Transitioning from TLD to OSLD for In-Vivo

Measurements

Michael Lometti MSc, DABRUniversity of California San Francisco

Helen Diller Comprehensive Cancer Center

• TLDs/OSLDs?

• Why TLDs/OSLDs?

• TLDs

• What are OSLDs?

• Transition to OSLDs

• Advantages of OSLDs

• Conclusion

Outline

TLD: ThermoLuminescence Dosimeters

OSLD: Optically Stimulated Luminescence Dosimeters

TLDs/OSLDs?

• 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

Why TLD’s/OSLD’s?

• TG-40 states “Institutions should have access to TLD or other in vivo systems.”

• 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 in-

vivo dosimetry measurements for years.

• Come in several forms; powder in capsules, disks, rods

TLDs• One-time use only (1 measurement & 1 read)

• Results are not immediate (~24 hours) No friday measurements!

TLDs• Can be difficult to handle and time consuming

(control TLDs, vacuum tweezers, scales, nitrogen, glow curves & annealing ovens)

Physics of TLD’s

• 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

3 Step Process

Physics of TLD’s

• Light is then detected by a photomultiplier tube

• Amount of light detected is proportional to amount of ionizing radiation absorbed

LiF:MgTi

Incident Radiation

Conduction Band

Valence Band

Energy Trap

Electron

Heat

Visible Light

What are OSLDs?

OSLD TLD

• Similar to TLDs but Optically Stimulated Luminescence Dosimeter

• Trapped energy is released not by heat as with TLDs but using light (much faster & more precise process than heating)

• As with TLDs, amount of light measured is proportional to amount of radiation absorbed

What are OSLDs?

475 nm

410 nm

• OSLDs use Al2O3:C (Aluminum Oxide doped with Carbon)

• Dosimeter is a plastic disk infused with Al2O3:C *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

What are OSLDs?• 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)

• No significant energy dependence in therapy 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

What are OSLDs? Transition to OSLDs(Therapy range in-vivo measurements)

• 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 calibrated prior to shipping

This means there is no need to expose a control set of OSLDs for every measurement

Transition to OSLDs• No calibration curve is provided for therapy

range, need to make curve yourself

• 0, 50, 150, 300 cGy for linear curve

• Can create non-linear curve for > 300 cGy

P. Jursinc, “Characterization of optically stimulated luminescent dosimeters, OSLDs, for clinical dosimetric measurments,” Med Phys. 24,

4594-4603 (2007)

Transition to OSLDs• Compare your results to your TLDs

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effective depth of ~0.37 mm

Transition to OSLDs• But is it right?

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Transition to OSLDsUCSF Setup

• 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

OSLD Advantages

• Simpler & Faster workflow than TLDs

• Optical stimulation means no heating (more accurate and faster)

• Dosimeters are re-readable and re-usable and can be stored

• No angular, temperature, dose-rate or energy dependence (for 6 and 18 MV)

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!