Outline
Transitioning from TLD
to OSLD for In-Vivo
Measurements
Michael Lometti MSc, DABR
University of California San Francisco
Helen Diller Comprehensive Cancer Center
TLDs/OSLDs?
• TLDs/OSLDs?
• Why TLDs/OSLDs?
• TLDs
• What are OSLDs?
• Transition to OSLDs
• Advantages of OSLDs
• Conclusion
Why TLD’s/OSLD’s?
TLD: ThermoLuminescence Dosimeters
OSLD: Optically Stimulated Luminescence Dosimeters
• TG-40 states “Institutions should have
access to TLD or other in vivo systems.”
•
Both TLDs & OSLDs contain materials that are
used to measure ionizing radiation by
measuring the amount of visible light emitted
from the material after exposure to ionizing
radiation
• Radiological Physics Center (RPC) changed
in 2010 from TLDs, in use since 1968, to
OSLDs for yearly remote verification of
radiotherapy units
TLDs
• TLD’s have been used for accurate and reliable invivo dosimetry measurements for years.
• Come in several forms; powder in capsules, disks,
rods
TLDs
• Can be difficult to handle and time consuming
(control TLDs, vacuum tweezers, scales, nitrogen,
glow curves & annealing ovens)
TLDs
• One-time use only (1 measurement & 1 read)
• Results are not immediate (~24 hours)
No friday measurements!
Physics of TLD’s
3 Step Process
• Incident radiation causes electrons in material to be
excited out of the ground state by absorbing
sufficient energy.
• Electrons can get trapped between valence and
conduction band due to impurities in material
• When heated, electrons may escape from trap and
return to valence band releasing energy in the form
of visible light
Physics of TLD’s
What are OSLDs?
• Similar to TLDs but Optically Stimulated
Incident Radiation
Luminescence Dosimeter
Heat
• Trapped energy is released not by heat as with TLDs
Conduction Band
LiF:MgTi
but using light (much faster & more precise process
than heating)
Energy Trap
Visible Light
• As with TLDs, amount of light measured is
Electron
Valence Band
proportional to amount of radiation absorbed
• Light is then detected by a photomultiplier tube
• Amount of light detected is proportional to amount
of ionizing radiation absorbed
OSLD
What are OSLDs?
What are OSLDs?
• OSLDs use Al O :C (Aluminum Oxide doped with
2
3
Carbon)
• Dosimeter is a plastic disk infused with Al O :C
2
3
*Don’t open your OSLD like this!
• Basic physics of OSLDs are very similar to TLDs
• OSLDs stimulated with a broad spectrum of light
w/ peak ~475 nm green light from LED or laser
• Emission occurs over broad spectrum w/ peak
~410-420 nm
475 nm
410 nm
TLD
• Therapy dosimeters available as
• 1.0 x1.0 x 0.2cm nanodot
• 1.0 x 2.5 x 0.2cm dot
• Multiple reads (non-destructive readout)
• Multiple exposures
• Range of µGy to 15 Gy
• Linear response up to 3-4 Gy
• High stability over time (can be stored)
What are OSLDs?
Transition to OSLDs
(Therapy range in-vivo measurements)
• No significant energy dependence in therapy
• Workflow is similar to that of TLDs
• Order your dots
• Regular or screened?
• ± 5% or ± 2% accuracy
• Each dot is exposed to small dose and
range (6 MV &18 MV)
• Electrons have ~ 5% diff compared to 6 & 18 MV
• No angular dependence
• No temperature dependence
• No dose rate dependence
• Ready to read dose 8-10 minutes after exposure
calibrated prior to shipping
This means there is no need to expose a
control set of OSLDs for every measurement
Transition to OSLDs
Transition to OSLDs
• No calibration curve is provided for therapy
• Compare your results to your TLDs
range, need to make curve yourself
P. Jursinc, “Characterization of optically
stimulated luminescent dosimeters, OSLDs, for
clinical dosimetric measurments,” Med Phys. 24,
4594-4603 (2007)
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Transition to OSLDs
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Transition to OSLDs
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Transition to OSLDs
UCSF Setup
OSLD Advantages
• Simpler & Faster workflow than TLDs
• Optical stimulation means no heating
(more accurate and faster)
•
•
Skin dose (breast, facial, genitals, scar, etc...), TBI,
Cyberknife, LINAC validation,
CT dose, fluoroscopy, radiation safety around vaults
Environmental whole body badges for emergency
preparedness
• Dosimeters are re-readable and re-usable
and can be stored
• No angular, temperature, dose-rate or
energy dependence (for 6 and 18 MV)
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Conclusion
•
OSLDs offer equal (or better) accuracy and
reliability of TLDs
• Readings are available in less time and with less
effort than TLDs
• Dosimeters can be reread, stored or reused
• Price/dosimeter may be slightly more for OSLD
than TLD, however time savings will be significant
• I have no affiliation with OSLD other than I being
a happy user
Thanks
• Jean Pouliot
• Bruce Faddegon
• Josephine Chen
• Martina Descovich
• Olivier Morin
• Cynthia Chuang
• Paula Petti
• Everyone who came out today!