L 18: Particle therapy

Question 1

Daily QA for Protons

Answer 1

• Dosimetry and beam delivery
• Proton beam output check for a defined operating condition to verify correct operation of beam monitoring system
  and monitor unit calibration
• For a scattered beam verification of the integrity of scatterers, alignment of scatterers and beam penetration
• Checks of Bragg peak width and lateral beam profile including flatness and symmetry, for scattered beam,
• uniform scanned (beam) and pencil beam scanning
• Back-up monitor constancy
• Mechanical
• Localization lasers
• Snout alignment
• Beamline inspection
• Distance indicator (ODI)
• Modulator wheel interlocks (barcodes etc.)
• Beam delivery system interlocks
• Safety
• Door interlocks
• Audiovisual patient monitors
• Treatment room area radiation monitors
• Function of motion stops on all moving systems (gantry, patient position, etc.)

Question 2

Weekly QA for Protons

Answer 2

• Dosimetry and beam delivery
• For a randomly selected patient, compare calculated planned dose at selected points to measured dose points in a phantom
• Respiratory gating equipment
• Mechanical
• Gantry/collimator angle indicators
• Imaging equipment
• Alignment of x-ray imaging devices relative to beam axis and/or isocenter (orthogonal imaging, cone beam CT, etc.)
• Quality of images

Question 3

Monthly QA for Protons

Answer 3

• Dosimetry
• Verify integrity of modulator system
• Mechanical
• Light/radiation field congruence
• Field size indicators (MLC)
• Jaw symmetry
• Cross-hair centering
• Patient positioner readouts and tolerances
• For gantry determine isocenter location and check tolerances
• Coincidence of collimator, gantry, and couch axes at isocenter
• Safety
• Emergency off switches

Question 4

Annual QA for Protons

Answer 4

• Dosimetry and beam delivery
• Extensive recalibration of output under a wide variety of operating conditions
• Checks of modulators, range shifters, or energy selection systems as appropriate
• Lateral profile flatness and symmetry as a function of gantry angle
• Check location of virtual source
• Primary MU linearity check
• Check beam monitors for saturation conditions
• Measure dose per MU for primary and backup channels as a function of gantry angle
• Check dose per MU against standard laboratory or other institution using independent standard (e.g., ion chamber or TLD)
• Mechanical
• Patient position tolerances including table sag
• Safety
• Calibrate area radiation monitors throughout facility
• Comprehensive test of all accelerator, beam line, gantry, and nozzle safety systems
• Imaging equipment
• X-ray kVp, mA, timer, and magnification
• CT unit HU calibration
• Full check of all simulation devices CT, PET /CT. MRI

Question 5

Protons

Answer 5

• Charged particles
• Better dose distribution
• Low LET
• RBE similar to photons

Question 6

Neutrons

Answer 6

• Better cell killing
• High LET: direct killing/DNA damage
• High RBE (4 times RBE of protons/electron/gamma)
• Used for inoperable, radioresistent tumors
• Dose to kill is 1/3 of dose required by photons
• Alpha particle causes DNA Damage

Question 7

Stopping theory for Protons

Answer 7

• Rate at which protons lose energy increases as they slow down
• So rate of energy depends on proton energy and stopping material used.

Question 8

Scatter theory for protons

Answer 8

• MCS = Multiple columb scattering
• MCS angles < 16deg
• Due to elastic columb interactions with the target nuclei

Question 9

How do protons interact

Answer 9

• Incoming proton enters the nucleus of the tumor and displaces the constituent proton/neutron/light nuclon clusters
• The secondary proton has much lower energy and larger angles than primary protons
• Outgoing proton retains characteristic non elastic collisions

Question 10

Bragg peak / Pristine Bragg peak

Answer 10

• Loses more energy per unit length
• Reaches a peak where it loses all its energy and then stopped.

