PDD - Percentage Depth Dose

Question 1

define PDD

Answer 1

amount of dose available along CA (cental axis) of beam

Question 2

PDD compares what?

Answer 2

absorbed dose at __ depth to a reference depth known as dmax

Question 3

What factors effect PDD DIRECTLY -

Answer 3

1. energy/mA/quality (due to more penetration)
2. field size/shape (due to more scatter)
3. SSD (due to inverse square law)

Question 4

what factors effect PDD INDIRECTLY -

Answer 4

1. depth (due to ATTENUATION)

Question 5

Dosimetric field sizes are defined where?

Answer 5

at 50% isodose line

Question 6

electron therapeutic defined where?

Answer 6

at 90% isodose line

Question 7

scatter occurs as _____ beam interacts with what?

Answer 7

primary beam interacts with collimating jaws

Question 8

scatter will __increase/decrease__ as depth within patient increases

Answer 8

scatter with increase

Question 9

any SSD greater than ___ cm is considered/acts as a Point Source

Answer 9

80

–these

Question 10

as SSD increases what is something that decreases? and vice versa

Answer 10

Dose Rate

Question 11

what is considered a “greater SSD”?

Answer 11

anything over 80 cm

Question 12

what is recommended minimum SSD?

Answer 12

80 cm

Question 13

larger SSDs allow for greater _______ before reaching dmax, where max dose will be deposited

Answer 13

beam buildup

Question 14

describe the distribution of a photon beam dose -

Answer 14

after photon enters patient at skin surface, it begins to build up until it reaches dmax.

after dmax, the photon energy will slowly reduce in intensity due to attenuation.

Question 15

describe Exponential Attenuation

Answer 15

after dmax is reached, the photon energy will slowly reduce in intensity due to attenuation.

Question 16

attenuation is effected by what 2 factors?

Answer 16

1. tissue type
2. density

Question 17

T/F - water will attenuate a beam more than air, but less than bone

Answer 17

TRUE

Question 18

density of LUNG (air)

Answer 18

.25-.33 g/cm^3

Question 19

density of WATER

Answer 19

1.0 g/cm^3

Question 20

density of BONE

Answer 20

1.8 g/cm^3

Question 21

______ is factor used for finding MU of an SSD technique

Answer 21

PDD