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Heterogeneities

QuestionAnswer
Human body heterogeneitites - Teeth - Bone - Air cavities - Lungs
Man-made heterogeneities - Mandibular plates - Prostheses - Spinal cord fixation devices - Surgical rods - Stents - Dental fillings - Tracheostomy tubes - Fiducial markers - Breast implants
CT-ED file - Converts CT numbers (HU) to electron density values for the TPS - Bone: 1000, water: 0, air: -1000, soft tissue: 1
Planning considerations - Don't place an entry beam in line with spinal implants - Contour and override the streaking artefact from dental fillings
High-Z - Material with an atomic number greater than cortical bone - e.g. tungsten, gold, gadolinium
6MV & heterogeneities - Increased dose proximal to the high-Z material - Reduced distal dose from attenuation
18MV & heterogeneities - Increased dose proximal to high-Z material - Increased dose distal to high-Z material also, due to pair productions in the metal -> overdosing patient (use lower energies when treating patients with metal implants)
iMAR - Metal artefact reduction - Occurs in CT - Improves image quality and therefore contouring and dose accuracy
3DCRT - Avoid beams passing through device - Can increase dose to OARs - Non-coplanar approach (depends on ease of planning and delivery, collision risks, portal verification and staff training)
IMRT - Avoid prosthesis - Contour prosthesis and make it a critical structure with a dose constraint - 5-9 beams
VMAT - Contour avoidance sector - Use avoidance sectors (Eclipse)
Contouring - Use different window levels - Check relative electron densities - Contour different components separately
AAPM recommendations - Investigate limitations regarding heterogeneity corrections - MDT must be informed of prostheses prior to sim - Use medical records, surgeon, manufacturer - Beam arrangements that avoid the prosthesis - Use the most physically rigorous algorithm
Created by: chloe.donaldson
 

 



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