radiation therapy rt 265 history zx-rays discovered in 1895 zbecquerel’s accidental experiment...
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Radiation Therapy RT 265HISTORY
X-rays discovered in 1895Becquerel’s accidental experiment
showed the first radiobiological effects of x-rays
Experimentation of ram testicles revealed radiosensitivity of different tissues
Higher energy units available in 1950s and advent of linear accelerators
Basis of radiation for therapy
Electromagnetic radiations release energy indirectly to cause cellular damage
X-rays and Gamma rays are similar in action; their production is different
X-rays are produced extranuclearlyGamma rays produced intranuclearly
Production of radiation to cause effect
Depth of irradiation depends on radiation beam
Lower energy beams affect skin Higher energy beams spares skin Difference between Cobalt-60 and
lower energy linear accelerators involves beam shape
Radiation Dose Quantification
Rad has generally been replaced by GrayMeasurement of dose is difficult directlyAbsorbed dose is calculated based on
indirect measurements of ionization of airPattern of energy deposition varies with
types of particles causing cellular disruption
Effects on Tumors
Both malignant cells and normal cells respond similarly to radiation
Both undergo repair of sublethal damage
Both cell types are more sensitive during the mitotic phase
Only malignant cells have areas of hypoxia - reason for fractionation
Systemic EffectsA value often used is LD50 which is the lethal dose for
50% of the population sampleDeaths due to total body exposureWhen TBI used before bone marrow transplant interstitial
pneumonitis is the limiting factorEffects on immune reactions varyDepressions generally occur only when large tumors are
irradiated or large surface areasNausea and vomiting secondary to irradiation or disease
processes Nausea that presents later during treatment may be
secondary to underlying disease process
Radiobiology
FractionationReassortment of cellsRepair of sublethal damageAccelerated repopulation
Fractionation
Single prolonged dose has profound effects on normal tissues
Studies on spermatogenesis of ramsReason for fractionation - allows
tumor cells to reassort into the mitotic phase
Reduces hypoxia while sparing normal tissues
Reassortment
Cells more radiosensitive in mitosis or late in G2
Survival curve is steep in these stages
Fractionation permits cells to reassort themselves into more sensitive phases of the cell cycle to allow better killing
Sublethal Damage Repair
Molecular basis not understoodDefined as increase in survival when a
dose of radiation is splitThis feature is ubiquitous among cellsBecause of ability to repair damage
quickly, melanomas have been thought of as “relatively radioresistant”
Rationale for fractionation
Reassortment allows for better cell killingRepair of sublethal damage should be
minimizedReoxygenation allows for better cell
killingHyperfractionation used to minimize the
late effects of irradiation while increasing dose and tumor control
Tumor volume and control by dose of irradiation
Difficult to extrapolate dataAssumptions must be made: - number of cells proportional to
volume - hypoxia does not vary with tumor size60 Gy leads to depopulation of
10,000,000,000 or regression of a 2 cm mass in 90% of patients
Sequelae of Treatment
Acute and late effectsMucositisDysphagiaOsteoradionecrosis increases with
irradiated volume and increased dose and proximity of dose to mandible
Lhermitte’s syndrome and transverse myelitis
Brachytherapy
Radioactive sources placed close to the target
Temporary and permanent implantsAdvantagesEntire tumor must be accessibleLymph node metastases preclude sole use
of brachytherapy
Radiotherapy Treatment Planning
PlanningSimulationTreatment
Every treatment using radiotherapy has to be rigorously planned. The planning process consists of three phases:
Radiotherapy Treatment Planning
PlanningThe cancerous tumour has to be located so that its size and position can be analysed. This information can be obtained from:
•X-rays•CT scans•MRI scans•Ultrasound images
Radiotherapy Treatment Planning
Simulation
Once the amount of radiation to be given has been accurately calculated, the patient then goes to the simulator to determine what settings are to be selected for the actual treatment using a linear accelerator.
The settings are determined by taking a series of x-rays to make sure that the tumour is in the correct position ready to receive the ionising radiation.
Radiotherapy Treatment Planning
Treatment
Irradiation using high energy gamma rays.
Irradiation using high energy x-rays.
Cancerous tumours can be treated using radiotherapy as follows:
Irradiation Using High Energy Gamma Rays Gamma rays are
emitted from a cobalt-60 source – a radioactive form of cobalt.
The cobalt source is kept within a thick, heavy metal container.
This container has a slit in it to allow a narrow beam of gamma rays to emerge.
Radiotherapy Treatment
Irradiation Using High Energy X-rays The x-rays are
generated by a linear accelerator (linac).
The linac fires high energy electrons at a metal target and when the electrons strike the target, x-rays are produced.
The x-rays produced are shaped into a narrow beam by movable metal shutters.
Treatment of Cancer
Radiotherapy
The apparatus is arranged so that it can rotate around the couch on which the patient lies.
This allows the patient to receive radiation from different directions.
The diseased tissue receives radiation all of the time but the healthy tissue receives the minimum amount of radiation possible.
Treatments are given as a series of small doses because cancerous cells are killed more easily when they are dividing, and not all cells divide at the same time – this reduces some of the side effects which come with radiotherapy.
Radiation Therapy
Brachytherapy This involves placing
implants in the form of seeds, wires or pellets directly into the tumour.
Such implants may be temporary or permenant depending on the implant and the tumour itself.
The benefit of such a method is that the tumour receives nearly all of the dose whilst healthy tissue hardly receives any.
Radiation Therapy
Brachytherapy
UterusCervixProstateIntraocularSkinThyroidBone
Brachytherapy is used to treat the following cancers: