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Phantoms from the Imaging and Radiation Oncology Core (IROC) show 8-17% of institutions fail to deliver the intended dose within established criteria. This work sought to quantify the source and magnitude of various sources of error in treatment delivery.

IROC lung (n = 41), head and neck (H&N) (n = 36) and spine (n = 63) phantoms irradiated by various institutions, as an end-to-end radiotherapy test, were evaluated for quantifiable errors that may occur in the planning and delivery phases of treatment. Plans were independently recalculated to assess dose calculation errors, delivery log files were used to evaluate delivery errors, and contemporaneously conducted beam output audits were used to evaluate machine output errors.

The errors from each source were used to characterize the dose deviations that exist between the institution’s TPS dose and measured TLD dose, to determine how much of that deviation was accounted for by the error sources investigated. A dose difference metric, D, was used to describe the result, where a positive D value indicated the presence of true error, and a negative D value did not. 

Phantom results were evaluated in 2 groups, separated by a threshold total dose deviation (3.2%) (Kirby, 1992) to account for measurement uncertainty in phantom TLD doses. No delivery error was assessed for spine due to lack of log file data.

Among all 3 phantoms, cases with absolute dose deviation within measurement uncertainty (< 3.2%), expectedly had a negative average D value, showing that not much, if any, of the error could be described by the categories investigated.

Phantom results with substantial dose deviation (> 3.2%) had positive D values in all categories, indicating that some of the existing dose deviations were accounted for by the error modes we evaluated. Output error was the largest category of error for the lung phantom (16%), whereas dose calculation error was the major contributor for the H&N (49%) and spine (40%). The total magnitude of error quantified among these phantoms, i.e., the % of absolute dose deviation accounted for, was 20.5%, 68.3% and 70.9% for lung, H&N, and spine respectively (Fig. 1). These findings were further emphasized by the positive correlations found between absolute dose deviation and error type per phantom: lung = output (Pearson correlation; r = .54, p < .01), H&N = dose calculation (r = .72, p < .01) (Fig. 2), spine = dose calculation (r = .59, p < .01).

Errors in radiotherapy remain prevalent and require attention and resolution in the community. For phantom results with non-negligible dose deviation, we identified 20.5%, 68.3% and 70.9 % of errors for the lung, H&N, and spine phantom respectively. Dose calculation and machine output errors both contributed substantially to the phantom error. As these are common sources of error, the radiation oncology community should pay particular attention to them.