tg-51 and clinical monitor unit calculations
TRANSCRIPT
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Treatment Machine Calibra0on (TG51)
and Monitor Unit Calcula0ons By
Vibha Chaswal, PhD
Mentors: Dr. Alfredo R Siochi, Edward Pennington
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ObjecBves
• Nuances of Clinical MU calculaBons • TG-‐51 protocol: clinical reference dosimetry procedures for high energy photons and electron beams
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Part 1: MU CalculaBons
• Hand-‐calculaBons performed over number of cases
• Performed using TPR technique
• MU equaBons:
€
MUTPR =Dose / fx * beam weight
K *TPR@ calc depth for fs@ calc * Spfs@ calc * Sccoll fs * ISF *OAR@ calc depth for off -axis distance *TF *WF
€
MUPDD =Dose / fx * beam weight
K *PDD@ calc depth for fs @ surface, SSD * Spfs@ surface * Sccoll fs * ISF *OAR@ calc depth for off -axis distance *TF *WF
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SAD
10x10
10 cm
10x10
10 cm
SSD
Clinical Reference dosimetry set-‐up: SAD and SSD
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Dose CalculaBon point Reference
calibraBon point
100 cm
Source Source
Dose to the Water Phantom at reference condi0ons => Dose at any point in the pa0ent
Hand Calcula0ons are a stepwise transla0on
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Hand MU calculaBons
i. Iso-‐centric calculaBons ii. Point off Isocenter in the iso-‐centric plane (AP/
PA) iii. Point off isocenter off the iso-‐centric plane (AP/
PA) iv. Whole brain v. TangenBal asymmetric fields – Breast plan vi. 4 field breast – PAB /super-‐calv and tangenBal
fields
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Error eliminaBon in hand calcs: correct field size esBmaBon
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….Or use % esBmaBon for blocked area
Rearrange MLCs to get simpler shapes….
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Error elimina0on: calc point depth
Either measure physical depth from pinnacle, or use effec0ve depth reported by Pinnacle TPS
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Other lessons
Account for missing Bssue Use correct ISF, big difference MU calculaBons in dmax region have more uncertainty
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TangenBal breast/chest wall tx and 4-‐field breast tx
Involves: -‐ Constantly changing breast contour and volume in the field -‐ Oblique surface contour -‐ Asymmetric Jaw -‐ Off-‐axis calculaBon point
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TG-‐51 CalibraBon
• TG-‐51 calibraBon for 6X photon beam – Elekta • TG-‐51 calibraBon for all Photon and Electron Energies on Oncor A
Lessons:
-‐ EffecBve point of measurement and point of measurement depths
-‐ What is real dmax
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Thank you!!!