ESTRO 2023 - Abstract Book

S1068

Digital Posters

ESTRO 2023

Nineteen patients with locally advanced stage III or oligometastatic NSCLC, treated with radical RT were included. Four treatment plans, with a prescribed dose of 30 x 2 Gy, were compared: photon and proton plans, on both a FB 4DCT scan and an IBH scan. For the photon FB plans, individual CTV-PTV margins were calculated for the CTV of the primary tumor (CTVp) [1]. For the IBH plans, this PTV-margin was 8mm. A 5mm margin was used for nodes (CTVn) in both plans. For the proton plans, a 3mm margin around CTVp and 0mm around CTVn were added for a GTV-amplitude on 4DCT <5mm, or an ITV was created with extra 2mm margin for >5mm; both combined with 3D robust optimization with an isotropic setup shift of 5mm and 3% density uncertainty. VMAT photon therapy plans were made for a Truebeam STx using 2 half-arcs. IMPT plans were made for a Mevion S250i Hyperscan system with 2-3 beam directions. The same beam arrangement was used for IBH and FB plans. Differences in doses to OAR and relevant NTCPs were evaluated [2-4]. Results For 6/19 patients the GTV amplitude exceeded 5 mm (large movers). The GTV amplitude was 6 ± 5 mm for the entire group and 11 ± 3 mm for the large movers. Dose and volume differences are reported in Fig 1/2A/2B; NTCP differences in Fig 2C/2D. IBH slightly improved the mean lung dose (MLD) for proton and photon plans. Although statistically significant NTCP differences were seen for radiation pneumonitis ≥ grade 2, the average difference FB-IBH only exceeded 5% for 2/19 photon and 3/19 proton plans. The expected 2 years-mortality decreased by >2% for 1 IBH proton plans while increasing for 4. For FB plans, 9/19 patients would have qualified for proton therapy in the Netherlands, based on the expected >2% difference in 2-years mortality, compared to 7/19 for the IBH plans. The subgroup of large movers did not stand out in terms of IBH dose (Fig 2A,B) nor NTCP decrease (Fig 2C, D).