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Enhancing Accuracy in In Vivo Dosimetry for Radiotherapy

In vivo dosimetry is crucial for accurate radiation dose delivery in radiotherapy, ensuring patients receive the prescribed dose. This process involves measuring the dose delivered to patients directly, accounting for individual variations like patient contour, organ motion, and treatment setup. Reliable dosimeters, such as semiconductors, TLDs, and EPIDs, are essential for precise readings. Calibration and correction factors, including temperature and beam energy, must be carefully managed to maintain accuracy. Novel techniques like dose-guided radiotherapy aim to improve precision and patient outcomes.

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Enhancing Accuracy in In Vivo Dosimetry for Radiotherapy

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  1. In vivo dosimetry Eirik Malinen Eva Stabell Bergstrand Dag Rune Olsen

  2. error Probalility Prescribed dose In vivo dosimetry • In vivo: In the living • Dosimetry: Estimates of radiation dose by theory and measurement • Verification of delivered • dose to individual patients • Radiotherapy requires • accurate dose delivery

  3. Errors in patient dose • Patient contour / planning basis (CT images) • Patient motion • Organ motion • Dose calculations (inhomogeneities, scatter) • Patient positioning • Transfer of treatment data from simulator to linac • Linac settings (energy, monitor units, field size) and calibration • Beam modifiers (blocks, wedges)

  4. Dose characteristics

  5. Dose measurements Point detector beam Entrance dose: wedge 2D detector array Output, SSD Wedge, curvature Patient curvature Exit dose: Thickness, density

  6. High accuracy Low precision Low accuracy High precision Desired in vivo dosimeter characteristics • Accurate and precise • Multiple readouts • Reusability • No cables • Non-destructive readout

  7. In vivo dosimetry principles • Point detector: • Semiconductors (diodes) • Thermoluminescent crystals • EPR (electron paramagnetic resonance) sensitive materials • …. • 2D detector, (electronic) portal imaging device; EPID: • Film • Arrays (ion chambers, semiconductors)

  8. Dosimeter reading → absorbed dose • Absorbed dose, D: • R: dosimeter reading • ND: calibration factor • Ci: correction factor

  9. Calibration • Under reference conditions: beam dosimeter Rcal dmax ion chamber Dcal water phantom

  10. Example – diodes spherical droplet

  11. Buildup cap

  12. Correction factors • Dosimeter reading may depend on: • Temperature • (Accumulated) Dose • Dose rate • Beam energy • Field size • ... • Accuracy may be reduced if dependence is not corrected

  13. Temperature and sensitivity, diodes Detector temperature after placing on patient Sensitivity dependence

  14. Accumulated dose and sensitivity, diodes • Regular calibration must be performed

  15. Field size and sensitivity, diodes 8 or 18 MV photons Entrance (in) or exit (out)

  16. Supralinearity, TLD

  17. Energy dependence, TLD

  18. Comparison

  19. Action level • Relative dose difference: • At what dose difference level should the treatment be revised? 1% ? 2.5 % ? 5 %? • Depends on: • dosimetric accuracy and precision • non-systematic errors • …

  20. Clinical example

  21. Methods Portal image profile

  22. Measured dose / prescribed dose Action level: 2.5% measured dose dose after correction

  23. Frequency distribution of relative dose

  24. Treatment planning algorithm Portal image Collapsed cone algorithm Location of normalization point 2D dose maps

  25. Novel methods – ”dose guided radiotherapy” prescribed isodose target Backprojection of filtered dose image into patient image →OK →correction dose image

  26. Corrections Novel methods – ”dose guided radiotherapy” bladder prostate rectum

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