Algorithms Used in Heterogeneous Dose Calculations Show Systematic Differences as Measured With the Radiological Physics Center's Anthropomorphic Thorax Phantom Used for RTOG Credentialing

Abstract

Purpose To determine the impact of treatment planning algorithm on the accuracy of heterogeneous dose calculations in the Radiological Physics Center (RPC) thorax phantom. Methods and Materials We retrospectively analyzed the results of 304 irradiations of the RPC thorax phantom at 221 different institutions as part of credentialing for RTOG clinical trials; the irradiations were all done using 6-MV beams. Treatment plans included those for intensity-modulated radiation therapy (IMRT) as well as 3D conformal therapy (3D CRT). Heterogeneous plans were developed using Monte Carlo (MC), convolution/superposition (CS) and the anisotropic analytic algorithm (AAA), as well as pencil beam (PB) algorithms. For each plan and delivery, the absolute dose measured in the center of a lung target was compared to the calculated dose, as was the planar dose in 3 orthogonal planes. The difference between measured and calculated dose was examined as a function of planning algorithm as well as use of IMRT. Results PB algorithms overestimated the dose delivered to the center of the target by 4.9% on average. Surprisingly, CS algorithms and AAA also showed a systematic overestimation of the dose to the center of the target, by 3.7% on average. In contrast, the MC algorithm dose calculations agreed with measurement within 0.6% on average. There was no difference observed between IMRT and 3D CRT calculation accuracy. Conclusion Unexpectedly, advanced treatment planning systems (those using CS and AAA algorithms) overestimated the dose that was delivered to the lung target. This issue requires attention in terms of heterogeneity calculations and potentially in terms of clinical practice.

Pencil Beam Algorithms Are Unsuitable for Proton Dose Calculations in Lung

“…The findings of this study is similar to what was previously discovered with the photon lung phantom (12): the dose calculated by Monte Carlo algorithms agrees better with measurements than the dose calculated by analytic algorithms. Overestimation of dose in the lung by up to 14% (13–15) has been observed with simple analytic algorithms (e.g.…”

Commissioning and initial stereotactic ablative radiotherapy experience with Vero

“…The shortcoming of PB algorithms to accurately predict dose in low density media has been described previously 27 , 32 , 45 , 46 , 47 , 48 , 49 . In a recent study of 304 irradiations performed at 221 different institutions, Kry et al (49) observed an overestimate of 4.9% in PB algorithms compared to measurement in the Radiological Physics Center (RPC) thorax phantom used for RTOG credentialing; these results agree favorably with those reported here.…”

National dosimetric audit network finds discrepancies in AAA lung inhomogeneity corrections

“…The study found that in-house IMRT QA failed to detect unacceptable plan delivery as measured independently in an IROC head and neck phantom and concluded that IMRT QA was not a reasonable replacement for a credentialing (audit) phantom irradiation. In a similar study to the work presented here, Kry et al [24] used a thorax phantom for a large (n ¼ 221 different institutions, 304 irradiations) credentialing study that looked at the impact of dose calculation algorithm and their accuracy when accounting for heterogeneities. Measured doses were compared to calculated doses for a lung target and analysed as a function of calculation algorithm.…”

“…In end-to-end tests, which are generally also performed as part of the commissioning, any algorithm rooted deviations might be diminished by other small deviations or might be within the larger allowed tolerances for such tests [24].…”