ESTRO 2024 - Abstract Book

S4174

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2024

Stockholm, Sweden), with 6MV FFF (flattening-filter-free) on an Elekta VersaHD linear accelerator. To initially determine residual inter-breath-hold target positioning error for iGTV (internal gross target volume) contouring, three planning CTs in DIBH were acquired; with no patient re-setup in between (except for first two exception patients). For all patients, iGTV to PTV margin was 10.0mm in sup/inf and 5.0mm in radial (except 7.0mm for patient #1) direction, and prescription dose 48Gy in 4 fractions. The lesions were in the right lower lobe (4x), left lower lobe (2x) or right upper lobe (2x), iGTV volumes varied from 0.99-12.00cc. Daily on-board Cone-Beam CT (CBCT) imaging and treatment were delivered in repeated DIBH, with breath-hold controlled by the Active Breathing Coordinator (ABC, Elekta). Breath-hold lung volume varied from 1.0 to 2.0l, and breath-hold length varied from 15 to 30s. IFI CBCT were acquired in 29 of total 32 fractions, with 4 of those 29 only recording partial (~90°) imaging arcs due to delivery interrupts. For 3 fractions, no IFI could be acquired. The retrospective patient set-up audit was independently performed by 5 radiation therapists, re-registering all IFI CBCTs to the respective reference planning CT for all 8 patients (translation only).

Results:

Statistical analysis of re-registration results confirmed all targets were within PTV during each fraction treatment, and the average residual error was (0.1±1.0)mm left/right, (1.0±2.4)mm sup/inf, and (0.2±1.5)mm ant/post (see Figure). Maximum (absolute) offset was 3.2mm left/right (patient #3), 7.6mm sup/inf (patient #3) and 4.9mm ant/post (patient #1). Patient #3, with maximum outliers in left/right and sup/inf, had a deep-inspiration lung volume of 2.0l and a breath-hold of 25s. Patient #1, who showed largest ant/post offset as well as next-largest outlier sup/inf of 6.8mm, was an exceptional, complex case with lower patient compliance. All other (absolute) sup/inf offsets were <5.0mm and thus well within PTV margin of 10.0mm. All other (absolute) offsets were <3.0mm in left/right, and in <4.1mm in ant/post, respectively, and therefore also within the radial PTV margin of 5.0mm.

Conclusion:

The overall results of the retrospective patient set-up audit via IFI CBCT show that all lesions were treated within their planning volume, which provides confidence in the lung SABR in DIBH treatment technique. While patient #1 and #2 were complex exceptions, patient #3 taught us most and initiated some process changes for the subsequent patients (#4 onwards): (1) reduction of breath-hold length to maximum 20s for pre-treatment CBCT imaging despite patients tolerating longer times. This reduces the risk of single breath-hold dominating the ~33s IFI CBCT acquisition potentially leading to a baseline shift. (2) phrase used to guide patient into breath-hold: aim for slow and steady going into breath hold rather than “taking a deep breath and hold”. This reduces the risk of so-called overachievers, exceeding the tolerance level pre-set for DIBH lung volume. Despite the average residual error being well below 5.0mm in all three