Development of a Decision Tree Analysis Tool for Optimal Delivery of Liver Stereotactic Body Radiation Therapy: Photons Versus Protons

Abstract

Stereotactic body radiation therapy (SBRT) is playing an increasingly prominent role in the treatment of liver tumors with excellent local control rates, but is often limited due to organs at risk (OAR) constraints. Characteristics of liver tumors that may derive maximal clinical benefit from liver SBRT are not well described. We performed a systematic dosimetric study to determine whether liver tumors of certain sizes and locations are more suited for photon vs proton SBRT. Six spherical tumors ranging from 1 to 6 cm in diameter were contoured on axial CT simulation images of one patient at each of four locations (24 total tumors): Segment 7/8 “Peripheral/Dome”, Segment 6 “Caudal”, Segment 2/3 “Left Medial”, Segment 1 “Central”. We generated and compared photon and proton plans for each of these tumors (48 plans) to deliver 50 Gy in 5 fractions of 10 Gy with a hot spot of ∼135% within the gross tumor volume (GTV). OAR dose constraints included liver minus GTV (mean < 14 Gy, volume receiving < 15 Gy to be ≥ 700 cc), cord, heart, chest wall, bowel, and stomach. Photon SBRT plans were optimized with OAR avoidance using volumetric modulated arc therapy (VMAT). Proton SBRT plans were created with two pencil beam scanning (PBS) fields using single field uniform dose technique. Student t-test was used to compare doses and volumes between photon and protons plans. All 24 proton plans were able to deliver PTV V95% = 100% while meeting all OAR tolerance constraints. Three of 24 photon plans (Left Medial 5 cm and 6 cm, Caudal 6 cm) required minor deviations in PTV coverage in order to meet non-liver OAR dose constraints. For tumors < 3 cm at any location or Caudal and Left Medial tumors of any size, there was no significant difference in normal liver sparing between photon and proton SBRT. For Peripheral/Dome and Central ≥ 3 cm tumors, there was a statistically significant increase in the volume of liver spared and decrease in mean liver dose with protons: Peripheral/Dome: 134 ± 21 cc (p = 0.03), 912 cGy vs 1303 cGy (p = 0.01); Central: 108 ± 4 cc (p = 0.002), 1179 cGy vs 1454 cGy (p = 0.001), respectively. Proton SBRT allowed dose escalation up to 80 Gy (16 Gy per fraction) for Central tumors ≥ 3 cm while meeting all OAR constraints. For all tumors, proton SBRT reduced the mean whole body integral dose by two-fold compared with photons (p < 0.001). Tumor size and location in the liver are important determining factors in generating deliverable liver SBRT plans. Compared with VMAT photon SBRT, PBS proton SBRT allows for lower integral dose as well as significantly increased sparing of normal liver tissue, particularly for tumors ≥ 3 cm in the dome or central locations. Improved liver sparing has the potential to allow for dose escalation in the treatment of large centrally located liver tumors.