Department of Radiological Sciences, Graduate School of Human Health Sciences, Tokyo Metropolitan University, Tokyo, Japan.
Department of Application Physics, Elekta K.K., Tokyo, Japan.
Radiol Phys Technol. 2024 Dec;17(4):834-842. doi: 10.1007/s12194-024-00835-5. Epub 2024 Sep 6.
In photon-collapsed cone convolution (pCCC) algorithm of the Monaco treatment planning system (TPS), the central-axis energy spectrum is assumed constant throughout the entire irradiation area. To consider lateral variations, an off-axis softening factor is applied to attenuation coefficients during the total energy released per unit mass calculation. We evaluated this method through comparison studies of percentage depth doses (PDDs) and off-axis ratios (OARs) calculated by Monaco and measured for a 6 MV photon beam at various off-axis angles and depths. Significant differences were observed, with relative differences exceeding ± 1%. Therefore, this method may not accurately represent lateral variations of energy spectra. We propose directly implementing energy spectra on both central-axis and off-axis to improve dose calculation accuracy for large field. To this end, we introduce reconstruction of PDDs from monoenergetic depth doses (MDDs) along off-axis angles, thereby estimating energy spectra as functions of radial distance. This method derives energy spectra quickly without significantly increasing the beam modeling time. MDDs were computed through Monte Carlo simulations (DOSRZnrc). The variances between reconstructed and measured PDDs were minimized using the generalized-reduced-gradient method to optimize energy spectra. Reconstructed PDDs along off-axis angles of 0°, 1.15°, 2.29°, 3.43°, 4.57°, 5.71°, 6.84°, 7.97°, 9.09°, 10.2° to estimate energy spectra at radial distances of 0-18 cm in 2 cm increments and OARs calculated using estimated energy spectra at 5, 10, and 20 cm depths, well agreed with measurement (relative differences within ± 0.5%). In conclusion, our proposed method accurately estimates lateral energy spectrum variation, thereby improving dose calculation accuracy of pCCC algorithm.
在 Monaco 治疗计划系统（TPS）的光子崩塌锥卷积（pCCC）算法中，假定整个照射区域的中心轴能谱是恒定的。为了考虑横向变化，在计算单位质量释放的总能量时，对衰减系数应用离轴软化因子。我们通过对在不同离轴角度和深度下，Monaco 计算和测量的百分深度剂量（PDD）和离轴比（OAR）的比较研究来评估该方法。观察到了显著的差异，相对差异超过了±1%。因此，该方法可能无法准确表示能谱的横向变化。我们建议直接在中心轴和离轴上实施能谱，以提高大野剂量计算的准确性。为此，我们引入了沿离轴角度的单能深度剂量（MDD）的 PDD 重建，从而将能谱估计为径向距离的函数。该方法无需显著增加束流建模时间即可快速获得能谱。MDD 通过蒙特卡罗模拟（DOSRZnrc）计算。通过广义最小梯度法最小化重建 PDD 与测量 PDD 之间的方差，以优化能谱。通过离轴角度为 0°、1.15°、2.29°、3.43°、4.57°、5.71°、6.84°、7.97°、9.09°、10.2°的重建 PDD 来估计 0-18cm 径向距离的能谱，增量为 2cm，并使用估计的 5cm、10cm 和 20cm 深度处的能谱计算 OAR，结果与测量结果吻合良好（相对差异在±0.5%以内）。总之，我们提出的方法可以准确地估计横向能谱变化，从而提高 pCCC 算法的剂量计算准确性。
