8.1/8.2 radiotherapy I & II Flashcards
what is radiation therapy?
using ionizing radiation to kill tumors
what are radiation doses delivered to, and how are these doses decided?
- delivered to malignant tumors
- since we know the threshold dose for different types of tumors, we can set this amount of radiation to them to kill them
what are “deterministic effects” of radiation? how do we use this to our advantage regarding tumors?
- deterministic effects are short-term, adverse tissue reactions resulting from a dose that is significantly high enough to damage living tissues
- the severity of a deterministic effect increases with radiation dose above a threshold, below which the detectable tissue reactions are not observed
- because doses are in the deterministic regime, a predictable fraction of the tumor cells die
what is a major disadvantage to the use of radiotherapy to kill tumors? explain
- healthy tissues are also exposed to high doses, along with tumors
- this is because with ionizing radiation, we cannot focus the radiation as we can do with light photons
what is a consequence of healthy tissues being exposed to high radiation doses?
- these treatments deliver a stochastic dose to the rest of the body
- which may crease an increased risk of cancer at some later time (this small risk is weighed against an existing life-threatening disease)
what occurs before a patient undertakes radiotherapy?
- we use the ‘justification principle’
- we weigh the future risk of using the radiotherapy with the benefit
what does it mean for cancerous tumors to be radiosensitive?
responding well to radiotherapy
why are tumors radiosensitive?
because most malignant tumors fall into the class of rapidly reproducing cells, they are easily affected by radiation
what are two reasons destroying enough of a tumor to prevent its regrowth is a difficult task?
- because if one cell is left behind, the tumor will reseed
- because we are exposing healthy tissues to radiation
if cancer cells aren’t necessarily confined in separate, compact masses, what does that mean?
it means they can be spread throughout the body and infiltrate vital organs
if cancer is easily spread and can infiltrate many organs, treating cancer often involves?
treating an entire region surrounded by healthy tissue
in radiotherapy, what do we try to choose when treating cancer?
the optimum dose of radiation
what is the optimum dose?
a dose that will kill the cancer, but take into consideration the surrounding healthy tissues
what does choosing an optimum dose infer?
that a compromise is made between the effectiveness in tumor killing and in sparing nearby healthy tissue
what plot or graph is used to find the optimum dose?
the dose-response curve
what does the dose-response curve show?
- that even small variations in treatment dose leads to undesirable consequences
- too high a dose kills too many healthy cells
- too low a dose spares some tumor cells
what are the significant features of the dose-response curve?
- there is a threshold dose for cell killing
- the fraction of cells killed increases with dose via a characteristic S-shaped curve
- there is a dose at which 100% of cells are killed thus no effect is seen if dose is increased
if the threshold for the healthy tissue exceeds that for the tumor, treatments can use a dose that:
- kills most of the tumor cells
- affects relatively few of the healthy cells
there are situations where there is an appreciable overlap between the two curves in the plot, what does this mean?
that killing appreciable numbers of tumor cells will inflict great damage on healthy tissue
what does the form of the dose-response curve depend on generally?
- the tissue from which the cells originate
- the type of ionizing radiation used (ex: gamma photons, electrons)
- the energy (of those particles)
- the rate at which the radiation dose is delivered
how does the dose-response curve depend on the rate at which the radiation dose is delivered?
- more rapidly delivered doses cause more damage (to tumors) with a lower threshold
- this can protect more healthy tissue (fractionation)
what process is very important for the protection of healthy tissues in radiotherapy?
fractionation
what aspect of healthy cells makes the use of fractionation beneficial?
the tendency to have better DNA repair mechanisms than cancer cells do
what is fractionation?
dividing the high doses required to kill tumor cells into small fractions, giving healthy tissues time to repair
what is then made with these “smaller doses” during fractionation?
the smaller doses are delivered with a pause of a day or do between fractions
what are the consequences of using fractionation?
- giving time to healthy cells to recover and repopulate
- which means also exposing cancer cells to radiation for longer periods
how else does using fractions damage tumor cells?
by changing the point in the cell cycle at which the tumor cells are irradiated
what does the cure rate and effectiveness of radiation greatly depend on?
- cancer type
- specifics of individual cases
how does cancer type differ when it comes to responsiveness of treatment?
- for regions of the body such as superficial skin cancers, radiation is especially effective
- other areas, the responsiveness of tumor cells differ little from that of neighboring healthy cells (less responsive)
if a cure is unlikely, how can radiation therapy be used?
as a palliative measure
what are the 4 R’s of fractionation?
