principles radiotherapy

8/10/2019 Principles Radiotherapy

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Basic Principles ofRadiotherapy


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Objectives

At the end of this presentation, you

should be able to answer the followingquestions:1) What 3 basic principles need to be

considered when recommendingradiotherapy (RT)

2) What are the 3 basic RT approaches forcancer treatment (ie. When and why is itused)


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3) What are some of the radiation treatment

modalities (list 5) available4) 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 andpelvis)


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Some general background

Radiation has been available as a treatment for

cancer for over 100 years. Ionizing radiation (X-rays) is a type of energyfound within the electromagnetic spectrum

(which also includes microwaves, radio wavesand visible light). The goal of radiation treatment is to deliver a

precisely measured dose of radiation to a target(tumour) with minimal damage to surroundingnormal tissue.


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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)


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Pre-Treatment Planning

Should this patient be treated withradiation? Patient Factors:

Previous therapy Relevant past medical history

Performance status and age Social situation Wishes / likelihood of compliance


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Should this patient be treated withradiation? Tumour Factors:

Type Extent

Natural history Treatment intent Treatment options, expected toxicities and

results


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Pre-treatment Planning

What are 3 radiotherapeutic treatmentintentions ? (part A of treatmentfactors)


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What are the 3 basic RT

approaches to cancer treatment1) Curative requires high doses, typically

above 60 Gy (the exceptionis lymphomas)2) Adjuvant requires intermediate doses,

typically in the range of 30-50Gy

3) Palliative low doses effective, notgreater than 30 Gy in mostcases


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Dose fractionation

Curative Usually delivered as 2 Gy once

daily, but there can be smaller fractionsizes (1.2-1.8 Gy) or slightly larger fractionsizes (2.2 Gy).

Adjuvant Also usually delivered as 2 Gyonce daily, but there can be the samevariations as for curative.

Palliative Much larger fraction size (3-8Gy) is standard.


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Examples of treatment delivery

Curative most often think of H&N

cancers where RT is theprimary treatment modality The patient requires an immobilization mask. The RO outlines the various target volumes

on CT images, and also outlines normalstructures that are in proximity to the tumour

Treatment planning can be very sophisticatedusing IMRT to target tumour and minimizedose to normal tissue.


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Adjuvant Typically think of breast

treatment. In these cases,the gross tumour has beenremoved. The RO outlinesthe CTV/PTV and treatmentvolume, using standard X-ray(fluoroscopy) or CT imaging.Treatment planning can be2D or 3D.


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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 toexpedite initiation of treatment.


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Questions/comments so far?


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What are some RTmodalities for treatment of

cancer?


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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 betemporary or permanent (eg, gynecologic tumours aretreated with temporary insertions while prostatetumours are treated with permanent seed implants)

3) Unsealed sources- These are radionuclides such as iodine which are

ingested or injected.


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Patient Education:

Rationale for treatment Expected toxicities of treatment

Process of treatment planning Rough time frame for starting treatment


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Treatment Planning

Goal:

Evaluate possible treatment approaches, andchoose one that:

Gives the best (or at least an acceptable) dosedistribution

Is reproducible Is verifiable


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Treatment Planning:

Simulation Mark-up

typically used for planning of RT ofsuperficial lesions (skin CA, breast boost,

palliative DXR for rib / sternal mets) also used for planning of palliative brain RT

Conventional Simulation CT-Simulation


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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 minuteswithout difficulty?

For a given site, avoid treating same patient in different

positions


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Treatment Planning: Simulation


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Treatment Planning: Simulation

CT-MRI fusion

used for planning of treatment of brain lesionsfairly routinely, as MRI and CT arecomplementary imaging modalities


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Beam Choices

Orthovoltage

Photons Co-60 MV

Electrons Exotica (you cant do that here)

Neutrons Protons


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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)


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TBCC Orthovoltage PDD Curves(8 x 10 cm field)

020406080

100120

0 5 10 15

depth (cm)

d o s e (

% )

75 kVp225 kVp250 kVp


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

Orthovoltage


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Absorption in BoneClin RT Phys, 2nd ed,Table 14-3:

ratio of mass-energyabsorption coefficientsfor bone/muscle showsimpact of photoelectriceffect at low energiesseen with orthovoltage

radiation


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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


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Co-60 Beam



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Basic Beam Characteristics

Megavoltage Photon Beam:

characteristics (PDD curve): decreased surface dose with gradual build-up to

dmax 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


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Megavoltage Beam



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PDD Curves, 10 x 10 cm field

02040

6080

100120

0 10 20

depth (cm)

%

d o s e Co-60

6 MV

18 MV

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


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Switching Horses


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TBCC Electron PDD Curves, 10 x 10 cm field

