ESTRO 2021 Abstract Book

S1497

ESTRO 2021

Figure 1. Experimental set-up. SRS MapCheck and radiochromic film on QUASAR platform. The system is assigned a gating window of 3mm, simulating the clinical conditions of our centre. It will perform 6 different deep inspiration breathing curves, with different number of beam interruptions in each one. The vertical profile on each film was analysed with ImageJ programme, in order to estimate the shift of the radiation centre with respect to the case with no interruptions. Likewise, with the SNC Patient Programme (SunNuclear), we compared the dose matrix obtained with the SRS MapCheck for each breathing curve to the case with no interruptions, and we evaluated the shift of each matrix on the vertical axis. Results Figure 2 represents the shifts obtained with each measurement method, relative to the case without interruptions. Both methods show a growing positive shift in the vertical axis of the dose distribution as the number of interruptions increases.

Figure 2. Vertical dose shift with radiochromic films and SRS MapCheck as a function of the number of treatment beam interruptions. The dosimetric impact of these shifts has been evaluated in 15 patients of our centre, all of them treated with DIBH on the left breast. The results for both measurement methods are summarized in the following table for the most unfavourable case of 6 interruptions. The results are given with respect to the case without interruptions. Note that those with radiochromic film are more reliable due to its higher resolution. A slight loss of coverage in PTV and a reduction dose in OARs can be seen.

Ipsilateral lung V18Gy PTV V95% Mean difference [range] (%) Mean difference [range] (%) Mean difference [range] (%) Heart V22Gy

Radiochromic films -0.35 [-0.30, -0.44]

-0.10 [0, -0.15]

-0.24 [-0.10, -0.38]

SRS MapCheck -0.33 [-0.14, -0.59] Table 1. Dosimetric differences with respect to the case without interruptions, arising from vertical dose shift in 15 DIBH patients, with both measurement methods. These results correspond to the most unfavourable case of 6 interruptions. Conclusion Both radiochromic films and SRS MapCheck showed that treatment interruptions with Catalyst – DIBH system cause a vertical dose shift in the positive direction. This results in a slight loss of coverage in PTV and dose reduction in OARs, which are both not clinically relevant. -0.46 [-0.4, -0.55] -0.10 [0, -0.19]

PO-1770 Robotic preoperative breast radiotherapy: tracking uncertainties and dosimetric implications