Topic 2 - Cobalt-60 Therapy & Brachytherapy

ONCOL 234 - Therapeutic and Imaging Equipment. University of Alberta

before WWII, radium was attempted to be used for radiotherapy because it could create energies of 2 MeV. Why was it not used?

it has a half-life of 1600 years, and there was limited amounts of it available

where were the first two cobalt-60 units built?

Saskatoon, Saskatchewan and London, Ontario

When was the first patient treated with cobalt-60 radiation

1957 in London, Ontario

How is Cobalt-60 Produced

⁵⁹Co(n,y)⁶⁰Co

• bombarding stable Co⁵⁹ with neutrons

t_1/2 of Co⁶⁰

5.3 years

Co⁶⁰ decay process

Beta minus decay

• produces 1.17 MeV and 1.33 MeV gamma

What are the two ways to turn the cobalt beam ‘on and off’

1. rotating cylinder

   • rotate the gamma beam 180 degrees into a lead shielding area

2. sliding drawer

   • most popular: push the source into lead shielding area

what is contained in a cobalt-60 unit’s source head

1. a steel shell with lead lining

2. a mechanism to bring the source in front of collimator

3. collimators to prevent unwanted scatter

What is a Co-60 fail safe device?

At the end of exposure or a breakdown, there is a mechanism to interrupt the beam and take it out of the ‘on’ position

if the source fails to return to the off position with the fail safe device, how is exposure interruption

using a manual return system

how does the size of a Co-60 machine compare to today’s LINACs

Co⁶⁰ machines are smaller as there is much less electronics involved

• no magnetron, klystron, accelerating chamber.

  • just cobalt sources in head

what is the purpose of the depleted uranium in this Theratron-780 source head?

it works like lead, absorbing radiation

how is the cobalt machine put into the ‘on’ position

the source drawer is pushed in line with the collimator

Did the Saskatoon Co⁶⁰ machine have a treatment head like a or b?

a

did A.E.C.L units have a treatment head like a or b?

b

how was the collimator used to prevent excess radiation from coming through if machine was stuck in on posiiton

it would make criss-cross pattern

How did the Co⁶⁰ machine compare to 200 kV x-ray machine in terms of isodose and depth

dose was much better at depth

• 200 kV: at 10 cm depth we got 35% of surface dose

• Co⁶⁰: at 10 cm depth, we got 55% of surface dose

  • compared to modern LINAC: 80%

radiation output was steady, easier to predict, and more controlled

How does the penumbra of the Co⁶⁰ machine compare to LINAC

Co⁶⁰ penumbra is much worse due to geometric penumbra

What are two modern applications of Cobalt therapy?

1. Cobalt-MRI

2. Gamma Knife

Cobalt-MRI system

cobalt source inside MRI magnet allows for simultaneous imaging and radiation for image radiation guidance

Gamma Knife

contains 201 Co⁶⁰ sources in a circular array which converge to focus a narrow high dose beam of 0.5 mm on to a small tumor volume

what treatments can the gamma knife be used for?

deep brain tumors and brain radiosurgery

• radiosurgery = 1 fraction

• tumors = multiple longer fractions

Why is the gamma knife great for small lesions?

due to narrowing of beam, the isodose can be very small

Brachytherapy Definition

a method of treatment in which sealed radioactive sources are used to deliver radiation at a short distance by interstitial, intracavitary, or surface application

Where does the term ‘brachy’ come from

from the greek word brachys meaning ‘short distance’

what cancers is brachytherapy commonly used to treat

cervical, prostate, breast, skin

interstitial brachytherapy

the sources are placed directly in the target tissue of the affected site such as the prostate or breast

contact brachytherapy

placement of radiation source in a space next to the target tissue. this space may be

• a body cavity (intracavitary)

• a body lumen (intraluminal)

• externally (surface

intravascular brachytherapy

a radiation source placed in blood vessels for the treatment of coronary stenosis

interstitial brachytherapy catheter technique

uses several flexible catheters inserted into treated tissue

• catheters are then attached to afterloader to deliver treatment

intracavitary brachytherapy catheter technique

involves placement of a single baloon catheter into the cavity left after the removal of the tumor.

• balloon is inflated and catheter is then connected to afterloader to deliver radiation into the balloon

LDR (Low Dose Rate) Brachytherapy

sources that emit radiation up to 2 Gy/hr

• used in oral cavity, sarcomas, prostate

MDR (medium dose rate) brachytherapy

rate of dose deliver between 2-12 Gy/hr

HDR (high dose rate) brachytherapy

rate of dose delivery exceeds 12 Gy/hr

• used in cervix, lungs, breast, prostate

why must automatic afterloading be used for HDR

because of the radiation source’s high activity

PDR (pulse dose rate) brachytherapy

short pulses of radiation given once an hour to stimulate rate and effectiveness of LDR treatment

• used in gynaecological and H+N cancers

Permanent implant brachytherapy

delivers a high total dose of 150 Gy at a very low dose rate over several months

Afterloading Unit

after applicator/catheter is placed into target position, the radioactive sources are loaded after by a machine

• the applicator cable containing the sources is preloaded, then moved by a stepping motor into the catheter

Remote HDR afterloader

used to deliver a single source of high activity (like IR¹⁹²)

how is the remote afterloader tested for QA?

before the active source is placed into the catheter, a dummy wire is placed to ensure no obstruction occurs

advantages of brachytherapy

• high localized dose to small volume

• reduced radiation to healthy organs

• shorter treatment time

• hypoxic tumor targeted easier

Disadvantages of brachytherapy

• invasive

• requires expertise, experience, equipment

• time consuming

Radium t_1/2

1600 years

What series is radium apart of?

uranium series

• starts with U²³⁸, ends with Pb²⁰⁶

radium decay mode

alpha decay

what does radium decay into?

Radon (a gas)

Radon half-life

3.83 days

how many gamma decays does it take to decay radon

49

what does radon decay into?

lead

Radium Source Encapsulation

radium sources have two capsules, as after radium decays, it decays into radon gas

• we don’t want this to break inside the patient

Iodine-125 half life

59.3 days

iodine-125 decay mode

electron (k) capture

what tumors is I-125 used in brachytherapy for?

prostate cancer

• can also be used in diagnostic nuclear medicine

Iridium-192 half life

73.8 days

is iridium a sealed or open source?

sealed

Is Ir-192 used for HDR or LDR therapy

HDR and also PDR

Ir-192 decay mode

Beta minus (95%)

K capture (5%)

Co-60 half life

5.26 years

when is Co-60 used in radiotherapy

rarely used, but can be used to replace Rad-226

what kind of afterloader is needed for Co-60

a remote afterloader, since it is HDR

Co-60 decay mode

beta minus

Cs-137 half life

30 years

when is Cs-137 used in therapy

LDR interstitial or intracavitary brachytherapy

Cs-137 decay mode

Beta minus

I-131 half life

8 days

is I-131 a sealed or unsealed source

unselaed, it is given orally for nuclear medicine

I-131 decay mode

beta minus

Sr-90 half life

29 years

is Sr-90 a sealed or unsealed source?

unsealed

how is Sr-90 used in brachytherapy

used to treat superficial ocular (eye) malignancies

Sr-90 decay mode

Beta minus

Pd-103 half life

17 days

When is Pd-103 used in brachytherapy

used similarly to I-125: prostate treatment

Pd-103 decay mode

K capture

temporary prostate implant method

radioisotopes like Ir-192 are implanted during surgery and left in for three days

• patients are hospitalized for this

permanent prostate implant medthod

radioisotopes with short half lives like I-125 or Pd-103 emit low energy photons are are left in the patient forever

• guided through TRUS

intravascular brachytherapy

used to reduce rate of restenosis to below 10% with high energy beta sources like Sr-90 or gamma sources like Ir-192