S1911
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
D50% = 54 Gy, corresponding to a BED = 151.2 Gy (de Jong et al, 2020) were investigated.A constant relative biological effectiveness (RBE) factor of 1.1 was used for the two proton plans.The treatment plans were renormalized from the original three fractions to different fractionation schemes, varying between single and seven fractions.From the standard BED formula an expression for the dose per fraction was derived. The new dose per fraction was calculated under the assumption of a constant BED = 151.2 Gy. Renormalized dose distributions with the new dose per fraction were received by multiplying with the calculated factor. Either the prescribed dose or the nominal value calculated from the treatment plan was used. This led either to a normalization to the prescribed BED or to a normalization to the planned BED. With these two methods, the dose distribution images were converted to BED and displayed as BED volume histograms (BVH). Results:
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Differences in fractionation scheme conversion using prescribed and planned biologically effective dose Katharine Ullrich 1,2 , Nils Olovsson 2,3 1 Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden. 2 Department of Immunology, Genetics and Pathology, Uppsala University, Uppsala, Sweden. 3 The Skandion Clinic, The Skandion Clinic, Uppsala, Sweden Purpose/Objective: The fractionation schemes for stereotactic body radiotherapy (SBRT) of lung cancer are plenty. At our institution three fractions are prescribed for peripheral tumors while four or five fractions can be prescribed to centrally located tumors or tumors adjacent to the chest wall. When comparing different fractionation schemes a conversion of the physical dose to a biologically effective dose (BED) is used.To achieve robust target coverage with regards to positioning and respiration a dose higher than the prescription might be needed for the nominal plan. In this study the effects of converting between different fractionation schemes using normalization to the prescribed or planned BED was investigated for proton and photon plans. Material/Methods:
When normalized to the prescribed BED, the CTV BVH and BED at D50% was pushed higher for all fractions lower than the reference fractionation scheme and pushed lower for all fractions higher than the reference fractionation scheme, Figure 2. However, when normalized to the BED given by the nominal plan dose, the BED at D50% stayed constant.The difference between the two normalization methods was larger for the proton plans. Conclusion: When changing to a different fractionation scheme the nominal plan dose, as opposed to the prescription
In this preliminary study, robust optimized treatment plans for proton and photon therapy from two patients, Figure 1, with a clinical target volume (CTV)
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