ESTRO 2021 Abstract Book

S1513

ESTRO 2021

The dose distributions acquired for the standard-dose and low-dose CTs were compared using the DVH curve and the gamma index test. Figure 1 compares OptHigh-DoseLow with OptHigh-DoseHigh (Ground Truth) for the PTV and brainstem where the DVH curves are on top of each other. Table 1 shows the gamma passing ratio and the mean value of the gamma index for the lower-dose CTs, showing the effect of reducing CT dose on dose calculation. Reducing the CT dose does not have a significant effect on dose calculation. The DVH curves are on top of each other and the gamma passing ratio (1%, 1mm) is 100% after a dose reduction by a factor of up to 10, confirming that the estimated dose distributions are clinically equivalent.

Conclusion Our preliminary results show that the CT dose reduction by a factor of up to 10 does not have a significant effect on dose calculation. A more systematic and quantitative analysis of the effect of CT dose reduction on proton dose calculation and plan optimization utilizing patient data is ongoing. PO-1788 Estimated risk of radiation-induced cancer after thymoma treatments with proton- or x-ray beams A. Lideståhl 1 , G. Johansson 2 , A. Siegbahn 2,3 , P.A. Lind 2,3 1 Karolinska Institutet, Department of Oncology-Pathology, Stockholm, Sweden; 2 Karolinska Institutet, Department of Oncology, Södersjukhuset, Stockholm, Sweden; 3 Karolinska Institutet, Department of Clinical Science and Education, Södersjukhuset, Stockholm, Sweden Purpose or Objective To compare the calculated risks of radiation-induced secondary malignant neoplasms (SMNs) in patients treated with 3D-CRT, IMRT or single-field uniform dose (SFUD) spot-scanning proton beam therapy for thymic tumors using a cancer-model based on the organ equivalent dose (OED) concept. Materials and Methods Twelve patients that previously received 3D-CRT for thymic tumors were included. For all patients, IMRT and SFUD plans were retrospectively prepared. The resulting DVHs for the OARs were extracted and used to estimate the risk of radiation-induced SMNs for the three treatment modalities. A RBE of 1.1 was assumed for the proton beams. The risk for SMNs was calculated using a cancer model developed by Schneider et al (2004). The OED concept is based on the assumption that an inhomogeneous 3D dose distribution in an organ, results in the same radiation-induced cancer risk as an OED, given homogeneously to the entire organ. The OED was calculated using a mechanistic model for carcinoma induction (Schneider, 2009; Schneider et al., 2011b), which accounts for cell killing and that the radiation dose is delivered in a fractionation regimen. In the present study, two limit cases were considered when applying the cancer model. The first of these was the linear-exponential model, in which the repopulation/repair of the cells is neglected, and the second limit case was the plateau model, in which full repopulation/repair of the irradiated cells is considered. The calculated risks for SMNs for the different radiation modalities and dose-relation models were used to calculate relative risks pair-wise. A Wilcoxon signed-rank test was used for pair- wise comparison. Results The relative risks for developing SMNs were significantly reduced for all OARs, and for both dose-relation models, if SFUD was used, compared to 3DCRT and IMRT (Table 1).