ESTRO 2023 - Abstract Book

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ESTRO 2023

SBRT treatments were performed on a Varian Clinac 2300 iX. This linear accelerator includes an On Board Imager unit to perform KVCBCT and a Millenium 120 MLC (5mm thickness at isocenter) for treatments performance (Varian Medical Systems, (VMS)). Treatment Planning System used for clinical dosimetry was Eclipse 15.6, and two algorithms available: Acuros 15.6 and AAA 15.6 (Analytical Anisotropic Algorithm) configured with a 2.5 mm calculation grid . PO 15.6 optimizer was used for VMAT treatments (VMS). 4DCT was acquired with Philips Brilliance CT, using a nominal slice thickness of 2mm. Respiratory cycle was binned into 10 phases and ITV was generated by means of merging 10 segmented GTV. Three partial arcs were used from 0° to 180° with collimation angles set to 30°, 330° and 30°, respectively. Photon energy was set to 6MV and isodose prescription ranged from 110% to 125%. Results Statistical analysis with Stata v.16 (StataCorp.) has been carried out to assess PI and GI correlation with: target lateral location (left/right), transversal location (grouped in three categories: chest wall (CW) , centered (C) and ‘Others’ (O) ); and ITV volume.

Paddick Index.

Positive association between ITV volume and PI has been found (p-value<0.0001), with an average slope of 0.0018 cm˜ ³ .

Neither lateral nor axial location has brought significant correlation.

Gradient Index.

When comparing with main location (CW) , ‘Others’ shows a lower GI (p-value=0.017), with an averaged difference of 0.250. GI of CW and C categories had no significant differences.

In addition, no significant differences have been found between lateral location and GI (p-value=0.277).

Negative association between ITV volume and GI has been observed (p-value<0.0001).

Table I.

Figure I. ITV size distribution.