ESTRO 2021 Abstract Book

S1587

ESTRO 2021

for comparative plans. All patients were replanned with 3 Gy/fr in 10 fraction dose in standard OLF, IMRT and VMAT. Parotid glands, eyes and lenses were contoured as OARs. In OLF planning, inferior border was between C1 and C2. MLCs were adjusted according to the bone structures as follows: No shielding of frontal sinuses, shielding upto the posterior border of orbita, anterior border of sphenoid sinus and anterior border of C1-C2 vertebrae (Figure 1). No OARs were taken into consideration in OLF dose calculation. IMRT plans were delivered in 5 fields in 70, 140, 0, 290 and 20 degrees. VMAT plans were delivered in two full arcs. OAR doses and treatment times for each modality were compared in relation to deparmental protocols (Table 1).

Results 43 patients who received previous WBRT were replanned. In OLF, protocol violation was observed in all patients for parotid glands and eye doses, in 28 (65%) for lens max doses. No patients achieved parotid max < 30 Gy. Mean parotid gland V25 was 29.2% (7.8-52.4). Compared to OLF plans, both IMRT and VMAT plans obtained significantly lower parotid mean doses, 14.1 (6.6-20.7), 4.2 (1.7-7.5) and 4.2 (1.6-10.7), respectively ( p <0.0001) (Table 2). Same sparing benefit was seen for eye max and lens max doses. Mean eye max dose was 31.8 (25.2-32.9) for OLF, 16.6 (9.9-26.3) for IMRT, and 13.5 (7.8-23.8) for VMAT, respectively ( p <0.0001). Mean lens max dose was 11.9 (2.4-31.6) for OLF, 4.0 (3.2-6.4) for IMRT, and 5.1 (3.3-10.9) for VMAT, respectively ( p <0.0001). Mean treatment time in OLF plans was 0.7 minutes. In IMRT and VMAT plans, treatment duration was 3.9 min and 3.2 min, respectively.

Conclusion Brain metastasis causes inferior survival for all cancer types. WBRT remains the standard of care for disseminated intracranial metastasis. Commonly referred standard for WBRT in OLF is a 2D concept and is unable to spare the OARs including parotid glands, eyes and lenses. Parotid gland V25 of 29.2% is similar to the doses attained in curative HNC RT, and could lead to xerostomia. Data suggests that around these dose levels, it can take up to 12 months for patients to recover from xerostomia, a survival period this group may not have. Additionally, eyes and lenses are not spared sufficiently in OLF, which could lead to acute visual deficits during patients survival period. Our results shows that OLF inadequate in sparing the OARs and could cause toxicity affecting quality of life. Additional 3 minutes treatment time in IMRT and VMAT is a time worth spending to provide patients low toxicity. We believe OLF WBRT should be avoided if conformal techniques are available. PO-1863 3D in-vivo dosimetry for novel inverse optimized 3D-CRT technique in craniospinal irradiation F. Biltekin 1 , G. Yazici 1 , G. Ozyigit 1 1 Hacettepe University, Faculty of Medicine, Department of Radiation Oncology, Ankara, Turkey Purpose or Objective Modern inverse optimization techniques are available for creating three-dimensional conformal radiotherapy (3DCRT) plans called as inverse optimized 3DCRT (i3DCRT) planning. In this study, we aimed to evaluate the accuracy of delivered dose at the field junction region in novel i3DCRT CSI technique using electronic portal imaging (EPID) based 3D in-vivo dosimetry system. Materials and Methods i3DCRT plan was created in RayStation v.8.1.0 treatment planning system (TPS) and treatment delivery for i3DCRT treatment plan was simulated in Alderson RANDO phantom. All measurements were performed in Elekta Versa HD linear accelerator and iViewDose™ software working in conjunction with the existing EPID panel was used as a in-vivo dosimetry system. As a gamma evaluation criteria 3 mm distance to agreement (DTA) and 3% dose differences (DD) was used for local dose. Gamma passing threshold (%γ ≤ 1) was set as 85% for 3D per fraction analysis. Additionally, γ mean (threshold ≤ 0.5 for protocol and ≤ 0.75 for minor variation) and γ 1% (threshold ≤ 2 for protocol and ≤ 3 for minor variation) were evaluated. Results An axial and sagittal view of the calculated and measured dose distribution in beam overlapping regions was shown in