ESTRO 2021 Abstract Book

S1357

ESTRO 2021

and lower-dose area (spared healthy skin) during three treatment sessions with uniform and three with individual bolus. Bolus-to-skin distances (folds) were determined from cone-beam CT images acquired for positioning control. Results The surface dose at the slab phantom changed insignificantly under conventional and cast boluses; increasing the surface-bolus distance from 5 to 10 mm decreased the surface dose by approximately 2 and 11% in the 6x6- and 3x3-cm² fields, respectively. For patient measurements a significant dose increase of 2.45 % (95%CI 0.0014-0.0477, p=0.038, N=30) was observed in the high dose area on the scar. Skin-to-bolus distances were reduced in mean by 3.4 mm with the customized bolus (95%CI 1.6-5.2, p<0.001, N=29). The limiting factor in the conformity of individual boluses was the immobilization mask, which itself created voids. In the area of skin surface to be spared by the cast bolus, there was a mean dose reduction of 25.9 % (95%CI 19.5-32.3, p<0.01, N=37). Conclusion By using individual boluses a significant dose reduction is achievable in the spared skin areas. For the high dose area near the scar, the better adherence of individualized bolus allows better dose distribution compared to the standard ones. Since less spacing between the surface and the bolus results in considerable dose reductions, which further increase at smaller field sizes, these finding are especially relevant for VMAT plans, which typically consist of multiple segments with small field openings. In this context the influence of the rigid mask on the bolus adherence should be considered. PO-1636 Isotoxic investigation of 18FDG PET/CT-guided dose escalation with IMRT for LA-NSCLC Y. Shao 1 , Z. Xu 1 1 Shanghai Chest Hospital, Shanghai Jiao Tong University, Department of Radiation Oncology, Shanghai, China Purpose or Objective This study compared the dosimetric and biological differences between PET/CT-guided isotoxic SIB-IMRT plans and conventional radiotherapy plans for patients with locally advanced non-small cell lung, and it also evaluated the factors that affect dose escalation. Materials and Methods Thirty patients with IIIA-IIIB NSCLC were included in this retrospective study. For each patient, planning target volume (PTVPET/PTV) was determined based on PET/CT and CT. SIB-IMRT (Plan_iso) and conventional radiotherapy (Plan_primary) plans were generated. Dosimetric indicators such as mean lung dose (MLD), mean heart dose (MHD) and other indicators in Plan_iso and Plan_primary were compared. Tumor control probability (TCP) of PTVPET and PTV and normal tissue complication probability (NTCP) of total lung and heart were calculated. The relationship between dose escalation and age, the volume of PTV, 3D length of PTV and other factors were analyzed. Paired-samples t-test, Mann-Whitney U test and Chi-Square test were used to check for statistically significant differences (p < 0.05) between dataset. Results The Dmean, D2, D98, V95, and V105 of PTV in Plan_iso were higher than those of PTV in Plan_primary, and there were significant differences (p < 0.001). Compared with Plan_primary, Plan_iso slightly increased MLD, V5, V10, V13, V15, V20, V30, V40 of total lung, MHD, V30, V40 of heart, Dmax of spinal cord and MUs. The absolute differences were small, though there were significant differences (p < 0.001). TCP of PTVPET and PTV in Plan_iso were 21.13% and 17% higher than those in Plan_primary, respectively, and there were significant differences (p < 0.01). NTCP of total lung in Plan_iso was higher than that in Plan_primary. There was a significant difference but the absolute difference was small. NTCP of heart in Plan_iso was slightly higher than that in Plan_iso, but there was no statistical difference (p > 0.05), and the absolute difference was also small. Conclusion For local advanced non-small cell lung cancer, the simultaneous integrated boost based on isotoxic radiotherapy can significantly increase TCP under the premise that the toxicity of OARs is comparable. PO-1637 Dosimetric characterization of a 12 cm IORT applicator of a Liac 12MeV linac R. Ayala 1 , G. Ruíz 1 , M.J. García 1 , M.S. Gómez 1 , R. Jiménez 1 , T. Valdivielso 2 , J.L. Navío 1 , E. Martínez 1 1 H.G.U. Gregorio Marañón, Servicio de Dosimetría y Radioprotección, Madrid, Spain; 2 Hospital de Terrassa, Servicio de Radioterapia, Terrassa, Spain Purpose or Objective The Liac 12 MeV accelerator (SIT) is supplied with cylindrical applicators with diameters ranging from 3 to 10 cm. It is however possible to purchase a 12 cm diameter applicator which is indicated for intraoperative radiotherapy (IORT) of large lesions, such as sarcomas of the extremities. The objective of this work is to provide a full dosimetric characterization of the 12 cm applicator with its four possible bevel endings and compare it with the regular applicators. Materials and Methods This study was carried out on a SIT accelerator model Liac 12 MeV with nominal energies of 6, 8, 10 and 12 MeV. The distal part of all applicators ends in 4 possible bevel angles: 0, 15, 30 and 45º. All measurements were performed in water with an IBA Blue Phantom 2 beam analyzer and the IBA PPC05 and PTW 60019 microDiamond detectors. The measurements performed comprise PDDs, OARs at maximum dose and R90 depths and field factors (FF) at maximum dose depth for the four accelerator energies. In addition to that, a Monte Carlo modeling of the accelerator/applicator assembly has been carried out with the code PENELOPE 2018 (Nuclear Energy Agency) that completes the previous modeling of the other applicators.