Main Session
Sep 29
PQA 07 - Head and Neck Cancer, Lung Cancer/Thoracic Malignancies, and Nursing and Supportive Care

3633 - DWI-Based Visual Non-Visible Lesion Assessment After Lung Stereotactic Body Radiotherapy: A Prospective Pilot Study

03:45pm - 05:00pm ET
Poster Hall - Exhibit Hall A
Screen: 27
POSTER

Presenter(s)

Osamu Tanaka, MD, PhD Headshot
Osamu Tanaka, MD, PhD - Asahi Univeristy Hospital, Gifu city, Gifu

O. Tanaka1, T. Taniguchi1, K. Ono2, S. Nakaya1, N. Funaguchi3, C. Makita4, and M. Matsuo5; 1Department of Radiation Oncology, Asahi University Hospital, Gifu, Japan, 2Department of Radiation Oncology, Gifu University Hospital, Gifu, Japan, 3Department of Respiratory Medicine, Asahi University Hospital, Gifu, Japan, 4Gifu University School of Medicine, Department of Radiology, Gifu, Japan, 5Innovation Research Center for Quantum Medicine, Graduate School of Medicine, Gifu University, Gifu, Japan

Purpose/Objective(s): Post-SBRT CT frequently shows persistent opacity from radiation lung injury, complicating early response assessment and differentiation of viable tumor from post-treatment change. We tested whether baseline FDG-PET SUVmax and early changes in ADCmean and tumor size predict a viability-oriented MRI surrogate, non-visible lesion on the ADC map at 6 months (NVL6M), after lung SBRT.

Materials/Methods: Patients treated with lung SBRT (40 Gy/4 fractions) were prospectively enrolled. CT/MRI were obtained as a same-day set at baseline and at 1 month, 3 months (±7 days), and 6 months (±14 days). MRI included DWI/ADC plus T1- and T2-weighted imaging; T2WI supported interpretation of radiation pneumonitis versus residual tumor. ADCmean was measured from a manual 2D maximal-slice ROI delineated on DWI and T2WI and applied to the ADC map. NVL was defined as inability to delineate a measurable lesion on the ADC map. All patients underwent baseline FDG-PET/CT within 2 weeks prior to SBRT. Six-month MRI was planned for all; if NVL was achieved at 3 months, 6-month MRI could be omitted at patient request, resulting in response-driven missingness. RECIST 1.1 was measured on CT and T2WI. CT surveillance was protocolized up to 3 years; this report reflects early follow-up (~1.5 years since study initiation). Comparisons used Mann–Whitney U tests (exploratory).

Results: Twelve SBRT patients were analyzed (NVL6M=7; non-NVL6M=5). Baseline SUVmax was lower in NVL6M versus non-NVL6M (median 7.0 vs 9.0; p=0.048). ?ADC% did not clearly separate groups (Pre?1M: 47.6% vs 40.3% [available n=6/3], p=0.795; Pre?3M: 90.1% vs 69.0% [n=5/3], p=0.571). ?LD% trended toward greater reduction in NVL6M (Pre?1M: -57.7% vs -18.3% [n=7/5], p=0.268; Pre?3M: -54.2% vs -38.5% [n=6/4], p=0.571). Among three small cell carcinoma cases, two developed mediastinal nodal relapse despite primary-lesion NVL at 3 months.

Conclusion: Lower baseline FDG-PET SUVmax was associated with NVL6M after lung SBRT, suggesting baseline metabolic activity may influence MRI-based lesion non-visibility. Early ?ADC% was not a robust discriminator in this small cohort and was limited by response-driven missingness as lesions became non-measurable. NVL should be interpreted as a local surrogate and does not exclude nodal relapse, particularly in small cell carcinoma. Larger cohorts with longer follow-up will assess whether NVL6M improves early post-SBRT response assessment beyond CT-based measurements and predicts =12-month clinical endpoints.