ESTRO 2020 Abstract book

S792 ESTRO 2020

agreement.

increase the risk of radiation-induced side effects. The analytical anisotropic algorithm (AAA) and the Acuros XB algorithm are both in the Eclipse treatment planning system (TPS) for volumetric modulated arc therapy (VMAT). To the extent of our knowledge, studies comparing the two algorithms in the brain area are limited to three studies of which two were conducted on phantoms, and one on clinical data with only a small subgroup of neuro patients with no specific focus on low dose regions. Therefore, this study aimed to explore in silico the dose differences of these algorithms with a focus on low dose regions in brain metastases patients. Material and Methods After obtaining the ethics approval and by using a retrospective design, ten neuro metastases patients were loaded to the TPS for the purpose of comparison. For each patient, the 3D dose distributions were re-calculated by both algorithms; AAA (version 15.5.11) and Acuros XB (version 15.5.11), using the pre-set monitor unit values (MU) of the original treatment plans. Since we could not re-optimize the AAA before calculating, the AAA doses were normalized to the Acuros ones using the mean dose in the PTV. A comparison between the dose volume histograms (DVHs) of the two algorithms was then performed to extract the mean and maximum doses for the relevant organs-at-risk (OARs), namely: brain-PTV, brainstem, spinal cord, right and left cochlea, right and left optic nerve, right and left retina. The agreement between the two algorithms was assessed by gamma analysis (MATLAB), and the Bland Altman (B-A) test (SPSS). Gamma criteria were set to 1 mm distance to agreement and 2% dose difference. Results The gamma pass rates of the ten-neuro patients show 100% similarity between the two algorithms in all but three of the nine OARs. The brain-PTV, brainstem and the right cochlea yielded rates equal to 99.21%/ SD= 0.96, 99.87%/ SD= 0.36, and 99.48/ SD= 1.65, respectively. The scatter plot and sixteen out of the eighteen B-A plots (nine OARs * two DVH metrics (mean and max doses)) show a good agreement at lower dose levels, however, the profile of points exhibit an increasing trend with dose within the tolerance limits (figure 1 and 2).

Figure 2: A B-A plot of the right cochlea (the horizontal line of zero= no difference) Conclusion The agreement level between the algorithms is high and almost all of the differences in each OAR are located well within the default tolerance limits, suggesting that AAA and Acuros could be used interchangeably in neuro patients for low dose regions. Future work may include adding more patients for stronger conclusion. PO-1401 Dental radiation dosimetry maps from IMRT planning for head and neck cancers G. Emile 1 , S. Polce 2 , J. Antone 3 , D. Frank 1 , J. Segal 1 , L. Potters 1 , B. Parashar 1 1 Zucker School of Medicine at Hofstra/Northwell Health, Radiation Medicine, Lake Success, USA ; 2 Stony Brook School of Medicine, Medicine, Stony Brook, USA ; 3 Northwell health, Radiation Medicine, Lake Success, USA Purpose or Objective Dental professionals often request radiation doses to teeth, maxillae and mandibles from radiation oncologists who treat cancers of the head and neck. This frequently occurs before patients undergo CT simulation, precluding precise dose calculations. The purpose of this study was to create dental radiation maps, using exact dental numbering to provide to dental professionals the mean RT dose to teeth, maxilla and mandible for commonly treated primary head and neck cancers using intensity modulated radiation therapy (IMRT). Material and Methods Nineteen common clinical settings were chosen for this retrospective study. Patients had received prior curative radiation treatment using IMRT. Radiation plans were extracted from ‘Velocity’ planning software for primary cancers that included nasopharyngeal, oropharyngeal, and oral cavity cancer among others, both in the definitive and adjuvant setting. Each tooth was contoured and labeled at the junction of the tooth and the gingiva based on dental numbering. Dental numbering was based on the Universal numbering system (American System). In addition, left lateral 3 rd , middle 3 rd and right lateral 3 rd of the mandible and lower maxilla were contoured. Dose distribution to each tooth and mandible, maxilla were obtained. Dose wash images in axial, sagittal and coronal were also captured for each plan. Results Dosimetric data was calculated for 19 patients. All were treated with prescribed doses of 50-70Gy in 1.8-2Gy/fxn using dose painting IMRT. Tables with mean doses to each 3 rd of the mandible and maxilla, along with each tooth were created, labeled 1-32. Two examples are provided in image format. Map 1: A 48 year old female with cT2N1 left tonsillar SCC. She was treated definitively with 70Gy to the primary and involved lymph node, 63Gy to left level II and 58.1Gy to left levels III-IV. The contralateral neck did not receive radiation. The ipsilateral 3 rd of the mandible and 2 teeth (#16 and #17) received mean doses >50Gy, while the

Figure 1: A scatter plot of one algorithm against the other comparing the mean absolute doses to the line of perfect