ESTRO 2023 - Abstract Book

S1654

Digital Posters

ESTRO 2023

where v i was the ture location of tumor at ith phase. ^ represented the estimated value. N was the number of test phases for one patient. || || meant Euclidean distance. Results The 20 test samples showed tracking error of 3.10±1.45mm (average ± standard deviation). The RMSEs of the 4 test sets were 3.40±1.54mm (range: 1.10~5.09mm). Their corresponding target motion ranges were 9.68~11.77mm. Conclusion The proposed cascade ensemble model shows promising in accurate tumor motion tracking without patient-specific training before treatment.

PO-1906 Dosimetric evaluation of an online adaptive strategy for prostate SBRT (PACE-B)

J. Calvo-Ortega 1,2 , C. Laosa-Bello 1 , S. Moragues-Femenía 1 , J. Torices-Caballero 1

1 Hospital Quirónsalud Barcelona, Radiation Oncology, Barcelona, Spain; 2 Hospital Quirónsalud Málaga, Radiation Oncology, Málaga, Spain Purpose or Objective To evaluate the dosimetric advantage of performing daily online adaptive radiotherapy (ART) using an ultra hypofractionated stereotactic body radiotherapy (SBRT) scheme for localized prostate cancer. Materials and Methods First 20 patients receiving ART-SBRT in our department were included. Prescription dose consisted of 5 fractions of 7.25 Gy according to the PACE-B trial (Lancet Oncol. 2019 Nov;20(11):1531-1543). Our ART technique is based on CBCT imaging acquired in a Varian TrueBeam linac. Once the patient is setup by using the room lasers, a CBCT scan is acquired and 6D-rigid registered with the simulation CT scan (sim-CT). The target (CTV: prostate), organs-at-risk (OARs) and the reference VMAT plan are mapped from the sim CT scan to the CBCT scan. The CTV and OARs are adapted to the actual anatomy by a qualified radonc. Then, the reference VAMT plan that was calculated on the sim-CT scan is re-optimized on the CBCT scan to obtain an adapted plan (ART plan), while the patient lies on the linac couch. In order to mimic each treatment session without this adaptive strategy, the reference plan was mapped to the CBCT scan according to the sim-CT/CBCT 6D-rigid fusion (focused on the CTV), and recalculated with the same original monitor units (non-ART plan). For each patient and treatment session, the ART and non-ART plans were compared (100 plan pairs) using the metrics and objectives shown in the Table 1, where Vx Gy is the % of volume receiving ≥ x Gy, Dx% is the minimum dose (in Gy) at the % of the volume, and Dmax is the point maximum dose. A 2 tailed Student t-test ( α = 0.05) was performed to evaluate whether there is a significant difference between adapting and non-adapting strategies. The required time for the online plan adaptation and total treatment time were accounted.

Results 1) Table shows the results found for each dosimetric metric included in this study:

a. Adapting the plan increased the target coverage significantly compared to non-adapt: 98.4% vs. 81.1% (p < 0.001) and 96.6% vs. 84.4% (p < 0.001), for V40 Gy of the CTV and V36.25 Gy of the PTV, respectively. b. Small differences were found for the three rectum metrics (D50%: 10.7 Gy vs. 10.2 Gy, D20%: 19.8 Gy vs. 18.9 Gy; D1 cm3: 33.2 Gy vs. 32.8 Gy), between the ART and non-adapted plans. Although all plans met the required objectives, the adapting plan technique only improved the D50% and D20% metrics in 37% of the 100 sessions included in this study, while D1 cm3 was improved in 47% of times.

c. Non-significant differences were found for the bladder metrics between adapting and non-adapting approaches.