ESTRO 2023 - Abstract Book

S1681

Digital Posters

ESTRO 2023

The average and standard deviations of the inter-fractional BH reproducibility of the tumor position in the LR, AP, and CC directions, and the 3D vector were 1.7 ± 0.5 mm (range of 0.8–2.7 mm), 2.0 ± 0.7 mm (range of 0.7–4.0 mm), 2.1 ± 0.7 mm (range of 1.0–3.7 mm), and 2.7 ± 0.7 mm (range of 1.5–4.8 mm), respectively. Ten patients exhibited inter-fractional displacements of the lung tumor >3 mm in the 3D vector. No displacement >5 mm was observed in any direction for all patients. Conclusion Our study indicated that the inter-fractional BH reproducibility of the tumor position was small for lung-cancer patients, using the Abches system. The VVH method confirmed that the expiratory BH status using the Abches system was sufficiently reproducible.

PO-1928 Feasibility study of the Transit-Guided Radiation Therapy technique: an interim analysis

A. Latorre-Musoll 1 , M. Marín de Castro 2 , S. Serrano 2 , G. Antelo 2 , G. Oses 2 , M. Mollà 2

1 Hospital Clínic de Barcelona, Servei d'Oncologia Radioteràpica (ICMHO), Barcelona, Spain; 2 Hospital Clínic de Barcelona, Servei d'Oncologia Radioteràpica (ICMHO), Barcelona, Spain Purpose or Objective Transit-Guided Radiation Therapy (TGRT) is a promising technique that can provide on-line corrections to position errors by assessing the transit portal images (TPIs) of the treatment fields ( Phys Med Biol 2022 67 155022). In this work we present the preliminary results of an ongoing prospective clinical study aimed to assess, for the first time, the performance of the TGRT technique in a clinical setting. Materials and Methods The TGRT technique uses the TPIs to assess the position error projected to the EPID plane, which is denoted by the shift vector s . To validate the TGRT technique, breast and whole-brain plans have been used to obtain TPIs from gantry-static fields with skin flash. Such fields directly irradiate a region of the EPID extending beyond the body contour, which defines a sharp edge on the TPIs that can be used as a surrogate of the patient position. An in-house code has been developed to identify the projection of the body contour onto a given TPI by using an edge-detection algorithm. Then, the code rigidly registers the body contours of a given TPI and the first TPI (used as reference), yielding the true shift projected at the EPID plane s_0 between these two fractions of that field. The TGRT-based shifts s have been compared to the edge-based shifts s_0 (which constitutes the gold standard set) for all fields and fractions. A total of 262 TPIs obtained from ten patients treated with 3DCRT have been evaluated in this interim analysis: 6 breasts with breath-hold (202 TPIs), 3 breasts in free breathing (42 TPIs) and 1 whole-brain (18 TPIs). The magnitude of the difference d = | s – s_0 | has been obtained for all fields and fractions. The differences d have been compared to the magnitudes of the corresponding true shifts s _0 ≡ | s_0 | by using a left-tailed paired-data t -Student test (stratified by site/technique) to check whether the TGRT technique, if applied, could have reduced the existing position error ( d < s _0) or not ( d ≥ s _0) (significance level α = 0.05). Results Figure 1( a ) shows a correlation of R 2 = 0.76 between the components of the TGRT-based shifts and the edge-based shifts. Figure 1( b ) shows that most of the TGRT-based shifts (92%), if applied, would have reduced the existing position error ( d < s _0). Most of the overcorrections (18 out of 21) corresponded to small true shifts below 5 mm, i.e., with a minor clinical impact.