Basic Principles of Radiotherapy - Department of Medicine

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Basic Principles of Radiotherapy - Department of Medicine

Basic Principles of

Radiotherapy


Objectives

At the end of this presentation, you

should be able to answer the following

questions:

1) What 3 basic principles need to be

considered when recommending

radiotherapy (RT)

2) What are the 3 basic RT approaches for

cancer treatment (ie. When and why is it

used)


3) What are some of the radiation treatment

modalities (list 5) available

4) How is radiation treatment delivered (be

able to describe a standard approach)

5) What are some site specific side effects

(describe 3 side effects for each of brain,

head&neck, chest, breast, abdomen and

pelvis)


Some general background

• Radiation has been available as a treatment for

cancer for over 100 years.

• Ionizing radiation (X-rays) is a type of energy

found within the electromagnetic spectrum

(which also includes microwaves, radio waves

and visible light).

• The goal of radiation treatment is to deliver a

precisely measured dose of radiation to a target

(tumour) with minimal damage to surrounding

normal tissue.


At the Clinic or Bedside

• Consultation with Radiation Oncologist

• History & Physical Exam (the patient factors)

• Staging (the tumour factors)

• Diagnosis

• Recommend treatment (the treatment factors)


Pre-Treatment Planning

• “Should this patient be treated with

radiation?”

– Patient Factors:

• Previous therapy

• Relevant past medical history

• Performance status and age

• Social situation

• Wishes / likelihood of compliance


Pre-Treatment Planning

• “Should this patient be treated with

radiation?”

– Tumour Factors:

• Type

• Extent

• Natural history

• Treatment intent

• Treatment options, expected toxicities and

results


Pre-treatment Planning

• What are 3 radiotherapeutic treatment

intentions ? (part A of treatment

factors)


What are the 3 basic RT

approaches to cancer treatment

1) Curative – requires high doses, typically

above 60 Gy (the exception

is lymphomas)

2) Adjuvant – requires intermediate doses,

typically in the range of 30-50

Gy

3) Palliative – low doses effective, not

greater than 30 Gy in most

cases


Gray

• SI unit for absorbed dose is Gray (Gy)

• 1 Gy = 1 J/kg

• Older term ‘rad’ is no longer used


Dose fractionation

• Curative – Usually delivered as 2 Gy once

daily, but there can be smaller fraction

sizes (1.2-1.8 Gy) or slightly larger fraction

sizes (2.2 Gy).

• Adjuvant – Also usually delivered as 2 Gy

once daily, but there can be the same

variations as for curative.

• Palliative – Much larger fraction size (3-8

Gy) is standard.


Examples of treatment delivery

Curative – most often think of H&N

cancers where RT is the

primary treatment modality

– The patient requires an immobilization mask.

– The RO outlines the various target volumes

on CT images, and also outlines normal

structures that are in proximity to the tumour

– Treatment planning can be very sophisticated

using IMRT to target tumour and minimize

dose to normal tissue.


• Adjuvant – Typically think of breast

treatment. In these cases,

the gross tumour has been

removed. The RO outlines

the CTV/PTV and treatment

volume, using standard X-ray

(fluoroscopy) or CT imaging.

Treatment planning can be

2D or 3D.


• Palliative – Covers a wide range of sites.

The set-up is kept as simple as possible.

• Volume delineation may be done using

surface landmarks (eg. Ribs, clavicle,

brain), fluoroscopic imaging (eg, spine,

hips) or CT (lung, H&N, pelvis)

• Planning is kept as simple as possible to

expedite initiation of treatment.


• Questions/comments so far?


What are some RT

modalities for treatment of

cancer?


What are some RT modalities for

treatment of cancer

• 1) External beam

– The commonest external beam utilizes photons

– Electrons are another type of external beam.

2) Sealed sources

- These are inserted into the patient and can be

temporary or permanent (eg, gynecologic tumours are

treated with temporary insertions while prostate

tumours are treated with permanent seed implants)

3) Unsealed sources

- These are radionuclides such as iodine which are

ingested or injected.


Pre-Treatment Planning

• Patient Education:

– Rationale for treatment

– Expected toxicities of treatment

– Process of treatment planning

– Rough time frame for starting treatment


Treatment Planning

• Goal:

– Evaluate possible treatment approaches, and

choose one that:

• Gives the best (or at least an acceptable) dose

distribution

• Is reproducible

• Is verifiable


• Mark-up

Treatment Planning:

Simulation

– typically used for planning of RT of

superficial lesions (skin CA, breast boost,

palliative DXR for rib / sternal mets)

– also used for planning of palliative brain RT

• Conventional Simulation

• CT-Simulation


Treatment Planning:

Simulation

• Get patient in optimal / acceptable

treatment position

– Allows reproducible and verifiable treatment of tumour

– Possible additional benefit: allows / increases sparing of

normal tissues

– Patient comfort is critical

• Pain control

• Use support devices and immobilization devices liberally

– Can patient maintain desired position for 15 – 30 minutes

without difficulty?

