[Feasibility of Effective Source-to-surface Distance Calculation Using an Electron Monte Carlo Algorithm for Electron...

In radiation therapy, the absorbed dose is corrected for changes in the nominal treatment distance using the inverse square law. However, in the case of electron beams, the inverse square law using the nominal treatment distance is invalid. Therefore, an effective source-to-surface distance (SSD) should be determined. The effective SSD must be measured for all electron beam energies and applicator sizes. Here, we calculated the effective SSD using a radiotherapy planning system with an electron Monte Carlo (eMC) calculation algorithm and evaluated its usefulness.

The effective SSD was calculated from the absorbed dose ratio at d_max at extended SSDs under 5 gap conditions, using both eMC calculations and LINAC measurements. The consistency between calculated and measured values was evaluated based on the absorbed dose ratio at d_max, effective SSD, and distance correction factor.

The difference in the absorbed dose ratio at d_max between eMC calculations and measurements at extended SSDs was within 1.38%, and the effective SSD values agreed within 5.40 cm. Larger discrepancies in effective SSD were observed under conditions of high energy with large field sizes and low energy with small field sizes.

The good agreement in absorbed dose ratio at d_max, effective SSD, and distance correction factor between eMC calculations and measurements indicates that effective SSD calculation using eMC is feasible and can be employed for comparative verification against measured values.