ESTRO 2024 - Abstract Book

S1728

Clinical - Lung

ESTRO 2024

The original CTV-PTV margin inside the MAGELLAN trial is 3 mm (PTV 3mm ), so that we generated larger PTVs with 5- and 6-mm margins (PTV 5mm , PTV 6mm ). Furthermore, we created exploratory PRVs for the PBT with 3- and 4-mm margins (PRV 3mm , PRV 4mm ). The different PTVs are geometrically compared with each other, and the PRVs are compared with the PBT all within a 20-mm region from the original baseline PTV.

Results:

The mean size of the original PTV 3mm is 38.2 cm³ (range 21.9 – 63.4), which increases to mean 55.8 cm³ (+ 48%) for a PTV 5mm and 63.6 cm³ (+ 70%) for a PTV 6mm , respectively. Due to interfractional motion, the PTV all amounts to 57.7 cm³ (range 30.8 – 90.3). Mean coverage of the PTV all is 91% (77 – 98%) by the PTV 5mm and 95% (84 – 100%) by the PTV 6mm . The PBT all demonstrated considerable interfractional changes that were pronounced contralateral to the target volume. Since the occurrence of serious bronchial bleedings depends on high doses to small volumes [1], we focused on interfractional PBT changes close to the target. In a region within 20 mm from the original PTV, the PRV 3mm covers 96% (92 – 100%) and the PRV 4mm covers 98% (94 – 100%) of the PBT all . Finally, we calculated an exemplary radiation plan for one patient using a PTV 5mm and creating a PRV 3mm around the PBT. This plan yielded > 95% coverage of the PTV by the prescribed dose of 10 x 5.5 Gy while complying with all organs-at-risk constraints, particularly a PBT D0.3cm³ < 57.75 Gy ( Figure 2 ).

Figure 2: Simulated SBRT plan for a conventional linac in deep inspiration breath hold-technique. A CTV-PTV margin of 5 mm and a PBT-PRV margin of 3 mm are used.

Conclusion:

Non-adaptive surface-guided SBRT of ultracentral lung tumors seems to be an alternative to MR-guided adaptive treatment. Larger CTV-PTV margins and a PRV concept for the proximal bronchial tree are required, which increase the treatment volume but could yield adequate PTV coverage while sparing the PBT. Currently, we plan inclusion of further patient data and dosimetric analyses to obtain the most reasonable PTV and PRV margins.

Keywords: SBRT, MR-guidance, Ultracentral Lung Tumors

References: