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Examining the Magnitude of Dose Calculation Errors as a Component of IROC Lung and Spine Phantom Failures

S Edward*, M Glenn , P Balter , J Pollard-Larkin , C Peterson , R Howell , S Kry , UT MD Anderson Cancer Center, Houston, TX


(Thursday, 7/18/2019) 1:00 PM - 3:00 PM

Room: 304

Purpose: From 2012 to present, 17% of IROC SBRT spine and 15% of IROC moving lung phantom irradiations have failed to meet established acceptability criteria. In this study, we looked at the contribution of dose calculation errors to these failing results.

Methods: We evaluated dose calculation errors by comparing the calculation accuracy of institutions’ treatment planning systems (TPS) versus IROC-Houston’s previously established independent dose recalculation system (DRS). Each calculation was compared to physical dose delivered to the phantom; cases where the recalculation was more accurate were interpreted as a deficiency in the institution’s TPS. 188 phantom irradiation plans (128 lung and 60 spine) were recomputed. The spine phantom simulates a highly modulated SBRT case; the lung phantom represents a low-to-none modulation moving target case.

Results: Although overall, the DRS performed better than the TPS in only half (52%) of the spine cases, the DRS was more accurate among failing spine cases 93% of the time. This difference was found to be over 2% on average, indicating errors in the institution’s dose calculation system. In contrast, the lung phantom DRS recalculations were superior for only 31% of all cases and 28% of failing lung phantoms, indicating that the TPS was more accurate on average for both the passing and failing lung phantom groups. Dose calculation errors were therefore not substantially present among lung phantoms.

Conclusion: For spine phantoms, the institutions’ TPS performed better overall, but the DRS performed remarkably better among failing phantoms. This indicates substantial room for improvement in institutions’ TPS calculations in the majority of cases where the institution failed the phantom. In contrast, lung phantom calculations indicated no direct dependence of failure on dose calculation.


Not Applicable / None Entered.


TH- Radiation dose measurement devices: Phantoms for dosimetric measurement

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