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On the Systematic Difference Between IROC TLD Measurements and Tomotherapy TPS Dose Calculation for Small Body Phantoms

A Kapulsky*, A Ndlovu, Cancer Center & Hackensak Medical Center, Hackensack, NJ, Hackensack University Medical Center, Hackensack, NJ


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Based on our own and other Tomotherapy users’ experience we noticed a systematic difference between in-house planned and IROC-measured doses to TLDs during annual machine calibration.
In the past 7 years the average difference between IROC TLD measurement and Tomotherapy Planning System (TPS) calculation in our institution was -2.6% compared to -2.3% reported previously (1).
We propose the following explanation for this systematic differences.

The “patient” treatment plan for small cylindrical phantom (diameter, d=2.8cm) comprised of two inserts from Tomotherapy Cheese Phantom similar to IROC phantom (diameter, d=3.5cm) was calculated on the Tomotherapy TPS.
The Delivery Quality Assurance (DQA) plan was calculated using the same “ patient” treatment plan for the small phantom but a Tomotherapy Cheese Phantom was used as a phantom, so the same plan was projected to a large phantom body ( diameter, d= 30cm).
Both the “patient” and the DQA plans were delivered 3 times sequentially to maintain similar measurement and delivery conditions.
Dose was measured using an ionization chamber and compared to the average TPS dose to a small ROI equal to a chamber’s measuring volume.

Results: The average measured dose difference from TPS between two sets of plans in the small and the large body phantoms was -3.2%

Conclusion: We have demonstrated that not only IROC TLDs but also chamber measurements for small volume phantoms show doses lower than calculated by the TPS.
The Tomotherapy system is normally calibrated based on several measurements of the dose delivered to the “big” Tomotherapy Cheese Phantom. During standard calibration small body phantom ion chambers measurements are never used for TPS verification and the “large” body TPS calculation models are never adjusted for these not-so clinically relevant “small” bodies.
We propose to use a "large" body Tomotherapy Cheese Phantom for TLD measurements.


Phantoms, Tomotherapy, TLDs


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