Accuracy and sources of error of out‐of field dose calculations by a commercial treatment planning system for intensity‐modulated radiation therapy treatments

Jessie Y Huang,David S Followill,Stephen F Kry,Xin A Wang

doi:10.1120/jacmp.v14i2.4139

Abstract

Although treatment planning systems are generally thought to have poor accuracy for out‐of‐field dose calculations, little work has been done to quantify this dose calculation inaccuracy for modern treatment techniques, such as intensity‐ modulated radiation therapy (IMRT), or to understand the sources of this inaccuracy. The aim of this work is to evaluate the accuracy of out‐of‐field dose calculations by a commercial treatment planning system (TPS), Pinnacle3 v.9.0, for IMRT treatment plans. Three IMRT plans were delivered to anthropomorphic phantoms, and out‐of‐field doses were measured using thermoluminescent detectors (TLDs). The TLD‐measured dose was then compared to the TPS‐calculated dose to quantify the accuracy of TPS calculations at various distances from the field edge and out‐of‐field anatomical locations of interest (i.e., radiosensitive organs). The individual components of out‐of‐field dose (patient scatter, collimator scatter, and head leakage) were also calculated in Pinnacle and compared to Monte Carlo simulations for a 10×10 cm2 field. Our results show that the treatment planning system generally underestimated the out‐of‐field dose and that this underestimation worsened (accuracy decreased) for increasing distances from the field edge. For the three IMRT treatment plans investigated, the TPS underestimated the dose by an average of 50%. Our results also showed that collimator scatter was underestimated by the TPS near the treatment field, while all components of out‐of‐field dose were severely underestimated at greater distances from the field edge. This study highlights the limitations of commercial treatment planning systems in calculating out‐of‐field dose and provides data about the level of accuracy, or rather inaccuracy, that can be expected for modern IMRT treatments. Based on our results, use of the TPS‐reported dose could lead to an underestimation of secondary cancer induction risk, as well as poor clinical decision‐making for pregnant patients or patients with implantable cardiac pacemakers and defibrillators.PACS numbers: 87.53.Bn; 7.55.D‐

Highlights

187 Huang et al.: Out-of-field treatment planning system (TPS) inaccuracies for intensity-m odulated radiation therapy (IMRT) (“collimator scatter”), and internal patient scatter
Out-of-field dose accuracy for IMRT treatment plans In Fig. 2, the thermoluminescent detectors (TLDs)-measured doses and the TPS-calculated doses are plotted as a function of distance from the treatment isocenter for each of the three IMRT treatment plans
The TLD-measured dose fell off approximately exponentially as the distance from the isocenter increased; this result is similar to conventional field data from the Report of American Association of Physicists in Medicine Task Group 36.(21) the TPS-calculated doses follow a similar trend for higher doses, the calculated doses do not demonstrate this exponential falloff behavior for locations far outside the treatment field; rather, they exhibited less predictable, almost random, behavior

Summary

Introduction

187 Huang et al.: Out-of-field TPS inaccuracies for IMRT (“collimator scatter”), and internal patient scatter. It has been found that the cumulative incidence of SPM could be as high as 20% of patients treated with radiation therapy The incidence of these secondary malignancies depends on the delivered dose distribution, size of the irradiated volume, dose, and dose rate, along with other patient-specific factors.[2] Diallo et al[3] found that the majority of these second cancers arise in the margin of the irradiated region or the “beam-bordering” region (from 2.5 cm inside to 5 cm outside of the irradiated volume) and that a sizeable number of cancers developed at distant sites far outside of the treatment field. Aside from research regarding secondary cancers, accurate knowledge of this peripheral dose is important in many clinical situations, such as the treatment of pregnant patients or patients with implanted electronic devices like pacemakers. Clinical decision-making is based on dose thresholds (e.g., 2 Gy for implanted cardiac pacemakers), and inaccurate dose knowledge could lead to poor decision-making.[4]

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