Abstract Book

S1039

ESTRO 37

structures. In the other six cases, only minor dosimetric differences (<2 Gy) were found. In all cases where a cumulative dose exceeded 100Gy, the volume receiving >100 Gy (V100) was larger for protons compared to photons. On average, the median V100 was 2.85 (0.64 – 4.74) cc larger in the proton plans, due to inferior dose gradients caused by the range shifter. The integral dose and the dose to the brain excluding the PTV were higher in all photon plans. The mean decrease of integral dose by protons was 41.8% (±12.5). The mean decrease of dose to the brain excluding the PTV was 52.1% (±14.0).

EP-1913 Dosimetric comparison of protons vs photons in re-irradiation of intracranial meningioma R. Poel 1 , S. Tanadini-Lang 1 , G. Lucconi 1 , A. Stuessi 1 , S. Kloeck 1 , M. Guckenberger 1 , R. Foerster 1 1 Universitätsspital Zürich, Radio Onkologie, Zürich, Switzerland Purpose or Objective Re-irradiation of recurrent brain lesions is highly controversial. There is a sincere risk of serious radiation induced toxicities, there are almost no standardized treatment guidelines and there is no full understanding of the biological effects and recovery abilities for a secondary treatment. However, re-irradiation has shown to be an effective salvage treatment. Proton therapy is often suggested as a possible modality for re-irradiation because of dosimetric advantages to non-target tissues. The aim of the study was to investigate the dosimetric differences of dose delivery with spot scanning protons vs VMAT photons in intracranial re-irradiation of meningiomas. Material and Methods Patients from our institute, that have received an initial dose exceeding 50Gy and were re-irradiated for a recurrent intracranial meningioma, were selected. Two plans were prepared, one with VMAT using photons and one with spot scanning using protons, in Eclipse treatment planning system (Varian Medical Systems), based on the clinically used recurrence targets. Prescription was 15x3 Gy. Constructed plans were transferred to MIM (version 6.6.1), and 2Gy equivalent dose distributions were calculated. Total dose was derived by adding the initial and recurrence dose distribution. For the accumulated dose distribution, we analyzed the maximum and mean dose to the OARs and the volume receiving more than 100Gy. Additionally, we analyzed the re-irradiation plans with respect to integral doses. Results Nine cases have been included in the study. In four cases, there was no overlap of the primary and secondary planning target volume (PTV). In the other five cases there was an overlap ranging from 11.6 to 68.2 %. In two of the nine cases, the max and mean doses to certain OARs were reduced by the proton plan. In one case, the photon plan showed dosimetric advantage to critical

Bar plot of the integral dose to the brain by the single photon and proton plans

Conclusion Dose delivery using protons as compared to photons for intracranial re-irradiation will reduce the dose to the brain by approximately 50%. However, proton delivery did not reduce the high dose volumes (V100). Sparing of critical structures is very case specific and depends on target volume and shape as well as the distance of OARs to the target. This indicates the need for patient- individual evaluation of the optimal treatment modality. EP-1914 hHybrid VMAT As A Feasible Technique In Internal Mammary Chain Post-Mastectomy Adjuvant Radiotherapy A. Bruno 1 , D. Becci 1 , M. Leo 1 , A. Terlizzi 1 , G. Lazzari 2 , G. Silvano 2 , D. Mola 1 1 Azienda Ospedaliera SS. Annunziata Presidio Osped, Medical Physics, Taranto, Italy 2 Azienda Ospedaliera SS. Annunziata Presidio Osped, Radiology, Taranto, Italy Purpose or Objective To evaluate dosimetric advantages using hybrid Volumetric Modulated Arc Therapy (h-VMAT) vs Dual-Arc VMAT (D-VMAT) plans for Internal Mammary Chain (ICM) irradiation in post-mastectomy implants adjuvant Radiation Therapy (RT).