Introduction to Medical Technology: Radiotherapy

Contents

• Why radiotherapy?• Some basic principles• Approaches to radiotherapy

– external beam: x-rays, electrons, p, n, heavy ions

– brachytherapy• Stages in the radiotherapy process

– QA, imaging, planning, simulation, treatment, verification, modelling outcome

Why Radiotherapy?

• Radiotherapy used to treat cancers– In conjunction with surgery and

chemotherapy• Proven benefit• Cells display strong dose-effect relationship• Physics, engineering, imaging, technology

based

Benefits of Radiotherapy

• Breast Cancer• Mastectomy• Compare surgery and

chemotherapy (CMF) with and without radiotherapy

• 10 year survival improvement at 10 years

Contents

• Why radiotherapy?• Some basic principles• Approaches to radiotherapy

– external beam: x-rays, electrons, p, n, heavy ions

– brachytherapy• Stages in the radiotherapy process

– QA, imaging, planning, simulation, treatment, verification, modelling outcome

Radiobiology:Linear-quadratic model

• Effects of radiation on cell survival described by:

linear-quadratic model• Linear

– single hit sufficient • Quadratic

– multiple events

Radiobiology:Tumour and normal tissue

• Different responses to radiation / different

• Deliver one large dose– effects similar

• Deliver many smaller doses– effects very different

• Fractionation– e.g. 60 Gy in 2 Gy

fractions

Radiobiology: Tumour and normal tissue

• Radiation effect vs. dose– sigmoid behaviour– stochastic process

• Tumour control lower dose than normal tissue damage– Makes radiotherapy

possible!• Radiotherapy goals and

research– separate two curves

Some Nomenclature

• Dose response curve– plot of radiation effect vs. dose

• Radical treatment– treatment with curative intent (cf. palliation)

• RBE (relative biological effect)=quality factor• Tumour staging: Tumour, Nodal, Metastasis• Necrosis and hypoxia

– regions of tumour dead or dormant due to low oxygen level

Contents

• Why radiotherapy?• Some basic principles• Approaches to radiotherapy

– external beam: x-rays, electrons, p, n, heavy ions

– brachytherapy• Stages in the radiotherapy process

– QA, imaging, planning, simulation, treatment, verification, modelling outcome

Approaches to Radiotherapy

• External beam– Fire radiation beam into patient

• Usually several beams from various directions• Cross-fire effect

• Brachytherapy– Place set of sealed radioactive sources or

seeds into patient• Often contained in applicator tube

External Beam Treatment

• Most commonly Cobalt 60 rays (1.25 MeV), x-rays, electrons– Co 60 radioactive source– x-rays and e-, electron linac

• Sometimes orthovoltage (up to 300 kV)– Superficial tumours

• More rarely hadrons (many MeV)– protons, neutrons, heavy ions

Electron Linac for X-ray and Electron Radiotherapy

• X-rays (4-50 MV), electrons (4-50 MeV)• X-rays polychromatic

– Bremsstrahlung• Electrons scatter in air

– Need for collimation• Cross-fire effect

– Several beams aimed at tumour/target– High dose to tumour/small dose to

surrounding tissue

Radiotherapy Linacs

X-Ray Beam Characteristics

Beam Shaping Devices

• Main Rectangular Collimators• Wedge• Multileaf Collimator

– Field edge shaping• Block• Compensator• Multileaf Collimator

– IMRT (intensity modulated radiotherapy)

Lead Compensator

• Sheets of lead– 0.5 mm thick

• Used in conjunction with wedge

• Shapes to breast and shields lung

Granulate Compensator

• Stainless steel granulate

• Used in conjunction with wedge

• Shapes to breast and shields lung

Intensity Modulated Radiotherapy

• Most common delivery method MLC (multileaf collimator)– Dynamic

• Scan MLC leaves across field with radiation on

– Multiple segment• Delivery set of irradiations at same position with

different field shapes

• Compensators

Patient Image

Cumulative Intensity

Field 1

Field 4Field 3

Field 2

Electron Beam Characteristics

Brachytherapy

• Sources placed in patient using metal tubes or applicators

• e.g. Ir192

• Afterloading• High dose rate (HDR) and low dose rate

(LDR), pulsed dose rate (PDR)• Fractionated• Prescription schemes for location of sources

Hadron Radiotherapy

• Large accelerators– High energies needed– Large magnetic rigidity

• Protons most common– Over 20 in the world

• Neutrons• Heavy ions