Effect of beam configuration with inaccurate or incomplete small field output factors on the accuracy of treatment planning dose calculation

Abstract

Purpose: To evaluate the effect of beam configuration with inaccurate or incomplete small field output factors on the accuracy of dose calculations in treatment planning systems. Methods: Output factors were measured using various detectors and for a range of field sizes. Three types of treatment machines were configured in two treatment planning systems. In the first (corrected) machine, the Exradin W1 scintillator was used to determine output factors. In the second (uncorrected) machine, the measured output factors by the A1SL ion chamber without considering output correction factors for small field sizes were utilized. In the third (clinical) machine, measured output factors by the Exradin W1 were used but not for field sizes smaller than 2 9 2 cm 2 . The dose computed by the anisotropic analytical algorithm (AAA), Acuros XB (AXB) and collapsed cone convolution/superposition (CCC) algorithms in the three machines were delivered using static (jaw-, MLC-, and jaw/MLC-defined), and composite [intensity modulated radiation therapy (IMRT), volumetric modulated arc therapy (VMAT)] fields. The differences between measured and calculated dose values were analyzed. Results: For static fields, the percentage differences between measured and calculated doses by the three algorithms in three configured machines were <2% for field sizes larger than 2 9 2 cm 2 . In jaw-and jaw/MLC-defined fields smaller than 2 9 2 cm 2 , the corrected machine presented better agreement with measurement. Considering output correction factors in MLC-defined fields, among the three configured machines, the accuracy of calculation improved to within AE0.5%. For MLC-defined field size of 1 9 1 cm 2 , AXB showed the smallest percentage difference (1%). In IMRT and VMAT plans, the percentage differences between measured and calculated doses at the isocenter, as well as the gamma analysis of different plans, which include field sizes larger than 3 9 3 cm 2 , did not vary noticeably. For smaller field sizes, using the corrected machine influences dose calculation accuracy. Conclusion: Configuration with corrected output factors improves accuracy of dose calculation for static field sizes smaller than 2 9 2 cm 2 . For very small fields, the robustness of the dose calculation algorithm affects the accuracy of dose as well. In IMRT and VMAT plans, which include small subfields, the size of the jaw-defined field is an important factor and using corrected output factors increases dose calculation accuracy.

The impact of scanning data measurements on the Acuros dose calculation algorithm configuration

“…The monitor units are mainly affected by different output factor measurements, and the goal of this work was to evaluate relative dose only. The effect of different output factor data has been the subject of other works, mostly in the frame of small field management [ 14 , 18 , 29 ].…”

“…The dose calculation accuracy is the ability of the algorithm to estimate the dose distribution (energy deposition) in the medium [ 5 , 9 – 15 ]. However, there is a lack of studies evaluating the accuracy of the implementation of an algorithm in the planning system, except in the specific case of small field, where a lot of publications describe in detail the accuracy and the tuning of specific parameters [ 14 , 16 – 21 ].…”

Extending the EclipseTM AcurosXB output factor table for small field radiosurgery

“…7 When field sizes are smaller than 1 × 1 cm, 2 the robustness of the dose calculation algorithm becomes important. 7 Larraga-Gutierrez et al 8 measured MLC defined field output factors in a TrueBeam STx following TRS-483 5 recommendations, using four radiation detectors, to evaluate the AcurosXB algorithm in Eclipse TPS. The backup jaws were set at 0.5 cm larger than the MLC shaped fields.…”

“…They concluded that when jaw or jaw/MLC defined fields are smaller than 2 × 2 cm, using the corrected output factors improves the accuracy of dose calculation in all three algorithms. 7 When field sizes are smaller than 1 × 1 cm, 2 the robustness of the dose calculation algorithm becomes important. 7 Larraga-Gutierrez et al 8 measured MLC defined field output factors in a TrueBeam STx following TRS-483 5 recommendations, using four radiation detectors, to evaluate the AcurosXB algorithm in Eclipse TPS.…”

“…2 The deviations increase with decreasing field sizes, particularly so for Eclipse. 2 Sendani et al 7 investigated the effect of small field OFs on the TPS calculation accuracy for three different TPS algorithms (AAA, Acuros XB, CCC) and three different field settings (jaw, MLC, and jaw/MLC). They concluded that when jaw or jaw/MLC defined fields are smaller than 2 × 2 cm, using the corrected output factors improves the accuracy of dose calculation in all three algorithms.…”