tg-51 and clinical monitor unit calculations
TRANSCRIPT
Treatment Machine Calibra0on (TG51)
and Monitor Unit Calcula0ons By
Vibha Chaswal, PhD
Mentors: Dr. Alfredo R Siochi, Edward Pennington
ObjecBves
• Nuances of Clinical MU calculaBons • TG-‐51 protocol: clinical reference dosimetry procedures for high energy photons and electron beams
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
SAD
10x10
10 cm
10x10
10 cm
SSD
Clinical Reference dosimetry set-‐up: SAD and SSD
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
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
Error eliminaBon in hand calcs: correct field size esBmaBon
….Or use % esBmaBon for blocked area
Rearrange MLCs to get simpler shapes….
Error elimina0on: calc point depth
Either measure physical depth from pinnacle, or use effec0ve depth reported by Pinnacle TPS
Other lessons
Account for missing Bssue Use correct ISF, big difference MU calculaBons in dmax region have more uncertainty
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
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
Thank you!!!