Geometric errors in 3DCRT and IMRT
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This document discusses geometric errors in 3D conformal radiation therapy (3DCRT) and intensity-modulated radiation therapy (IMRT). It explains that IMRT is more sensitive to geometric errors due to its higher conformality. There are two main types of errors: mechanical errors related to equipment and set-up errors from inaccurate patient positioning. Set-up errors can be random or systematic. Systematic errors have the largest impact on treatment accuracy. Minimizing errors requires reducing both random errors through improved immobilization and systematic errors through careful documentation and quality assurance procedures. Accurate patient positioning is crucial for optimal treatment.
Geometric errors in 3DCRT and IMRT
- 1. Geometric Errors in 3DCRT And IMRT Prof Amin E AAmin Dean of the Higher Institute of Optics Technology & Prof of Medical Physics Radiation Oncology Department Faculty of Medicine Ain Shams University
- 2. Goal of Radiation Therapy • Precise delivery of high dose to the target volume: – Destruction of tumor tissue. • Limited dose to surrounding normal tissue/organs: – Minimize acute and long term toxicity risk.
- 3. Achievement of Treatment Goal To Achieve the treatment goal we need to: • Accrately position patient for each treatment. • Eliminate body movement and minimize organ motion during treatment. – Inadequate dose to tumor may lead to local recurrence. – Limit target and critical normal tissue movement.
- 4. Clinical Rationale of 3D and IMRT • Achievement of dose escalation: – To improve loco-regional control – To increase overall survival • Reduction of normal tissue complications: – To improve quality of life • This require a high level of accuracy
- 5. Accuracy And Precision • Accuracy: is the closeness of the measurement value to the true value. • Precision: is the closeness of measurement from one another.
- 7. Treatment Accuracy • Dosimetric accuracy • Geometric accuracy • Accurate treatment requires understanding of Errors
- 8. Geometric Error • The impact of geometric errors increases with the conformality of the radiation therapy treatment. • IMRT is more sensitive to geometric errors because of its higher conformality than that of 3DCRT. • IN 3DCRT; the larger the number of fields is the higher the conformality and accordingly the higher the expected geometric error.
- 9. Types of Errors There are essentially 2 different errors; • Mechanical errors • Set-up errors – Discrepancy between intended and actual treatment position. • Random Errors: Inconsistent deviation • Systemic Position: Errors: Recurring error.
- 10. Set-up Error • Systemic errors have the largest impact. • Set-up error determined relative to the isocenter, the field borders or both. • Smaller random set-up error means more accurate treatment • Smaller systematic set-up error means more precise treatment
- 11. Random Errors Random Errors: Inconsistent deviation • Patient movement and organ motion • Inconsistent repositioning • Variables in equipment and devices • Inconsistent interpretation of skin marks
- 12. Systematic Error • Systematic positioning errors: Recurring error • Systematic errors have the largest impact on the margin needed to adequately dose the target. • The greatest benefit is gained from treatment strategies when we reduce systematic errors.
- 13. Systematic Error • Target delineation error • Change in target position and/or shape • Transfer error • Misinterpret set-up instructions • Blocks incorrectly cut/prepared • Discrepancies between simulator, treatment unit, etc. • Treatment plan transcription errors
- 14. Systematic Error • where no immobilization device is used to stabilize the patient • The careless use of an immobilization device • Inaccurate recording of positioning co-ordinates or a badly constructed immobilization system that has poor configuration and freedom of movement
- 15. Minimizing Errors Mechanical Errors • Intensive acceptance testing • Rigorous maintenance • Good equipment quality assurance protocol • Regular equipment quality control in a daily, weekly, and monthly basis
- 16. Minimizing Errors Random Errors • Improved patient immobolisation • Proper education of RTT • Better set-up procedure
- 17. Minimizing Errors Systematic Errors • Detailed documentation for individual patient set-up • Good checking system and work procedures
- 18. Importance of Patient Positioning • Careful consideration of patient positioning will allow optimal treatment planning • Corollary – modern treatment planning cannot compensate for sub-optimal patient positioning
- 19. Importance of Patient Positioning • Limb sarcoma: –Need to re-position contralateral leg
- 20. Principles of Accurate Set-Up • Place patient in comfortable, relaxed position • Always check that the patient is straight • Use modern laser system • Extend the surface markings superiorly and inferiorly • Reference the patient position (e. g. tilt of head) and field to external anatomical landmarks (and to bony landmarks wherever possible) and record carefully • Make use of immobilization devices
- 21. Principles of Accurate Set-Up • Be sure that all set-up and positioning instructions are adhered to for every treatment • Never work alone and be sure that every set-up is checked by minimum of 2 qualified RTTs • Only start the treatment when absolutely sure the set-up is correct
- 22. Three Co-Ordinate System 1. Patient co-ordinate system 2. Room co-ordinate system 3. Beam co-ordinate system
- 23. 1- Patient co-ordinate system • Defined during localization and (virtual) simulation • Comfortable, reproducible position that will become treatment position • “pretreatment” fiducial marks on skin or immobilization device – Choose taut and relatively immovable skin • Avoid soft and flabby ereas – Reference skin marks to bony landmarks (manual or via imaging system)
- 24. 2- Room co-ordinate system • Origin is the isocentre of the external beam unit. • All indicators relate to the isocentre and corresponding horizontal and vertical planes of the central axis
- 25. 3- Beam co-ordinate system • Central ray of the radiation beam and X and Y axes in a plane orthogonal to the central ray • Origin is the isocentre of the external beam unit • Rotates relative to the room co-ordinator system
- 26. Process of Positioning Patient • Place patient on couch in correct position and with correct devices/systems • Align patient and room co-ordinate systems (superimpose orthogonal room lasers with fiducial skin marks) • Confirm target Surface Distance (TSD) with Optical Distance Indicator (ODI)
- 27. Process of Positioning Patient • Implement treatment plan for each field (align beam with room co-ordinate system) – Rotation of gantry collimator and couch (most complex planning and set-up) – Field size and beam modifiers