2944 - Association Between Peak-to-Valley Dose Ratio and Tumor gEUD in Large Liver Tumors Treated With Lattice SFRT
Presenter(s)
R. K. Badkul1, H. Li1, S. Badkul2, F. Wang3, R. C. Chen4, and D. Akhavan5; 1University of Kansas Medical Center, Kansas City, KS, 2Kansas Health Science University, Wichita, KS, 3Department of Radiation Oncology, The University of Kansas Medical Center, Kansas City, KS, 4Department of Radiation Oncology, University of Kansas Medical Center, Kansas City, KS, 5Department of Radiation Oncology, City of Hope National Medical Center, Duarte, CA
Purpose/Objective(s): Lattice spatially fractionated radiotherapy (SFRT) delivers highly heterogeneous dose distributions characterized by high-dose vertices embedded within lower-dose valley regions. While peak-to-valley dose ratio (PVDR) is commonly reported, its relationship to biologically relevant tumor dose metrics remains unclear. We evaluated the association between PVDR and tumor generalized equivalent uniform dose (gEUD) in patients with large liver tumors treated with lattice SFRT.
Materials/Methods:
Five patients with large liver tumors (GTV range 573–2013 cc; four >1300 cc) treated with single-fraction lattice SFRT with prescription dose of 27Gy were retrospectively analyzed. Spherical vertices (1 cm diameter) were placed within the GTV with 2–3 cm center-to-center spacing. PVDR was calculated using D10_peak/D90_valley and Peak_mean/Valley_mean. Tumor gEUD was computed from cumulative DVHs using Niemierko’s generalized EUD model with parameter a = -5. Peak volume fraction (f_peak), valley D90, and gEUD were evaluated descriptively.
Results:
Median GTV volume was approximately 1326 cc (range 574–2013 cc). Peak Dmean was consistent across cases (~27 Gy), with peak volume fraction ranging from 2.2–3.5% of the GTV. Valley D90 ranged from 6.38–9.23 Gy. PVDR (D10_peak/D90_valley) ranged from 3.31–4.80 (median 3.79). Tumor gEUD (a = -5) ranged from 3.08–10.58 Gy (median 8.83 Gy). Higher PVDR values were associated with lower valley D90 and correspondingly lower tumor gEUD. The patient with the highest PVDR (4.80) demonstrated the lowest gEUD (3.08 Gy), suggesting that increasing modulation strength reduces effective tumor dose despite stable peak dose levels.
Conclusion: In large liver tumors treated with lattice SFRT using photon VMAT technique, tumor gEUD appears primarily driven by valley dose rather than peak dose magnitude. Increasing PVDR was associated with reduced effective tumor dose, reflecting the dominant contribution of valley coverage in highly heterogeneous distributions. These findings emphasize the importance of balancing modulation strength with adequate valley dose when optimizing lattice SFRT plans for bulky disease.