– For a given site, avoid treating same patient in different

positions


Treatment Planning: Simulation


Treatment Planning: Simulation

• CT-MRI fusion

– used for planning of treatment of brain lesions

fairly routinely, as MRI and CT are

complementary imaging modalities


Treatment Planning: Simulation

• CT-PET fusion


XBRT: Beam Choices &

Characteristics


Beam Choices

• Orthovoltage

• Photons

– Co-60

– MV

• Electrons

• Exotica (you can’t do that here)

– Neutrons

– Protons


Basic Beam Characteristics

• Orthovoltage Beam:

– characteristics (PDD curve):

• full dose at surface

• rapid attenuation in tissue (~8%/cm with 250 kVp)

– slightly slower with higher energy beams

– compared to higher energy photons:

• increased absorption in bone

• increased scatter when bone in way of path to

tumour (i.e. decreased dose to tissue beyond)

• shorter SSD (typically 50 cm)

• Slow delivery (typically 10-15 minutes/field)


TBCC Orthovoltage PDD Curves

(8 x 10 cm field)

dose (%)

120

100

80

60

40

20

0

0 5 10 15

75 kVp

225 kVp

250 kVp

depth (cm)


Orthovoltage

Clin RT Phys, 2nd ed, Fig. 15-2


Absorption in Bone

Clin RT Phys, 2nd ed,

Table 14-3:

•ratio of mass-energy

absorption coefficients

for bone/muscle shows

impact of photoelectric

effect at low energies

seen with orthovoltage

radiation


Basic Beam Characteristics

• Cobalt-60 beam:

– characteristics (PDD curve):

• ~50% surface dose, with d max at 0.5 cm depth

• slower attenuation in tissue than orthovoltage

(~5%/cm)

– not a point source geometric penumbra

contributes to total penumbra

– Treatment time typically 2-4 minutes


Co-60 Beam

Clin RT Phys, 2nd ed, Fig. 15-3


Basic Beam Characteristics

• Megavoltage Photon Beam:

– characteristics (PDD curve):

• decreased surface dose with gradual build-up to

d max

– surface dose decreases as increase photon energy

– depth of d max increases as increase photon energy

• slower attenuation in tissue than Co-60

– rate of attenuation decreases as increase photon energy

Treatment delivery time typically 1-2 minutes/field


Megavoltage Beam

Clin RT Phys, 2nd ed, Fig. 15-4


PDD Curves, 10 x 10 cm field

% dose

120

100

80

60

40

20

0

0 10 20

depth (cm)

•Co-60: past dmax (0.5 cm), lose ~ 5%/cm

•6 MV: past dmax (1.5 cm), lose ~ 4%/cm

•18 MV:past dmax (3 cm), lose ~ 3%/cm

Co-60

6 MV

18 MV


Switching Horses


Basic Beam Characteristics

• Electron Beam:

– characteristics (PDD curve):

• relatively high surface dose (75- 95%)

– surface dose increases with increased electron

energy

• broad region of maximum dose

– this region widens with increased electron energy

• rapid dose fall-off beyond region of maximum

dose

– slower with increased electron energy

• low dose tail (x-ray contamination of electron

beam)


120

100

TBCC Electron PDD Curves, 10 x 10 cm field

dose (%)

80

60

40

20

6 MeV e-

9 MeV e-

12 MeV e-

16 MeV e-

20 MeV e-

0

0 5 10 15

depth (cm)


Exotica

• Available in a few highly specialized centers

only


Protons


Neutrons

• Finally have ability to build treatment machines

which would be suitable for clinical use, but

interest in neutrons has waned because:

– no additional benefit over traditional photon or

electron radiation for most tumours

– depth-dose characteristics are at best like 6 MV

photons (most like DXR – 4 MV)

• Only rationale for neutrons = radiobiological

– late effects often far worse than expected for given

dose neutrons


Questions?


Designing the treatment


2D-RT

• Conventional simulator used to design

beam portals based on standardized beam

arrangement techniques and bony

landmarks visualized on planar

radiographs


Volume delineation for external

beam and sealed sources

• The gross tumour volume (GTV) is outlined

• A margin is included around the GTV to include

areas at risk for microscopic involvement, this is

the clinical target volume (CTV)

• A margin is added onto the CTV to allow for

differences in internal organ motion or day-today

set up variations, this is the planning target

volume (PTV)

• There is a margin added to the PTV to allow for

physical characteristics of the beam (penumbra),

this is the actual treatment volume.


