232 - A Deep Quantitative Assessment of Metabolic Mucosal Changes with Variable Relative Biological Effectiveness of Head and Neck Proton Therapy
Presenter(s)
S. Yarlagadda1, M. Acosta1, N. Carvallo1, K. A. McConnell2,3, and N. S. Kalman3; 1Department of Radiation Oncology, Miami Cancer Institute, Baptist Health South Florida, Miami, FL, 2Miami Cancer Institute, Baptist Health South Florida, Miami, FL, 3Herbert Wertheim College of Medicine, Florida International University, Miami, FL
Purpose/Objective(s): We had previously observed higher mucosal uptake on post-treatment PET scans for oropharyngeal carcinoma (OC) patients treated with protons, with possible association with variable relative biological effectiveness (RBE). The current study expands on these findings in a larger proton cohort with more granular data comparing different regions of interest (ROIs).
Materials/Methods: We reviewed patients with OC who received definitive or adjuvant (post-surgery) proton therapy between 2018 and 2022. All patients received = 50 Gy to oropharyngeal mucosa and were followed up for = 6 months to confirm there was no recurrence. 3-month post-treatment PET scans were registered with planning CTs. Mucosal treatment volume (CTVp) and its isometric expansions CTVp+5 mm, CTVp+10 mm, CTVp+20 mm were created, with SUVmax recorded. The corresponding volumes were categorized as enhancing and non-enhancing based on FDG uptake (1.1X and 1.5X) higher than mean liver standard uptake value (SUV). Mean planning dose, LET, and RBE (using 3 models: McNamara, Carabe, and Wedenberg) were recorded for the ROIs. Wilcoxon signed-rank test was used to compare mean RBE between each ROI’s enhancing and non-enhancing volumes. Pearson correlation was used to assess the association between RBE and SUVmax in each ROI.
Results: 25 OC patients were included in the current analysis (14 definitive RT and 11 adjuvant RT). Table 1 depicts mean planning dose and RBE computed per the 3 models and LETd for various ROIs based on the mucosal treatment volume. Across all the models used, the ROIs had RBE 116.8%-125.61% higher relative to the planning dose (p < 0.01). For enhancing high FDG uptake ROIs, mean RBE exceeded that of non-enhancing ROIs across all models (p < 0.05). LETd increased with greater volume expansion (5mm of 3.2-3.3 keV/µm vs 20mm of 4-4.3 keV/µm. Moderate-strong correlations were observed between RBE mean (all models) in 1.5X enhancing CTVp+10mm and CTVp+20mm with SUVmax in CTVp, CTVp+5mm, CTVp+10mm (Pearson coefficient [r]= 0.38-0.6, p < 0.05). The SUVmean CTVp+5mm/liver was similar for definitive and adjuvant groups (median: 1.33 vs 1.27).
Conclusion: This analysis demonstrated a statistically significant correlation between RBE and post-RT PET uptake in head and neck cancer patients. The association between variable proton RBE and imaging changes was observed in both definitive and adjuvant treatment. Further study is needed to evaluate these effects in a larger patient cohort.
Table 1:| Planning dose [Gy] (Physical dose x 1.1) | RBE [% of planning dose] | LETd [keV/µm] | ||||
| McNamara | Carabe | Wedenberg | ||||
| CTVp+5mm (1.1X) | Enhancing | 62.9 | 120.32% | 117.69% | 119.62% | 3.2 |
| Non-enhancing | 61.5 | 119.66% | 116.80% | 118.76% | 3 | |
| CTVp+5mm (1.5X) | Enhancing | 63 | 120.83% | 118.38% | 120.30% | 3.3 |
| Non-enhancing | 62 | 119.94% | 117.27% | 119.23% | 3.1 | |
| CTVp+20mm (1.1X) | Enhancing | 48.7 | 123.78% | 121.29% | 124.23% | 4 |
| Non-enhancing | 42.1 | 122.80% | 119.41% | 123.33% | 3.4 | |
| CTVp+20mm (1.5X) | Enhancing | 44.4 | 124.86% | 122.64% | 125.61% | 4.3 |
| Non-enhancing | 43.2 | 122.99% | 119.93% | 123.50% | 3.6 | |