0

20

40

60

80100

120

0 5 10 15

depth (cm)

d o s e

( % ) 6 MeV e-

9 MeV e-12 MeV e-16 MeV e-20 MeV e-


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Exotica

Available in a few highly specialized centers

only


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Protons


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Neutrons

Finally have ability to build treatment machineswhich would be suitable for clinical use, butinterest 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 givendose neutrons


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Designing the treatment


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2D-RT

Conventional simulator used to design

beam portals based on standardized beamarrangement techniques and bonylandmarks visualized on planarradiographs


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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 isthe clinical target volume (CTV)

A margin is added onto the CTV to allow fordifferences in internal organ motion or day-to-day set up variations, this is the planning targetvolume (PTV)

There is a margin added to the PTV to allow forphysical characteristics of the beam (penumbra),this is the actual treatment volume.


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Organs At Risk

(Part B of treatment factors) organs at risk := normal tissues whose

radiation sensitivity may significantlyinfluence treatment planning and / orprescribed dose

class I organs : radiation lesions are fatal orresult 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 nosignificant morbidity (muscle)


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3D - Conformal Radiotherapy

3D-CRT: method of irradiating target volume(defined in 3D imaging study) using array ofbeams individually shaped to conform to 2Dprojection of target

Beam orientations selected to minimizeoverlap with neighbouring OARs

Beam characteristics and modifiers selectedto produce dose distribution that is uniformthroughout target(s) and as conformal as

possible, consistent with dose constraints tonormal tissue


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3D - Conformal Radiotherapy

Iterative changes to weights, beam modifiers,

number and directions of beams untilsatisfactorily uniform dose to target is achievedwithout 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 inincreased local tumour control probability

ConformalConformal


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ConformalConformalTreatmentTreatment

vs.vs.ConformalConformal

Avoidance Avoidance


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

Can compare DVHs generated forcompeting 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 todose distribution information

Perez, 4th

ed, Fig 8.20 A& B


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Limitations of 3D-CRT

3D-CRT cannot conform well to 3D shape oftarget unless: Large numbers of beams are used Target has relatively simple shape

3D-CRT cannot give a satisfactory treatmentplan if: Concave tumour wrapped around sensitive

structure Angles required to avoid / minimize dose tonormal tissues are difficult or impossible toachieve clinically

target surrounded by different OARs:e.g., nasopharyngeal cancer

Wh t i I t it M d l t d


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What is Intensity Modulated

Radiotherapy (IMRT)?

IMRT: method of irradiating target volume(defined in 3D imaging study) using array ofbeams, where the intensity of the beams

varies across each treatment field

Does this really help?

Wh t B k d Ab t


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Traditional

forward planning:

Whats Backwards About

Inverse Planning?

Choose treatment parametersChoose treatment parameters

Produce dose distributionProduce dose distribution

Assess dose distribution Assess dose distribution Satisfied ?Satisfied ?

Accept treatment plan Accept treatment plan

Yes Yes

NoNo

Wh t B k d Ab t


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Inverse planning:

Whats Backwards About

Inverse Planning?

ChooseChoose dose volume constraintsdose volume constraints

for target &for target & OARsOARs

Set treatment parametersSet treatment parameters

Create dose distributionCreate dose distribution Satisfies constraints ?Satisfies constraints ?

Accept treatment plan Accept treatment plan Yes Yes

No


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IMRT- 9 Beams


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Coronal & Sagittal Slices at Iso


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Side effects from radiation

Side effects are grouped into acute, delayed andlate; severity is related to overall dose as well aspatient 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


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2) Delayed Lung is the classic organ for a delayed response

(pneumonitis) 2-6 months post RT3) Late

Brain: Necrosis, pituitary dysfunction, hearing loss

H&N: Xerostomia, dental decay, thyroid dysfunctionLung/esophagus : Esophageal stricture, lungfibrosis/dyspnea, coronary artery disease

Breast: Altered skin pigmentation, firmness of breast,arm edema Abdomen or pelvis: Bowel obstruction, infertility,proctitis


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Objectives

At the end of this presentation, you

should be able to answer the followingquestions:1) What 3 basic principles need to be

considered when recommendingradiotherapy (RT)

2) What are the 3 basic RT approaches forcancer treatment (ie. When and why is itused)

What factors need to be considered


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What factors need to be considered

when recommending RT 1) Patient factors (age, performance

status, co-morbidities [particularlyconnective tissue diseases], surgery) 2) Tumour factors (extent of disease [ie.

stage] 3) Treatment factors (has there been

previous RT, what normal structures are inproximity to the tumour)


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3) What are some of the radiation treatment

modalities (list 5) available4) 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 andpelvis)


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Thank you. Any questions?

principles radiotherapy

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