On the Use of 4D-PET/CT for the Safe SBRT Re-Irradiation of Central Lung Recurrence Within Radiation-Induced Fibrosis: A Clinical Case

<b>Background:</b> The re-irradiation of centrally located lung tumors poses substantial risks due to prior dose exposure and proximity to critical structures. Accurate target delineation is crucial to minimize toxicity and ensure tumor coverage. Four-dimensional positron emission tomography/computed tomography (4D-PET/CT) integrates respiratory motion and metabolic data, offering improved delineation over static imaging. Its clinical utility in re-irradiation remains under-reported. <b>Methods:</b> A 67-year-old male presented with the central recurrence of squamous cell carcinoma in the right upper lobe, embedded in radiation-induced fibrosis, following prior chemoradiotherapy. Delineation using static PET underestimated tumor motion. A 4D-PET/CT-guided Stereotactic Body Radiation Therapy (SBRT) plan was developed with a prescription of 60 Gy in eight fractions. A comparative plan using static PET was generated to assess the dosimetric differences. <b>Results:</b> The internal target volume (ITV) from 4D-PET/CT was nearly double the size of the GTV from static PET, with a 5.1 mm discrepancy in the craniocaudal axis. The 4D-PET-based plan achieved 95.0% PTV coverage, while the static PET-based plan covered only 61.7%, illustrating the risk of underdosage without motion-resolved imaging. The patient completed the treatment without acute or late toxicity and showed a sustained metabolic response at one year (SUVmax from 13.4 to 5.8). <b>Conclusions:</b> This case demonstrates the clinical value of 4D-PET/CT in the SBRT re-irradiation of centrally located lung tumors, particularly in fibrotic regions where anatomical imaging is insufficient. It enabled accurate delineation, improved dosimetric coverage, and safe, effective retreatment. These findings support its integration into planning for complex thoracic re-irradiation.