Effect of Different Pvdr Definitions on Dose Distribution In Photon and Proton Lattice Radiotherapy for Bulky Tumors
Abstract
Purpose
Spatially fractionated radiotherapy (SFRT) treats bulky tumors by creating heterogeneous dose distributions. Although both photon- and proton-based SFRT are accessible, various definitions of the peak-to-valley dose ratio (PVDR) may result in distinct dose distribution characteristics for different radiation modalities. This study compared photon and proton lattice plans and assessed the applicability of different PVDR definitions.
Methods
The study retrospectively analyzed 18 patients with large tumors, including 11 cases of lung cancer, 4 liver cancer, and 3 abdominal sarcoma. Sphere vertices (high dose region) were delineated using an in-house RayStation script. Both photon VMAT and proton IMPT lattice plans were generated. Three treatment planning strategies were designed: LR-S (single-field optimization), LR-M (multi-field optimization), and LR-V (photon VMAT). PVDR was calculated using four separate definitions: （Ⅰ）AVG-PVD=∑nPVDRi/n（Ⅱ）PVDR-K=Dmean of the vertices/Dmean of the valley（Ⅲ）PVDR10/90=D10%/D90%（Ⅳ）PVDR-WU=Davg(95-100)/Dpre. Dose metrics of normal tissue and plan motion robustness were also investigated.
Results
Across the four PVDR definitions, LR-S plans generally achieved higher PVDR than the other treatment planning strategies. AVG-PVDR resulted in reasonable PVDR values in range of 2.37-6.08 for all strategies. PVDR-K provided PVDR values and trends similar to AVG-PVDR . PVDR10/90 resulted in relative reasonable values (2.95-15.86) only for LR-V, while resulted in extremely large PVDR values in range of 13.15-576.00 for LR-S and LR-M. PVDR-WU resulted in smallest PVDR values in range of 1.02-1.11 for all strategies. LR-V plans generally showed less high-dose spill in the target, and higher doses to OARs. LR-S plans showed the highest PVDR, but also delivered higher doses to OARs. LR-M plans were more motion robust.
Conclusion
This study evaluated four PVDR calculation methods in VMAT and IMPT lattice plans. AVG-PVDR and PVDR-K were more stable and better reflected the dose distribution characteristics in lattice radiotherapy. Overall, multi-field proton lattice plans demonstrated higher PVDR and better OAR sparing.