- repair
- redistribution
- re-oxygenation
- repopulation
describe repair
- lasts for: few hours
- healthy cells are allowed to repair sub-lethal damage
describe redistribution
- lasts for: few hours - days
- tumor cells are given time to redistribute themselves into more radiosensitive phases of the cell cycle between radiation fractions
describe re-oxygenation
- lasts for: few hours - days
- tumor cells are poly-oxygenated
- when exposed to oxygen, the oxygen makes the cancer cells more radiosensitive
- tumor hypoxia and greater radio-sensitivity is seen
what does the use of the 4 R’s determine?
the success or failure of standard clinical radiation treatments
describe repopulation
- lasts for: few weeks
- the tendency of healthy and cancer cells to repopulate after irradiation
what are the types of fractionation schemes?
- conventional
- hyper-fractionation
- accelerated fractionation
- hypo-fractionation
what do we usually choose the type of fractionation scheme based on?
based on the type of cancer
describe conventional fractionation
- dose/fraction: 1.8 - 2.2 Gy
- total dose: 45.0 - 50.4 Gy
- fractions/week: 5 (treatment given once a day)
- issues: can be too slow for fast growing tumors, dose too low for resistant cancers
describe hyper-fractionation
- dose/fraction: 1.1 - 1.3 Gy (v low)
- total dose: 70 - 80 Gy (v high)
- fractions/week: 10 (patient is sometimes irradiated twice a day)
- issues: burden on patients, staff & equipment, no clear benefit has been demonstrated
describe accelerated fractionation
- dose/fraction: 1.4 - 2.5 Gy
- total dose: 40 - 50 Gy
- fractions/week: 10 (treatment 2x a day)
- issues: burden on patients, staff & equipment, acute reactions
describe hypo-fractionation
- dose/fraction: above 2.5 Gy (relatively high)
- total dose: 20 - 55 Gy (low)
- fractions/week: 1 - 5 (treatment given once a day or less)
- issues: acute reactions (due to high dose/fraction)
why do some fractionation types with high dose/fraction cause acute reactions?
with high dose/fraction, in most cases this is a threshold for deterministic effects, which cause acute reactions
which type of fractionation is used for fast growing tumors?
accelerated fractionation
which type of fractionation is used to avoid late normal tissue complication?
hyper-fractionation
usually delivered to the chest wall & regional lymph nodes
conventional fractionation scheme
what is an advantage regarding hyper-fractionation?
- because dose/fraction is lower in this type, this results in avoiding deterministic effects
- lower doses = below deterministic threshold doses
- avoid normal tissue complications in the future
compare accelerated fractionation with conventional fractionation
- conventional: 4.6 weeks for dose to be delivered to tumor
- accelerated: 2 weeks for dose to be delivered
- the same amount of dose is delivered in twice the amount of time in conventional
- this is due to the 10 fractions/week for accelerated fractionation
what are the factors that can be controlled in radiotherapy?
- means of delivering radiation (external beam radiotherapy, LINACs, brachytherapy, proton therapy etc)
- radiation type
- total dose
- fractions into which the total dose is divided (we can choose the fractionation scheme)
- time between fractions
what are two essential issues in radiation therapy?
the type of particle used and its energy
what is the ideal option when it comes to using a particle in radiotherapy?
β(-) particles
why are β(-) emitters the ideal option for radiotherapy and not any other type of particle (alpha, gamma, neutrons)?
- using β(-) emitters or electrons is ideal as β rays can travel a shorter range inside the body (compared to gamma rays) so they can localize the radiation into the tumor
- we don’t use alpha particles because they have a very short range & can travel short distances which prevents them from reaching the diseased region
- neutrons cannot be used as they require a nearby nuclear reactor for their production
- gamma rays CAN be used but for specific cases
when can gamma photons be used for radiotherapy?
in some cases where the tumor is located very deep inside the body, we use gamma radiation
_____ are used for tumors with ranges 1-6cm within the body, while _____ are used for tumors deeper than 6cm
- electrons with energies from 4 - 20 MeV
- gamma photons
what are the 3 alternative approaches for delivering ionizing radiation for therapy?
SOS
(1) beam sources external to the body
(2) brachytherapy sources
(3) unsealed sources
what do beam sources external to the body include?
