ESTRO 2024 - Abstract Book

S3975

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2024

During weekly CBCT, images were matched using the chest wall as a surrogate of the treatment volume. The resulting couch translations were applied and the MHD was calculated based on the position of the heart on CBCT. A simulated scenario was then created where no CBCT-guided translations were allowed. The MHD was recalculated based on the position of the heart in each CBCT without translations. This reflected the MHD in the scenario where the patient is aligned using skin tattoos only, without the aid of CBCT. To determine if CBCT decreased the interfractional variation of the MHD, the standard deviation (SD) of the MHD in the scenario with CBCT-guided translations was compared to those in the scenario without CBCT-guided translations using linear regression model. We also assessed whether CBCT-guided translations had any impact on the average MHD, by calculating the average of the MHD based on the heart position in each CBCT.

Results:

A total of 55 patients (vDIBH = 16, ABC = 19, SGRT = 20) were included in the analysis. The dose fractionation used are described in Table 1. The average MHD with CBCT-guided translations applied was 1.85Gy, 1.88Gy and 2.09Gy in the vDIBH, ABC and SGRT groups, respectively.

Table 1

vDIBH (n=16)

ABC (n=19)

SGRT (n=20)

Dose fractionation

50Gy/25fr

6.3%

15.8%

15%

42.56Gy/16fr

75%

84.2%

85%

40.05Gy/15fr

18.8%

0%

0%

Sequential

tumour

Included

56.3%

52.6%

40%

bed boost

Patients using vDIBH had greater interfractional variation of MHD in the scenario where CBCT-guided translations were not applied, compared with the scenario with translations applied (difference in SD: 22.6cGy, 95% CI: 9.2 – 35.9, p=0.0013). In patients using ABC or SGRT, there was no difference in the interfractional MHD variation when comparing fractions with CBCT-guided translations applied and not applied (see Figure 2).