Week 4TBI and 3D conformal

Question 1

TBI tx follwing statements true

Answer 1

Dosimetry is designed to give superficial dose =/> 90% of the prescribed dose
Treats leukemia, anaplastic anemia, lymphoma, multiple myeloma, autoimmune diseases, and inborn errors of metabolism

Question 2

TBI: 4 structures that are shielded in lats

Answer 2

Head, neck, lungs, legs

Question 3

Isodose normalizing:

Answer 3

If renorm to 92%–then orignal value/.92 becomes new isodose value at that line
Renorm to 105%–then value/1.05 becomes that lines new value

Question 4

TCP

Answer 4

Tumor control probability=probability that tumor is eradiacated or controlled at a prescribed dose

Question 5

NTCP

Answer 5

Normal tissue complication probability=the liklihood that normal structures will experience complication at a rx dose

Question 6

Cumulative DVH

Answer 6

The volume of an organ receiving a dose within a specified dose interval

Question 7

Differential DVH

Answer 7

Volume receiving a percent of dose

Question 8

3 main uncertainties of 3D planning

Answer 8

Clinical-limitations in visualizing microscopic extent of tumor, patients may vary in response to tx.
Physical-variation in target position due to breathing, change in weight, volumes in organs (bladder, rectum, stomach),
Dosimetric-calculation models may not be exactly correct

Question 9

TBI tx delivery=beam energy

Answer 9

can be ap/pa or laterals-more thicness variation with lats
6MV<35cm sep
>6MV for >35cm separations
Lats: Prescription at midpoint of body (umbilicus), comps for head, neck, lungs, legs
AP/PA: standing or lying down on side, can shield lungs and brain (with blocks), and boost with e-

Question 10

TBI tx delivery =dose buildup

Answer 10

normal dose build up does not apply due to extended distances (+/-400cm SAD)
Bolus or beam spoiler used to build up surface dose to atleast 90% of rx dose. 1-2 cm thick acrylic, placed as close to the patient as possible

Question 11

Dosimetry of TBI

Answer 11

Direct output of calibration can be performed in a water phantom (40 x 40 x 40 cm)
0.6 cm3 Farmer-type ionization chamber
Fixed TBI distance
Max field size
Chamber depth is varied by moving the chamber and the phantom while keeping the source to chamber distance constant.
Generate a table of output factors (dose/MU) as a function of depth
Then measure midline depth for each patient

Question 12

TBI: In VIVO measurements

Answer 12

Recommend that in vivo measurements be performed on the first 20 or so patients.
TLDs & bolus
Compare measured doses with expected doses
±5% agreement is good (±10% is acceptable)

Question 13

TBI Rx:

Answer 13

Most common dose schedule for myeloablative TBI:
12 to 15 Gy given in 8 to 12 fractions over 4 days, with 2 to 3 treatments daily.
Doses > 15 Gy have been shown to decrease relapse rate, but also increase the incidence of graft vs. host disease and decrease 2-year survival.
Dose rates are often 6 to 15 cGy/min
dose rates < 20 cGy/min help reduce complications