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Intro Radiotherapy

Organisation of the Body

QuestionAnswer
Scale of cancer Number one fear of British people More that 200 type each with different causes, symptoms and treatments Breast, lung, prostate and bowl cancers make up over half of cases Male incidence rates have rise by 23% compared to 43% in women
Grade of tumours Differentiation is incomplete to some extent Described in terms of tumour grade Grade I - well differentiated Grade III - poorly differentiated Has prognostic value
Stage of tumours Described on the TNM scale (tumour size 1-4, lymph node involvement 0-3 and metastasis 0 or 1 ) Or on an overall scale of 1 (small localised) to 4 (metastasis) Has prognostic value and often determines course of therapy
Survival rates of tumours Different cancers have different clinical outcomes Judged by survival or progression free survival Early stage cancers have the best prognosis whilst late stage cancers have the worst Linked to treatment options
Treatment options for cancers Surgery 49% - can cure local cancers by removing the tumour Radiotherapy 40% - large contribution Chemotherapy 11% - relatively little use except in certain tumour types
Outcomes of DNA damage Detected by damage sensors e.g. ATM Can lead to cell cycle arrest, DNA repair or apoptosis
Ionising radiation High energy low frequency wavelengths Can ionise material E.g. UV, X-ray and Gamma Triggers a free radical cascade starting with water in molecular tissues This generates DNA damage by many mechanisms
Iodine - 131 A radioactive isotope of iodine used to treat thyroid cancer and hyper-thyroidism (Grave's disease) Thyroid is a natural scavenger of iodine, so it concentrates in the thyroid gland Repurposed for other purposes e.g. target CD20 receptors on B cells
Brachytherapy Directly implanting metallic pellets into a patient A radioactive material releases ionising radiation directly into the tumour E.g. Iridium - half life 74 days
Radiotherapy Treatment 60% of patients receive this (alone or alongside chemo/surgery) High energy X rays delivered with a linear accelerator Localised against the tumour to avoid normal tissue Patient is immobilised Planning CT performed Can be palliative or curative
Linear accelerator Electrons accelerated using synchronised microwaves and a voltage gradient X-rays produced by electrons striking a metal target Energy of x-rays dependant on electron energy Isocentric set up - machine rotates around the tumour
The Compton effect Ionisation of water forces out electrons The scattered photons then have an increased wavelength Electron has a lower wavelength - depth of the effect is pronounced -
Distribution of dose The peak delivery is around 2 1/2 cm depth At greater depths the ionising potential is lost Changing voltage has very little effect Due to Compton effect Limits therapeutic potential
Organs at risk Many structures in the body are radiosensitive E.g. heart, lungs, spinal cord Dosage to these areas must be limited to prevent damage
Beam modification - multi leaf collimation Lead plates that can be adjusted to shape the x-ray beam The radiotherapy fields can be conformed to the shape of the tumour Enables shielding of some surrounding normal tissue
Wedges Thick end absorbs the beam more that the thin part Alters dose distribution of the beam Steepness of the wedge determines the change in beam characteristic Dynamic wedges more common than manual wedges
Gross tumour volume Extent of disease detectable by imaging or other clinical methods
Clinical target volume Contains the GTV and surrounding areas considered likely to contain subclinical disease e.g. adjacent tissues, lymph nodes
Planning Target volume Contains the CTV and a margin to allow for physiological and technical variations Accounts for limitations in imaging and motion
Immobilisation Treatment must be delivered to the intended area otherwise the tumour is missed and normal tissue inadvertently irradiated Need to be able to place patient in a position where they will remain still Precisely matched to patient anatomy
What does the level of immobilisation depend on How much the organ moves e.g. lung tumours Level of importance of immobility e.g. radiosurgery of brain tumours
Planning CT Patient undergoes a CT wearing any immobilising structures They are in the same position as they will be for the radiation - markers are placed on the patient to ensure the same position is used each time
3D conformal radiotherapy - Planning Patients scanned in treatment position Set up is identical to treatment room Tattoo made on skin which is then covered with radio-opaque marker for visualisation on CT Correlation between marker and CT images
3D conformal radiotherapy - process Involves a high dose of radiotherapy to patients with curable disease 3 or more intersecting beams Enables precise decisions to be made regarding treatment volumes Homogenous dose across tumour Beams can be aligned, shaped and wedged
Work plan of radiotherapy Oncologist plans treatment Each Ct slice outlines tumour and organs Grow to PTV Decide on dose and tell physicists what tolerances will be accepted by tissues Physicists optimise beam arrangements
Stereotactic radiotherapy Beams delivered from many directions Lots of beams provide low dose from each direction to give a highly specific higher dose Limits dosage to surrounding tissue SABR (A-ablative) at non-cranial sites
Intensity modulated radiotherapy Using varying intensities of hundreds of small radiation beams produce dose distributions that are more precise compared to 3DCRT Difference in physics Allows irradiation of local lymph nodes Lowers exposure to surrounding tissue
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