ICRU 50 Volume Definitions

Gross Tumor

Volume

Clinical Target

Volume

Planning Target

Volume

Treated Volume

Margins

GTV -> CTV: local sub-clinical

CTV -> PTV: setup variation

- patient movement

- organ movement

- variations in organ shape &

size

PTV -> IV: penumbra

Irradiated Volume


Organs At Risk

(Part B of treatment factors)

• organs at risk := normal tissues whose

radiation sensitivity may significantly

influence treatment planning and / or

prescribed dose

• class I organs : radiation lesions are fatal or

result in severe morbidity (spinal cord)

• class II organs : radiation lesions result in

mild to moderate morbidity (bowel)

• class III organs : radiation lesions are mild,

transient and reversible, or result in no

significant morbidity (muscle)


Treatment Planning:

Dose Distribution

• Optimal Dose Distribution:

– Cover target volume : appropriate dose &

homogeneity

• ICRU 50 recommends that dose coverage of PTV

be kept within +7% and -5% of prescribed dose; if

not possible, RO to access if acceptable

– Avoid significant dose to sensitive structures :

Conformal Avoidance

– Minimize dose to surrounding normal tissues:

Integral Dose


3D - Conformal Radiotherapy

• 3D-CRT: method of irradiating target volume

(defined in 3D imaging study) using array of

beams individually shaped to conform to 2D

projection of target

• Beam orientations selected to minimize

overlap with neighbouring OARs

• Beam characteristics and modifiers selected

to produce dose distribution that is uniform

throughout target(s) and as conformal as

possible, consistent with dose constraints to

normal tissue


3D - Conformal Radiotherapy

• Iterative changes to weights, beam modifiers,

number and directions of beams until

satisfactorily uniform dose to target is achieved

without exceeding dose tolerance of

neighbouring OARs

• Allows safe escalation of dose to targets in a

variety of areas in the body (prostate,

nasopharynx) that is expected to result in

increased local tumour control probability


Conformal

Treatment

vs.

Conformal

Avoidance


Treatment Planning: DVH

• Can extract dose stats from this data,

for both targets and normal tissues:

– Maximum or minimum point dose

– Mean dose, standard deviation

– V x (e.g., V 20 for both lungs – PTV)

• Can compare DVHs generated for

competing plans to try to decide on

best plan

• Can look at DVHs for individual plan to

assess if acceptable

• Does not provide any spatial

information therefore complementary to

dose distribution information

Perez, 4 th ed, Fig 8.20 A

& B


Limitations of 3D-CRT

• 3D-CRT cannot conform well to 3D shape of

target unless:

– Large numbers of beams are used

– Target has relatively simple shape

• 3D-CRT cannot give a satisfactory treatment

plan if:

– Concave tumour wrapped around sensitive

structure

– Angles required to avoid / minimize dose to

normal tissues are difficult or impossible to

achieve clinically

• target surrounded by different OARs:

e.g., nasopharyngeal cancer


What is Intensity Modulated

Radiotherapy (IMRT)?

• IMRT: method of irradiating target volume

(defined in 3D imaging study) using array of

beams, where the intensity of the beams

varies across each treatment field

• Does this really help?


What’s Backwards About

Inverse Planning?

“Traditional” forward planning:

Choose treatment parameters

Produce dose distribution

No

Assess dose distribution Satisfied ?

Yes

Accept treatment plan


What’s Backwards About

Inverse Planning?

Inverse planning:

Choose

Choose dose volume constraints

for target & OARs

Set treatment parameters

No

Create dose distribution Satisfies constraints ?

Yes

Accept treatment plan


IMRT- 9 Beams


Coronal & Sagittal Slices at Iso


Side effects from radiation

• Side effects are grouped into acute, delayed and

late; severity is related to overall dose as well as

patient factors.

• 1) Acute (fatigue is common to all)

– Brain: Headache, nausea, alopecia

– H&N: Xerostomia, mucositis, dysphagia

– Lung and esophagus: Dysphagia, cough, hoarseness

– Breast: skin erythema, breast discomfort

– Abdomen or pelvis: nausea, diarrhea, dysuria


• 2) Delayed

– Lung is the classic organ for a delayed response

(pneumonitis) 2-6 months post RT

3) Late

Brain: Necrosis, pituitary dysfunction, hearing loss

H&N: Xerostomia, dental decay, thyroid dysfunction

Lung/esophagus: Esophageal stricture, lung

fibrosis/dyspnea, coronary artery disease

Breast: Altered skin pigmentation, firmness of breast,

arm edema

Abdomen or pelvis: Bowel obstruction, infertility,

proctitis


Objectives

At the end of this presentation, you

should be able to answer the following

questions:

1) What 3 basic principles need to be

considered when recommending

radiotherapy (RT)

2) What are the 3 basic RT approaches for

cancer treatment (ie. When and why is it

used)


What factors need to be considered

when recommending RT

• 1) Patient factors (age, performance

status, co-morbidities [particularly

connective tissue diseases], surgery)

• 2) Tumour factors (extent of disease [ie.

stage]

• 3) Treatment factors (has there been

previous RT, what normal structures are in

proximity to the tumour)


What are the 3 basic RT

approaches to cancer treatment

1) Curative – requires high doses, typically

above 60 Gy (the exception

is lymphomas)

2) Adjuvant – requires intermediate doses,

typically in the range of 30-50

Gy

3) Palliative – low doses effective, not

greater than 30 Gy in most

cases


3) What are some of the radiation treatment

modalities (list 5) available

4) How is radiation treatment delivered (be

able to describe a standard approach)

5) What are some site specific side effects

(describe 3 side effects for each of brain,

head&neck, chest, breast, abdomen and

pelvis)


• Thank you.

• Any questions?

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