CHP 7 Treatment delivery

Dmax value

-depth where the peak treatment dose is deposited

-It increases with increasing energy

Superficial Energy Dmax Value

0

Orthovoltage Dmax value

0

Cobolt 60 Dmax

0.5 cm

4 MV Dmax

1 cm

6 MV Dmax

1.5 cm

10 MV Dmax

2.5 cm

15 MV Dmax

3.0 cm

20 MV Dmax

3.5 cm

25 MV Dmax

5.0 cm

KV equipment

low voltage and low treatment

disadvantages of KV equipment

-doesn't penetrate deep for deep-seated tumors

- No skin sparring

Superficial Therapy

40-150 kV

no treatment table

At what depth does Superficial Therapy treat?

no deeper than 0.5 cm

SSD for Superficial Therapy

15-20 cm

Orthovoltage Therapy

100-500 kVp

At what depth does Orthovoltage Therapy treat?

2.3 cm

SSD for Orthovoltage

50 cm

what does orthovoltage therapy use

cones and movable diaphragms (collimators)

What is Orthovoltage therapy used for?

To treat skin, mouth, rectal cancer

Cobalt 60

-Artifically produce from cobalt 59 (stable) to Cobalt 60 (unstable)

-uses Mega Voltage

- always emits radiation

The effective energy for Cobalt 60 is?

1.25 MeV

Half-life

When radioactive materials decay to 50% of their original activity.

Cobalt 60 has a half life of ?

5.26 years

What id the monthly output correction factor for Cobalt 60?

1%

How does a linear accelerator (LINAC) machine function?

uses charged particles traveling in a straight line while gaining energy from electromagnetic field

Advantages of LINAC

-Uses high energy= more skin sparring

-forward scattering= normal skin sparring

-Less penumbra= crisp field edge

properties of Early accelerators

-bulky and extremely large

-limited gantry motion

2nd gen accelerators

-360 gantry rotation around the isocenter, more motion

-lots of maintenance

3rd gen accelerators

-current

-Wide range of dose rates, field size, and beam energies

-computer driven

more reliable

Drive Stand of LINAC contains

- kylstron/ magnetron

-circulator

-water cooling system

-wave guide

klystron

Amplifies low-frequency microwaves created by RF (radio-frequency driver)

What types of accelerators use klystrons?

Higher energy accelerators (greater> than 10 MeV)

Magnetron

Both an amplifier and produces radiowave frequencies

What types of accelerators use magnetrons?

Low energy accelerator (less > than 10 MeV)

Waveguide

carries microwave power from klystron/magnetron to circulator, and from circulator to gantry

Circulator

Acts like a valve, prevents back-flow of microwave into klystron/ magnetron

water-cooling system

keeps the temperature of the LINAC machine constant to prevent breakdown

How often does water temp need to be taken?

Every morning at morning warmup

Gantry's primary responsibility

-Directing the beam towards the patient

-Controlled by a hand pendant.

Gantry of LINAC components

-pressure system

-vacuum system

- electron gun (e-)

-accelerator structure

- gantry head

Vacuum System

-allows for no collision with air molecules in the electron's path

-low pressure

pressure system

-Pressurizes waveguide with Sulfur hexafluoride gas (SF 6)

-Prevents electrical breakdown

Electron Gun (e-)

-produces and injects electrons into accelerator structure

-made of tungsten

Accelerator structure/accelerating waveguide

Microwaves from klystron/megnatron travel through the accelerator structure at about the same time as the electron gun fires off the electrons.

What is accelerator structure made of?

-A series of COPPER cavities with alternating positive and negative charge, accelerates electrons to target

- MORE CAVITIES=MORE ENERGY

where is accelerator structure located?

HORIZONTALLY for HIGH ENERGY machine; VERTICALLY for LOW ENERGY machines

Bending Magnet

-Only necessary for Horizontal accelerator structure

-bends electron stream towards target (PHOTON treatment)/scattering foil (ELECTRON) and patient

-bends e-stream 90-270

Target

- e-stream smashes into it to CREATE PHOTON BEAM

- tungsten material

Primary collimator

Determines that maximum field size of the phone beam (cannot be changed)

What is a flattening filter?

A conical metal absorber that absorbs more photons from the beam's central axis and fewer at the periphery.

What is the purpose of a flattening filter in radiation therapy?

It evens out the beam.

When is a flattening beam used?

ONLY FOR PHOTON treatment

What is a scattering Foil

Broadens e- beam

Purpose of scattering foil?

Flattens the field

When is the Scattering Foil used?

ONLY FOR ELECTRON treatment

where are the flattening filter or the scattering foil located?

carousel

monitor/Ion chamber location

located in gantry head

monitor/Ion chamber

-has 2 chambers in case one fails

-MU read by the 2 monitor chambers are displayed in a cabinet in the control area

-

Purpose of monitor/Ion chamber

-measures radiation dose and dose rates

-shuts off beam after all planned monitor units (MU) are delivered

-measures beam flatness and symmetry

Secondary Collimator

"field size" of the treatment field, adjustable with hand pendant and the control console

light field

Mirror

directs field light down to be visible of patient

How often does the mirror alignment need to be checked and by whom?

-Checked by a physicist as part of QA

-after a lightbulb has been changed or a collision occurs.

Coincidence test

A QA test performed by the physicist to ensure light field aligns with radiation field

Optical Distance Indicator (ODI)/range finder

light that projects numbers the CA to set and read the SSD.

Multileaf collimators (MLC)

shape PHOTON beam with LEAD LEAVES

Cerrobeams

Shape ELECTRON (e-) beam

Beam modifiers

-fit into slots just below the secondary collimators and MLC

Examples of beam modifiers

wedges, electron cone, compensators, or blocks.

Dynamic wedge

"virtual wedge" computer controlled and it replaces 15, 30, 45, or 60° hard wedges.

How is treating with e- beam different than photon?

1. Target must be retracted

2. Use a SCATTERING FOIL and not a flattening filter.

3. Use cerrobend cutout shapes instead of MLC

Interlocks

A safety mechanism designed to protect patients, staff and equipment from hazards

What does the Emergency Off button do?

Terminates all power to the machine.

When should the Emergency Off button be hit?

Only if there is an electrical malfunction!

What type of shutdown does the Emergency Off button perform?

Hard shutdown.

requires start up procedure

What is a key feature of the treatment couch related to its movement?

It is mounted on a rotational axis around the isocenter and moves vertically, horizontally, longitudinally, and rotationally.

What additional movements may new treatment couches offer?

Pitch and roll.

What is the maximum weight some treatment couches can support?

Up to 550 lbs.

Why should the width of a treatment couch be similar to the simulation table?

For reproductive purposes.

What material is commonly used to make treatment couches to reduce scatter?

Carbon fiber.

What feature do most treatment couches have to enhance treatment setup?

Indexing, which allows for increased accuracy and reproducibility.

Modulator cabinet location

In treatment room or console area

Modulator cabinet

-Houses primary power distributor for the LINAC