2011 - Volume-Scaled Maximum Standard Uptake Value (SUVmax) as an Indicator for Successful Biology-Guided Radiotherapy Delivery

02:30pm - 04:00pm PT

Hall F

Screen: 8

POSTER

Presenter(s)

Murat Surucu, PhD - Stanford University School of Medicine, Palo Alto, CA

M. R. M. Ashraf¹, D. Pham², G. Bal³, and M. Surucu²; ¹Department of Radiation Oncology, Stanford University, Palo Alto, CA, ²Department of Radiation Oncology, Stanford University, Stanford, CA, ³RefleXion Medical, Inc., Hayward, CA

Purpose/Objective(s): To establish the relationship between target size and required diagnostic PET maximum standard uptake value (SUVmax) thresholds needed for a successful multi-target Biology-guided Radiotherapy (BgRT) delivery on a medical technology company’s X1 PET-linac. Current clinical eligibility criteria require SUVmax =6 at simulation, but the medical technology company’s system subsequently evaluates Activity Concentration (AC), which must exceed 5 kBq/ml for successful BgRT planning and delivery.

Materials/Methods: Custom 3D-printed phantom containing 6 spherical targets of 8 mm, 9 mm, 11 mm, 13 mm, 16 mm and 20 mm diameter was integrated within patient-specific quality assurance (PSQA)-compatible cylindrical insert. To achieve different SUVmax ranges, targets injected with varying target to background ratios (5:1, 10:1, 15:1 and 20:1) using 18F-FDG, maintaining same background activity in each of four experiments. Images acquired on medical technology company’s X1 for BgRT planning (AC quantification), and technology company Biograph mCT for SUVmax quantification. AC quantifies target's detectability by calculating difference between mean activity of highest 80% of voxels within Biological Tracking Zone (BTZ) and mean activity in surrounding background shell. BTZ encompasses GTV plus 10 mm margin, while planning target volume (PTV) was defined as GTV plus 5 mm margin. BgRT treatment plans (10 Gy/fx for 5 fractions to cover PTV) generated using RefleXion SCINTIX system, with delivery accuracy validated using a PSQA (gamma criteria: 3%/2mm and 3%/3mm). Overall, twenty-four BgRT plans were evaluated to establish operational thresholds for small tumors.

Results: The SCINTIX system successfully planned treatments for 13 of 24 BgRT experiments, all of which achieved AC>5 kBq/ml. For 16 and 20 mm targets, SUVmax>6 consistently yielded AC>5 kBq/ml, enabling successful BgRT planning and delivery. For 13 mm targets, while SUVmax>6 was achieved under most conditions, the AC>5 kBq/ml criterion was only met when SUVmax exceeded 10. The 11 mm targets required substantially higher SUVmax values (>15) to achieve AC>5 kBq/ml. Smaller targets (8 and 9 mm) could not meet the AC>5 kBq/ml threshold despite achieving SUVmax values up to 14. Analysis of successful plans revealed that a product of target volume (cc) and SUVmax greater than 11 consistently predicted successful BgRT planning across all target sizes. All successful BgRT deliveries maintained clinically acceptable accuracy, with a mean gamma passing rate of 92.4% ± 5.0% (range: 81.9% to 100%) for 3%/2mm and improved to a mean pass rate of 97.6% ± 1.9% (range: 93.8% to 100%) using 3%/3mm criteria.

Conclusion: Target size significantly influences BgRT treatment eligibility, with a non-linear relationship between target size and required SUVmax. We propose a new simple and robust criterion —Volume × SUVmax > 11—that more accurately predicts treatment eligibility across all target sizes without requiring prior knowledge of AC values.