Grad class notes II

Question 1

pulsed dose rate

Answer 1

low dose rate, but radiation is delivered for only short part of the hour
-nurse can attend to patient during breaks

Question 2

BED

Answer 2

BED = nd(1+g*d/(alpha/beta))
-g is repair function
-depends on 1/2 time for cell repair and fractionation

-if HDR or EBRT, g = 1

Question 3

BED for LDR

Answer 3

-includes g factor
-BED = Rt[1+(2R/(mu *alpha/beta)) * (1- ((1-e^-mut)/mut))]
t= implant time for each LDR fraction
R= dose rate
mu = ln2/T1/2 (T1/2 is repair time of tissue)

Question 4

BED for PDR

Answer 4

-repair between successive fractions and during the whole fraction

Question 5

why do we prefer pure gamma emitters in brachy?

Answer 5

absence of alpha (don’t penetrate) and beta emissions simplifies geometry and reduces encapsulation thickness required

Question 6

why do we care about specific activity in brachy?

Answer 6

the higher the specific activity, the smaller the source can be
-determines max possible dose rate. Higher dose rates may be desirable to reduce patient treatment times

Question 7

why do we care about energy of emitted radiation in brachy?

Answer 7

-affects tissue penetration
-determines dose distribution
-important for radiation protection

Question 8

effect of photon energy on absolute dose rate

Answer 8

E > 200 keV - all sources have same dose rate constant regardless of medium
E < 100 keV - photoelectric effect results in up to 2-fold heterogeneity corrections (Z dependent)

Question 9

apparent activity

Answer 9

Sk/ (tau * (W/e))

Question 10

air kerma strength

Answer 10

Sk = K * d^2

K is air kerma rate in vaccuo due to photons with energy greater than 5 keV
-cutoff designed to exclude low energy photon-emitting radiation (would not contribute to dose in tissue beyond 0.1 cm but would increase K)

Question 11

NIST calibration standard for brachy source > 2 mCi

Answer 11

-series of graphite cavity ion chambers
-source of unknown strenght is calibrated by intercomparison with the working standard using 2.5 L spherical aluminum ion chamber in air positioned at 1 m

Question 12

NIST calibration standard for weaker source

Answer 12

-WAFAC- directly determine strength of unknown strength source
-Ti characteristic xrays are filtered out

Question 13

Sk for well-type ion chamber and electrometer

Answer 13

Sk = ImassPionpolPtpPelecNsk

Question 14

difference between TG43 and previous models

Answer 14

TG43 uses dose rate constants and other dosimetric parameters that depend on the specific source design, and are directly measured or calculated for each source design

Question 15

issue with Sv integral

Answer 15

only looked at IS law and some attenuation
-did not consider the media

Question 16

how is dose rate constant determined?

Answer 16

-measured or Monte Carlo

Question 17

what does dose rate constant depend on?

Answer 17

-internal design of source
-experimental methodology used by primary standard to realize Sk

Question 18

3 things that can cause anisotropy

Answer 18

-self filtration
-oblique filtration of primary photons through encapsulating materials
-scattering of photons in the medium

Question 19

dependency of anisotropy factor

Answer 19

F decreases as:
-r decreases
-theta goes to 0 or 180 degrees
-encapsulation increases
-photo energy decreases

Question 20

total dose delivered

Answer 20

1.44 * T1/2 * initial dose rate

Question 21

other dose limiting structures in manchester technique

Answer 21

< 40% of total dose to A can be delivered to ovoids
-rectum and bladder must get less < 80% of dose to pt A

Question 22

where is rectal pt

Answer 22

5 mm from most posterior aspect of system/packing

Question 23

where is bladder pt?

Answer 23

most inf pt of foley balloon surface

Question 24

why use MR vs CT for brachy planning?

Answer 24

MR can differentiate between uterus, periuterine tissues, and tumor whereas CT cannot
-both require special applicators