- the linear accelerators for the production of electrons & gamma photons
- they produce a beam of ionizing radiation aimed at the tumor during treatment sessions
- includes teletherapy machines that consist of radionuclides that decay, and these gamma rays are absorbed by the body
what do brachytherapy sources include?
sealed radioactive sources are placed in proximity with the tumor
what do unsealed sources include?
- radionuclides are taken into the body in liquid form either by injection or swallowing
- ex: iodine131 for treatment of thyroid cancer
what is the most widely used type of approach for delivering ionizing radiation for therapy?
beam sources
what does the beam source deliver?
it delivers a
- uniform
- well-defined
- stable beam
how do we ensure that the beam (1) reaches only the regions intended and (2) delivers the right dose?
(1) the particles are pre-selected for correct penetration depth as per the needs of each individual case
(2) since the exact dose value delivery is crucial, we use dosimeters to monitor the dose received by patients
what two machinery does external beam radiotherapy include?
- linear accelerators (LINACs)
- cobalt machines (or teletherapy machines)
what is the most commonly used device in external beam radiotherapy?
LINACs
why do we not use X-rays in radiotherapy and LINACs instead?
because in radiotherapy, we want rays of VERY high energy, so we use LINACs
what do LINACs offer?
- excellent versatility
- provides either electron or megavoltage X-ray therapy with a wide range of energies
electron and X-rays radiotherapy is carried out with other types of accelerators, along withLINACS, which are?
- betatrons
- microtrons
what are betratrons and microtrons?
- types of circular electron accelerators
- both accelerate electrons to high energies using electromagnetic fields
- betatrons are larger & operate on higher energies
- microtrons are smaller & operate at lower energies
what are 2 other types of X-ray machines used for radiotherapy?
- superficial X-rays (50 - 150 kV)
- orthovoltage X-rays (150 - 500 kV
(also known as deep X-rays or dXR)
the superficial x-ray machines are used for? the orthovoltage x-ray machines are used for?
(1) for more superficial cancers
(2) for deeper cancers
what are the main components of a radiotherapeutic X-ray machine?
- an X-ray
- a ceiling or floor mount for the X ray tube
- a target cooling system
- a control console
- an X-ray power generator
what are treatment machines that incorporate gamma ray sources for use in external beam radiotherapy called?
teletherapy machines
in practice, how do these teletherapy machines work?
they consist of radionuclides that decay & emit gamma photons (which are then directed through the body)
what are the main components of a teletherapy machine?
- a radioactive source
- a source housing with beam collimator and source movement mechanism
- a gantry and stand or a housing support assembly in stand-alone machines
- a patient support assembly
- a machine console
what are the collimators used for in a teletherapy machine?
they are used in order to direct the gamma photons to a specific point inside the body
what does the most widely used teletherapy source use?
Co-60 radionuclides
where are these Co-60 radionuclides contained and sealed?
- inside a cylindrical stainless steel capsule
- sealed by welding
why is a double welded seal used for Co-60 radionuclides?
(1) to prevent any leakage of the radioactive material
(2) to control the direction of the emitted radiation
what is the typical (1) diameter of the cylindrical teletherapy source and (2) typical height?
(1) diameter: 1 - 2cm
(2) height: 2.5cm
what os a penumbra?
- an important parameter in radiotherapy
- refers to the shadow area, or the rapid decrease at the edges of the radiation beam
what is the relationship between the source diameter and its physical penumbra?
the smaller the source diameter, the smaller its physical penumbra, and the more expensive the source
why do we aim to minimize the penumbra?
because in practice, the presence of a penumbra results in the exposure of healthy surrounding tissues to radiation
a diameter of ____cm is chosen as a compromise between the cost and penumbra
1.5 cm
typical source activities are of the order of _____-_____ Ci (185-370 TBq)
5000 - 1000 Ci (v high activities)
how often are teletherapy sources replaced?
often within one half-life after they are installed
in teletherapy, the maximum energy of the electrons is _____ keV, what happens to those electrons?
- 320 keV
- they are not directed into the patient, instead they are absorbed by the cobalt source, or the source capsule
in teletherapy, the __________ constitute the therapy beam
emitted gamma rays
why advantage does teletherapy have over LINACs?
cost-effectiveness
what is conformal radiation?
why do we use in radiotherapy LINACS and not X-ray tubes?
because in this type of accelerator, we use electrons or protons of VERY high energy, as we need to deliver this high energy to tumors
irrespective of the accelerator type, what are the two basic conditions that must be met for particle acceleration?