Question 11

Modulators

Answer 11

• Range wheel modulator
• Can adjust the beam length and bragg peak with this
• If the wheel stopped spinning, tumor would be partially overdosed and partially underdosed; pristine bragg peak would be delivered with inhomogeneous dose to the target

Question 12

Surface dose depends on

Answer 12

Modulator factor
* Higher the dose, deeper the tumor, higher the skin dose

Question 13

Compensator

Question 14

Proton Beam Path components

Answer 14

1. First scatter
2. Second Scatter
3. Range shifter : determines where the beam stops
4. Range Modulator: spreads the peak: SBP
5. Patient Aperture: finds the edges of the beam, made of brass
6. Patient compensator: gives conformality, reduces scatter in beam, made of wax
7. Tumor target

Question 15

Spot scanning
Pencil beam scanning

Answer 15

Advantage:
* Less interplay of patient motion with beam

Question 16

S.I Unit

Answer 16

RBE = 1.1equivalent

Question 17

Disadvantages of protons

Answer 17

• Complex margins
• Motion effects are more important
• RBE effect
• If the tumor shrinks the plan changes
• IGRT technology differs

Question 18

PTV margins for protons

Answer 18

• beam dependent
• non-Isotropic
• extend in the beam direction

Question 19

Ion chamber used

Answer 19

Multi layer ion chamber
* Zebra
* Giraffe
* Matrixx PT

Question 20

Carbon Ion

Answer 20

• Has 12 protons in the nucleus
• These are called heavy ion beams
• High LET, so high dose beyond bragg peak
• Smaller penumbra than protons
• Less MCS than protons
• RBE = 3 or larger equivalent

Question 21

Proton scattering

Answer 21

Active Scanning:
* Uses orthogonal magnets
* deflects proton pencil beam
* delivers dose in dmall spots
* susceptinle to missing parts of the tumor as it moves.
* There is proximal dose conformality

Passive Scattering technique:
* USes filters of varying thickness such as modulation wheel to spread the bragg peak.
* Scattering foil in place to spread the beam laterally
* Patient specific aperatures and compensator is designed

Question 22

Deflector

Answer 22

• Generates electrostatic field to redirect protons out of centripetal motion

Question 23

Scattering foil

Answer 23

Placed directly into the beam line to widen the beam

Question 24

Range

Answer 24

• The depth of the material at which half of the protons undergo Electromagnetic interactions.
• Depth at 50%

Question 25

Robust planning

Answer 25

TO minimize uncertainty from tissue heterogeneity during IMPT planning

Question 26

Fast Neutron therapy

Answer 26

* Due to bombarding protons into target * Used for adenoid cyctic carcinomas of H&N * Higher RBE

Question 27

Range Shifter

Answer 27

~ to bolus Used for treating superficial tumors

Question 28

Recoil Protons

Answer 28

They are a result of neutrons interacting with nuclei, hydrogen nucleus It is due to HIgh LET particles resulting form collision of a neutron with a proton.

Question 29

Neutron shielding

Answer 29

* Hydrogen rich materials such as water, concrete, polyethylene, borated polyethylene, compacted earth are used for shielding. * Hydrogen nucleus is approximately the same mass of a neutron, so collision with neutrons will transfer energyvia collision. * After slowing down, these neutrons can be absorbed. * Lead is NOT recommended for shielding Neutrons

Question 30

Proton range

Answer 30

DIstal side of the spread out bragg peak

Question 31

Modulation width

Answer 31

Width of the spread out bragg peak

Question 32

Robustness of proton treatment is insensitivity of PTV and OAR changes in

Answer 32

* Range uncertainty * Patient setup uncertainity * Internal Anatomy

Question 33

Inter play

Answer 33

* Interplay effect occurs when scanning proton beams interact with moving patient anatomy, breath hold and respiratory gating

Question 34

Damaging Neutron range

Answer 34

100 Kev - 2 MeV * They have an RBE from 5 to 20.