radioactive materials

Answer 14

NRC - Nuclear regulatory commission (both)

Question 15

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

Answer 15

States regulate operation FDA regulates manufacture

Question 16

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

Answer 16

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

Question 17

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

Answer 17

Absorbed dose at isocenter per week (Gy/week)

Question 18

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

Answer 18

Fraction of time primary beam is directed at barrier in question

Question 19

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

Answer 19

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

Question 20

What do the NCRP/ICRP regulate?

Answer 20

trick question - they make recommendations only!

Question 21

What does the IAEA regulate?

Answer 21

trick question - they make recommendations only!

Question 22

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

Answer 22

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)

Question 23

What is dose equivalent? What are the units?

Answer 23

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

Question 24

What is effective dose equivalent?

Answer 24

Reflects organ sensitivity for given dose equivalent (Sv)

Question 25

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

Answer 25

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

Question 26

What is the D_2%? D_98%?

Answer 26

D_2% = near-maximum dose; minimum dose delivered to hottest 2%

D_98% = near-minimum dose, dose received by 98% of PTV