ESTRO 2023 - Abstract Book

S1676

Digital Posters

ESTRO 2023

Figure 1.B shows it can be seen that in treatments with IMRT the values of GPR(%) are lower in all treatment schemes. As expected, the IMRT, having fixed gantry position, is more sensible to detect changes in the patient with portal dosimetry than VMAT, where the image is integrated throughout the entire gantry arc. The results of the statistical test among the treatment techniques IMRT and VMAT showed statistical significance for treatment regimens A and B (p < 0.001), but not for the most hipofractionated treatment C (p = 0.065). Conclusion Portal transit dosimetry along lung cancer treatments revealed a different behaviour depending on the fractionation scheme. The results also depend on the treatment technique except for the most hypofractionated regime. In view of these results, different protocols to check the patient variation along the treatment can be set based on the treatment dose scheme. 1 Karolinska University Hospital, Medical Radiation Physics and Nuclear Medicine, Stockholm, Sweden; 2 Karolinska University Hospital, Radiation Oncology, Stockholm, Sweden Purpose or Objective The aim of this study was to determine if rotational uncertainties for gynecological cancer patients can be reduced by surface imaging (SI), as compared to aligning three markers on the patient body with in-room lasers (marker-laser). As rotational deviations from the planned position can have severe impact on the set-up accuracy for patients with the para aortic lymph nodes (PALN) included in the CTV, the quantification of positioning accuracy is of high importance. Materials and Methods Fifty gynecological patients treated with external radiotherapy were retrospectively included in this study, 25 patients positioned with marker-laser and 25 patients positioned with SI. The values of rotational (pitch and roll) deviations of the patient positions between treatment planning CT (TPCT) and online CBCT were collected for both subcohorts, and all treatment fractions, after performing an automatic registration between the two image sets. The statistical analysis of the difference between the two set-up methods was done with the Mann-Whitney U-test. Yaw rotation and translations were not included in the quantification of positioning accuracy as those parameters were corrected for in the online registration between TPCT and CBCT. Each patient that required re-imaging was recorded, as an indication of large residual set-up errors after initial set-up. For all patients, the extent of the CTV was measured in the cranio-caudal (CC)-, anterior-posterior (AP)- and lateral-medial (LM) directions, from the isocenter to the most distant point on the contour, to allow quantification of the potential impact of errors, in pitch and roll, on target misalignment. PO-1923 Quantification of pelvic rotations of gynecological cancer patients positioned with surface imaging M. Bolin 1 , M. Falk 1 , M. Hedman 2 , G. Gagliardi 1 , E. Onjukka 1

Results