- the particle to be accelerated must be charged
- an electric field must be provided in the direction of particle acceleration
what distinguishes the various types of accelerators?
- the way they produce the accelerating electric field
- how the field acts on the particles to be accelerated
as far as the accelerating electric field is concerned, what are the 2 main classes of accelerators?
(1) electrostatic
(2) cyclic
what is/are example(s) of electrostatic accelerators?
- superficial X ray tubes (low voltage)
- orthovoltage X ray tubes (high voltage)
- neutron generators
what is/are example(s) of cyclic accelerators?
- the linear accelerator (LINAC) is the best known example
- other examples are microtrons, betatrons, cyclotrons
what is the LINAC process?
- linacs work by speeding up electrons to deliver therapeutic X-rays or electrons to a patient’s tumor
- these treatments can be designed in a way that they destroy the cancer cells while sparing nearby surrounding normal tissue
- they accelerate electrons in a part of the accelerator called the “wave guide,” then allows these electrons to collide with a heavy metal target to produce high-energy x-rays
describe how the LINAC process works
- medical LINACS are cyclic accelerators
- they accelerate electrons to kinetic energies from 4 - 25 MeV (VERY high energies)
- in LINACS the electrons are accelerated following straight trajectories
- they do so in special evacuated structures called accelerating waveguides
explain the process inside a LINAC
- electrons are brought using an electron gun
- this creates an electron beam
- the electrons from the beam are directed into the waveguide
- inside, we have the acceleration of the electrons using radiofrequencies
- the electrons then collide with a heavy metal target to produce high-energy x-rays
inside a linac, describe electron motion
electrons follow a linear path through the same, relatively low, potential difference several times
what is used for electron acceleration?
high power RF fields
(RF = radiofrequency)
what do different types of LINACs produce?
- some provide x-rays in the low megavoltage range (4 - 6 MV)
- others provide both x-rays (photons) and electrons at various megavoltage energies
why do we produce different energies of electrons or photons when using LINACs?
- the concept is the penetrating power of each electron / photon
- electrons of 1 - 4 MeV range 1-6cm deep inside the body, and are therefore suitable for more superficial tumors (neck, head)
- as for tumors deep within the body, we have to use photons of higher energies to give higher penetrating power for the photons to reach the tumor
what will a typical modern high energy LINAC provide?
two photon energies (6 and 18 MV) and several electron energies (6, 9 ,12, 16, 22 MeV)
we select the energies provided by the LINAC depending on?
the location of the tumor
[near the skin = electrons of lower energies]
[deep inside the body = photons of higher energies]
since the complexity of modern LINACs raises concerns regarding operational safety, what statement was given?
the International Electrotechnical Commission (IEC) published a statement on the safety of LINACs
what precautions are taken based on the IEC statement?
- many quality assurance controls were made regarding safety in radiotherapy rooms
- physicists made plans and measurements before deciding the location of the LINAC in the room
why is it crucial to make sure the LINAC measures the correct dose?
because an incorrect dose measured by the LINAC will be delivered to the patient and could result in serious injury, increased complications, genetic effects, and compromised tumor control
what are the 3 categories of safety issues does the IEC document address?
in the LINAC rooms, there could be electrical, mechanical, and radiation safety issues
how are LINACs usually mounted? explain
- they are usually mounted isocentrically
- an isocentric technique is where all beams used in a radiation treatment have a common focus point, (the isocenter)
- isocentric techniques require less patient repositioning as multiple field arrangements can be delivered with gantry and collimator movements, reducing treatment times
the operational systems of LINACs are distributed over 5 major and distinct sections of the machine; what are they?
- gantry
- gantry stand or support
- modulator cabinet
- patient treatment table
- control console
what is the function of the gantry?
it rotates around the patient to deliver the radiation from various angles to produce a 3D volumetric dose inside the patient
what is the function of the modulator cabinet?
this cabinet contains a fan control to cool the power distribution compartments
what are the major components housed in the gantry stand?
the klystron, circulator, and the cooling water system
where is the control console located?
- outside the treatment room
- this is because the technician operating the LINAC is outside the room, for safety purposes, as there are very high doses of radiation being delivered inside the room
what is the function of the klystron?
- produces the microwaves used in order to accelerate the electrons in the accelerator
- used as an amplifier
what is the function of the circulator?
what is the function of the cooling water system?
maintains constant temperature in the LINAC, and to cool the various structures within
