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TU‐FF‐A1‐02: IMRT Verification with 3 Min/plan, 50 Plans/week, 1 Therapist and Only EPID Dosimetry
Author(s) -
McDermott LN,
Mans A,
Wendling M,
Stroom J,
van Herk M,
Mijnheer BJ
Publication year - 2008
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2962639
Subject(s) - dosimetry , imaging phantom , image guided radiation therapy , nuclear medicine , medical physics , medicine , radiation treatment planning , attenuation , pinnacle , radiation therapy , computer science , radiology , physics , optics
Purpose: To provide an overview of 3 years' clinical experience with EPID dosimetry as our only form of plan‐specific IMRT verification, used for all curative patient treatments. Method and Materials: Dose is checked in 2D per field (back‐projected to the isocentric plane perpendicular to the beam) or in 3D per plan (summed back‐projected images to a volume). The algorithm includes corrections for EPID and patient scatter, attenuation (using CT data) and the inverse square law. Open images for each field are used to calculate the attenuation. For cases with large inhomogeneous regions (e.g. lung), plans are re‐calculated with inhomogeneity corrections “off” and compared with an adjusted dose reconstruction. With γ = 1 for 3.0% (of isocentre dose) and 3.0 mm differences, an alert is raised if γ mean> 0.5 ,γ max 1 %> 2.0 ,P γ > 1> 15 % or Δ D isoc> 3.0 % . Results: This year, 50 (IMRT) plans per week are verified by 1 therapist. Physicists perform the initial EPID dose calibration and are available in case errors arise. Checks are performed in vivo , except one‐fraction treatments and large fields (> EPID), which are verified by EPID in a phantom. Typical total analysis time is 3 minutes (3 fractions in vivo ). Between 2 and 4 serious errors have been prevented per year, i.e. plans changed based on discrepancies in 2D, or if necessary, 3D dose distributions. Conclusion: As the sole form of plan verification for a large radiotherapy department, the combination of in vivo and pre‐treatment EPID dosimetry allows a holistic approach to IMRT verification. Three years of clinical experience has proven the system to be highly efficient, sufficiently accurate and provides a safety net as the final check that all patients are correctly treated. EPID dosimetry solves time and management problems, and should become ubiquitous throughout radiotherapy centres worldwide.