L 21: SRS, GK, PSI

Question 1

Winston-Lutz Test

Answer 1

checks with the small SRS cone for
* gantry isocentricity
* Laser alignment to the isocenter

Question 2

Picket fence test
Garden fence test

Answer 2

Checks the MLC accuracy

Question 3

Homogenity Index

Answer 3

Max PTV Dose / Prescription Dose

Question 4

Conformity Index

Answer 4

• Volume encompassed by prescription / target volume
• Ideally between 1.0-2.0

Question 5

Gradient Index

Answer 5

volume of 50% prescription dose / Volume of total prescription dose
* also known as the dose fall off outside the target

Question 6

SRS elements

Answer 6

• Planning with high spatial resolution (MRI 1mm slices)
• Using narrow multiple beams, multi non-coplaner isocentric arc (improves dose conformality)
• Multiple isocenters, dyanmically shaping the field during arc roatation with mini MLCs
• Rigid immobilization
• Optimization is very important
• Highly accurate target (+/- 1mm)
• Isocenter displacment within +/- 0.1- 0.3 mm.

Question 7

Trigeminal Neuralgia

Answer 7

dose gradient can be high of 16Gy/mm

Question 8

Focused Cones:

Answer 8

• Reduces Penumbra
• long, narrow metal cones placed below the jaws to bring collimator closer to the patient surface: extends SDD so reduces teh penumbra
• Isocenter remains centered within the cone
• Not used now with mini MLCs

Question 9

Pitch

Answer 9

Question 10

Tilt

Question 11

Yaw

Answer 11

Question 12

Roll

Answer 12

Question 13

SBRT CT-Sim

Answer 13

Include 15 cms above and below

Question 14

SRS

Answer 14

• Single fraction
• Intracranial
• Narrow multiple beams, non co-planerIsocentric arcs
• High degree of dose conformality
• Multiple isocenters
• Mechanical accuracy +/- 1mm error acceptable

Question 15

SRT

Answer 15

• Multiple fractions
• Intracranial

Question 16

Types of frame
Not important for exam

Answer 16

• Leksell: Used here for GK
• Richard-mundinger
• Todd-wells
• Brown-Robert-wells
• Gill- Thomas- Cosman: Uses bite block

Question 17

CO sources in GK

Answer 17

192-201 sources
6000Ci

Question 18

Gamma Knife

Answer 18

• More than 1 isocenter
• multiple arcs
• can be with frame or frameless
• Dosimetry: %DD, TMR, Cross beam profiles, output factors

Question 19

GK Questions

Answer 19

• Source: Co-60
• Half life: 5.26 years
• What energies: 1.17Mev -1.33 Mev avg 1.25 MeV
• What collimator sizes: 4/8/14/18: old machine; 4/8/16 mm new machine
• How many sealed sources: 192
• Source to focus distance: 40..3 cms
• Mechanical precision at isocenter: +/- 0.3mm
• Plugs: Made of tungsten alloy, 6cm thick, old machine, external
• Co decays to Ni and gives out gamma-rays

Question 20

PSI (Prostate seed Implant)

Answer 20

• Can be used for early stage , confined to prostate gland, no EPE or SVI
• some patients might need PSI + EBRT
• Transperineal approach under the guidance of US.
• Inter-Seed placing: 1cm
• Inter-Needle spacing: 1cm
• Abstain from sexual activity for 2 weeks/use condoms
• Vendor certificate: +/- 10%
• Sample check: +/- 5%
• Basic requirement to release a permanent implant patient from the hospital does not exceed ** 0.5 rem (500mrem)**
• No contact with preg women or young children for 2 months
• No sexual intercourse in 2 weeks, use condoms

Question 21

Temporary PSI

Answer 21

• Radio isotopes with relatively longer half life
• sufficient dose rate to deliver prescribed dose in 3-4 days

Question 22

Permanent PSI
More popular

Answer 22

• Radio isotopes with shorter half life
• Photons of low energy
• I-125 or Pd-103

Question 23

Prostate Nomogram

Answer 23

• Prostate volume: using Ultrasound, 5mm intervals
• # of seeds
• Strength of the seed
• Interseed spacing 1cm
• interneedle spacing 1cm

Question 24

Seed Strength

Answer 24

• I-125: 0.3 mCi &
  Pd-103: 1.7 mCi

Question 25

Co 60

Answer 25

1% per month decay rate: constant

Question 26

Light and radiation field coincidence

Answer 26

* 2mm or 1% on symmetric side * 1mm or 1% on asymmetric side * Same for LINAC and SBRT/SRS

Question 27

Answer 27

Question 28

SRS checks routinely done

Answer 28

* TReatmetn planning * Winston-Lutz test * daily output checl * safety interlock check

Question 29

SRS check NOT done routinely

Answer 29

Cone output factor measurement

Question 30

SRS for mutliple targets

Answer 30

We are worried about the distance from isocenter to each target as we are worried about set up errors

Question 31

Monthly checks for SRS/SBRT tolerance

Answer 31

SRS 1mm SBRT 1.5 mm

Question 32

SRS/SBRT SRS frame or IGRT should be performed

Answer 32

Initial commissioning and annualy

Question 33

Dosimeters used for GK

Answer 33

* Ion chambers: **most precise**, least energy dependent * Film: **best spatial resolution** * TLD: **small size** * Diodes: **small size**