Vertebral SABR presents a challenging dose calculation scenario due to heterogeneities in material, high dose gradients and small fields. As a result, dose calculation with Monte Carlo or linear boltzman transport equation (LBTE) algorithms is an attractive option due to its improved dose calculation accuracy. Compared with Type B algorithms however, LBTE algorithm presents a number of key differences which may impact reporting of dose distributions in vertebral SABR.
Reporting dose to medium or water results in substantial differences in reported GTV dose, which depends on the density of the bone in the target. Lytic lesions exhibit minimal difference between AAA and either reporting mode for AXB; however increased HU in blastic lesions results in differences of up to 3.5% with dose to medium and 6% with dose to water in median GTV dose. This is a result of the increased difference in stopping power between bone and water, which is increasing with density of the bone. These differences translate into small reporting differences in PTV coverage metric. Zhen et. al. compared AAA and AXB Dm (v10.0.28), and showed increased dose in the vertebral body GTV and PTV with AXB Dm compared with AAA by 1–2%, in opposition to our finding 5. The cause of this discrepancy is unknown, but may be related to the different versions. Usmani et. al. compared Monte Carlo Dm with Dw for IMRT vertebral plans in iPlan 16. Increases in dose in high density bone were observed with Dw compared with Dm. The CTV mean dose was increased by approximately 5% with Dw compared with Dm a similar magnitude to that observed in our study.
The spinal cord PRV near maximum dose is a hard constraint in vertebral SABR planning. During optimisation, the spinal cord PRV is typically taken to the constraint, and the PTV coverage is maximised while meeting this constraint. When using AXB Dm, the spinal cord PRV near maximum dose was on average 3% lower than that with AAA. This may result in a systematic increase of spinal cord PRV dose when planning with AXB Dm. There was no systematic difference however when using AXB Dw. In Zhen et. al., the spinal cord maximum dose and D0.5 cc was lower with AXB Dm compared with AAA 5. Miura et. al. the near maximum spinal cord dose was on average 1% higher with Dw compared with Dm 16. Head and neck cancer radiotherapy presents a similar geometry, with the target volume wrapping around the spinal cord. Munoz-Montplet et. al. 17 compared AXB (v13.0.26) with AAA for 140 H&N plans. The near maximum dose was on average 1.07 Gy lower with AXB Dm compared with AAA, but 0.17 Gy higher with AXB Dw compared with AAA. Zifodya et. al. 18 compared AXB (v11.0.3.1) with AAA for head and neck plans; AXB Dm resulted in systematically lower mean spinal cord dose and near maximum spinal cord dose compared with AAA, however AXB Dw did not show the same trend, and was much closer to AAA.
The dose at the centre of the spinal cord was significantly lower than AAA when calculated with AXB, reporting either Dm or Dw. It is hypothesised this is a result of energy cut-off used in dose calculation. The AXB algorithm uses 200 keV for electrons and 1 keV for photons. The spinal cord is predominantly in the gradient region of the apertures, therefore selection of the electron cut off energy will have a large impact on the dose in this region. As demonstrated by Hughes et. al., electron energy is deposited proximal to the field edge, and as a result the dose calculated further from the field edge, in the centre of the cord, is reduced 19. The selection of energy cut-off may also be the cause of lower near maximum cord doses, due to a resultant sharper dose gradient with AXB. Given the serial nature of the spinal cord with respect to toxicity, the dose at the centre of the spinal cord may be of limited clinical importance. In treatment plan verification measurements however, the dose in the spinal cord is often measured. Further work is required to determine the accuracy of AXB compared to measurement; due to electron energy cut-off, substantial underestimates of the spinal cord dose may be observed.