L 22: SBRT + TBI + Fetal Dose

Question 1

Lethal fetal dose

Answer 1

• 3.6-5 Gy to the abdomen during embryonic or organogenesis phase (8-56 days of pregnancy)

Question 2

< 1Gy fetal dose

Answer 2

Sterility in males and females once they grow

Question 3

Principle dose outside of PTV for pregnant patient, where the fetus can get the dose form

Answer 3

1. Scatter from head of LINAC
2. Scatter from collimators : 20-40% of total peripheral dose
3. Scatter from patient
   1+2 is comparable to point 3

Question 4

Fetal Dose Threshold

Answer 4

• < 0.05 Gy - Low risk
• 0.05-0.1 Gy- Uncertain Risk
• 0.1-0.5 Gy 1st trimester sig risk
• > 0.5Gy high risk all trimesters

Question 5

Intra-Operative RT

Answer 5

TG-72
Needs shielded room
Max energy - 10-12 MeV

Question 6

Cardiac Pace Makers

Answer 6

They can fail due to radiation damage
Imp to keep the dose lower than 10 Gy to prevent failure & 2 Gy to prevent functional damage

Question 7

TBI

Answer 7

• To destroy bone marrow and tumor cells and cause immunosupression prior to transplant to avoid rejection
• Only 6 MeV
• If the patient’s thickness is <35cm then SSD=300cm can be used.
• Peripheral dose should not be more than >110% of the midline dose
• Bolus or beam spoiler is used to bring the surface/skin dose to 90% of the prescribed dose.

Question 8

Tissue lateral effect

Answer 8

Thicker patients and lower energy treated with parallel opposed beams can lead to higher dose to subcutaneous tissues compared to mid point dose

Question 9

TBI Beams

Answer 9

• SSD= 300cms
• 6 MV beams
• Parallel opposed beams
• 110% to midpoint dose

Question 10

SBRT

Answer 10

• TG - 101
• Dose > 5Gy / fraction
• 1-5 fractions
• Well circumscribed tumors upto 5cm diameter
• Needs 15 cm sup and inf at CT-Sim
• Use 4D CT for tumors that move & need ITV
• No CTV margins GTV-PTV
• Isotropic grid of 2mm
• High dose gradiets near the periphery
• One qualified physicist is present from the start to end of 1st treatment fraction.

Question 11

Dose uniformity for TBI

Answer 11

+/- 5-10%

Question 12

TBI Dose

Answer 12

<10cGy/ Min
ASTRO says < 20cGy / Min
Typical SSD is 300-400cms
This is prescribed to mid-plane

Question 13

Advantage of bilateral beams than AP/PA for TBI is

Answer 13

Lungs blocks are not required

Question 14

TBI, characteristics for extended SSD

Answer 14

• They can cover the entire patient
• They have increased dose uniformity
• They have decreased dose rate
• they have increased treatment time

Question 15

Min distance from Linac

Answer 15

Question 16

Beam spoiler in TBI use

Answer 16

To increase the skin dose to treat blood vessels in the skin

Question 17

Total skin electron therapy
patient stands in different positions to

Answer 17

• reduce x-ray contamination
• Low energy electrons are used
• Beam arrangement is typically at the central beam axis
• Central beam axis = fluence of contaminant x-rays is greatest
• Gantry is angled in sucha way that the central axis is above patients head or below the patient’s feet.
• Depending on location of the disease, hands may or maynot be shielded

Question 18

Beam spoiler brings the surface dose in TBI to

Answer 18

90% of the prescribed dose

Question 19

Techniques to reduce fetal dose

Answer 19

• Shielding above diaphragm and on lower extremities
• Bridge over patient, 5-7cm lead or 6-8cm cerroband