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Dosimetric Impact of Intrafraction Motion During Peripheral Lung Cancer Stereotactic Radiotherapy: Is a Second Cone Beam Computed Tomography of Added Value?

par Benkhaled, Sofian ;Koshariuk, Olga;Van Esch, A.;Remouchamps, Vincent
Référence International journal of radiation oncology, biology, physics, 111, 3, page (S140)
Publication Publié, 2021-11

Article révisé par les pairs

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Résumé :

PURPOSE/OBJECTIVE(S): In our department, during lung SBRT, all patients (regardless of tumor location) receive an additional intra-fractional CBCT, allowing us to adjust the beam delivery accordingly to the intra-fractional variation (IFV). It is to be expected that each IFV exceeding the planning target volume (PTV) margins could lead to potential discrepancies between planned and delivered plans. A posteriori robust dosimetric evaluation will help us to quantify the actual impact on the target's volumes coverage. MATERIALS/METHODS: Six hundred sixty-four CBCTs from 166 consecutives free-breathing lung SBRT patients were retrospectively analyzed. Treatments were based on the internal target volume (ITV) approach with a 4D-CT at simulation and performed using volumetric modulated arc therapy. An isotropic 0.5 cm margin was used to create the PTV around the ITV. The prescribed dose was 48 Gy in 4 fractions to the PTV (80% isodose line). Patients were divided into two groups: patients for whom the 3D-IFV was below 0.5 cm (low-risk group: 105 patients, 573/664 fractions) and patients with at least one 3D-IFV ≥ 0.5 cm (high-risk group: 61 patients, 91/664 fractions). Plans simulating the dosimetric impact of the IFV were created as follows: the original two arcs were copied into a new plan consisting of 4 times ARC1 and 4 times ARC2. The delivery of ARC1 was always assumed to have occurred with the isocenter coordinates as those in the original plan, whereas the positions of ARC2 were modified for each arc by the measured the 3D displacement for that fraction according to the intra-fraction CBCT. RESULTS: The IFV reduces both the minimal (D99%) and mean (Dmean) dose to both the PTV and the ITV. For the PTV, we obtained: D99(%) = 45.2 vs 48.2 Gy (P < .0001); Dmean = 53 vs 54 Gy (P < .0001) for the reconstructed vs planned dose values, respectively. For the ITV, the changes are less pronounced: D99(%) = 52.2 vs 53.6 Gy (P = .0007); Dmean = 56 vs 56.8 Gy (P = .0144). Maximum dose (Dmax) was not much affected by the IFV for neither the PTV nor the ITV (PTV = 60.3 vs 60 Gy (P = .3686), ITV = 60.2 vs 60 Gy (P = .5125)). The V48(Gy)-ITV coverage did not statistically change between the delivered vs planned dose (P = .1803). For the high-risk IFV group, 4 patients did not achieve 100% coverage of the prescribed dose to the ITV in the reconstructed dose. Two of these were clear outliers (ITV-V48(Gy) = 82.4 and 81.9%). Those were the patients for whom the 3D-IFV > 0.5 cm was observed for 3 and 4 fractions, respectively. Regarding the organs at risk (D99%; Dmean; Dmax): for both the low- and high-risk groups, dose-volume histograms were near-identical (no significant difference). CONCLUSION: Although IFV results in a significant change between the delivered vs planned dose distributions for the PTV (D99%; V48Gy; Dmean) and on the ITV (D99%; Dmean), the V48Gy-ITV coverage did not statistically change. Based on our data we demonstrated that a single CBCT is sufficient and reliable to manage the IFV during peripheral lung SBRT.