ESTRO 38 Abstract book

S494 ESTRO 38

proton therapy (IMPT) plans separately so that each modality delivers the prescribed dose per fraction to the target volume. The aim of this study is to demonstrate how both modalities can be combined to optimally exploit the protons’ ability to reduce the integral dose in normal tissues, also addressing the issue of the robustness of combined treatments against proton range uncertainties. Material and Methods We consider treatment sites (a spinal metastasis, a sacral chordoma, and an atypical meningioma) in which organs at risk (OARs) are located within or near the tumor. As an example, Figure 1 shows the spinal tumor case where the target volume entirely surrounds the cauda. Protection of the cauda requires fractionation in the planning risk volume (PRV). Hence, proton and photon fractions should deliver similar doses to this region. Meanwhile, the remaining target volume can be hypofractionated with protons. Such a proton-photon combination was planned by simultaneously optimizing IMRT and IMPT plans while accounting for the fractionation effect through the biologically effective dose (BED) model. A BED 10 corresponding to 35.2 Gy in 5 fractions was prescribed to the target volume while limiting the maximum dose in the cauda to the BED 2 -equivalent of 20 Gy in 5 fractions. Stochastic optimization was applied to mitigate range uncertainties. Results Figure 1 illustrates an optimized combination with 1 IMPT and 4 IMRT fractions. Figures 1a and 1b show the IMRT and IMPT dose distributions per fraction. Figure 1c displays the cumulative equieffective dose (EQD) and shows that protons and photons together yield the prescribed BED to the target volume. Protons and photons deliver similar doses to the PRV to protect the cauda through fractionation. Meanwhile, parts of the tumor are hypofractionated with protons. Therefore, the total dose delivered with photons is reduced leading to a reduction of the integral dose to normal tissues. In fact, optimized combinations with 1 or 2 proton fractions achieve 63% and 84% of the integral dose reduction in normal tissues that a 5-fraction IMPT plan yields, compared to 20% and 40% for the simple combinations. However, combined treatments which are optimized without accounting for proton range uncertainties, are very sensitive to errors (Figure 2a). Stochastic optimization leads to robust combinations (Figure 2b) without compromising the benefit over simple combinations. Conclusion A limited number of proton fractions is optimally used in combined proton-photon treatments if protons hypofractionate parts of the tumor while near-uniform fractionation is maintained in serial OARs.

PTV ART

and PTV 1

. Then, the prescribed dose of the PTV 1

was chosen as soft-constraint for D mean shell, with lower weight compared to PTV ART constraints, optimized to obtain a high dose gradient around PTV ART without losing its coverage. The resulting automatic plans were generally better than or equivalent to clinical plans. We reported the comparison of the plan sum (first+second phase) mean-DVH for both internal and external validation (figure 1). In closed-loop, PTVs coverage and homogeneity were comparable; OARs sparing for RP was slightly improved. In open-loop, coverage and homogeneity of PTV ART were improved and OARs sparing for RP was always better with most of the improvements statistically significant ( p <0.05): of note, V 30Gy for bowel was improved of 9% and V 40Gy for bladder of 8%. Moreover, a reduction of the planning time was obtained in RP plans. of the adaptive

Conclusion The suggested KB strategy for automatic planning in the case of ART early-regression guided boosting technique in neo-adjuvant radio-chemotherapy for rectal cancer was found to be satisfactory. The replacement of conventional planning is currently ongoing in our clinical activity. PO-0923 How can a limited number of proton slots be optimally used in combined proton-photon treatments? S. Fabiano 1 , M. Bangert 2 , N. Andratschke 1 , M. Guckenberger 1 , J. Unkelbach 1 1 University Hospital Zurich, Department of Radiation Oncology, Zurich, Switzerland ; 2 German Cancer Research Center, Department of Medical Physics in Radiation Oncology, Heidelberg, Germany Purpose or Objective Currently, proton treatment slots are a limited resource. Combined proton-photon treatments, in which most fractions are delivered with photons and only a few with protons, may represent a solution to optimize the allocation of proton resources over a patient population. Nowadays, institutions performing combined treatments optimize intensity modulated radiation therapy (IMRT) and