Assessment of out-of-field dose calculation algorithm by commercial treatment planning systems in IMRT and 3DCRT

Document Type: Conference Proceedings

Authors

1 Department of Medical Physics, Faculty of Medical Sciences, Shahid Beheshti University of Medical Science, Tehran, Iran

2 Department of Radiology, Faculty of Allied Medical Sciences, Shahid Beheshti University of Medical Sciences, Tehran, Iran

3 Department of Radiotherapy, shohada-e-Tajrish Hospital, Shahid Beheshti University of Medical Science.Tehran, Iran

Abstract

Abstract
Introduction: The accuracy an assessment of out- of- field dose due to secondary cancer risk is clinically important. Actually radiotherapy treatment planning systems are not commissioned for the out-of-field dose calculations, so the estimation of dose distributions by TPSs beyond the borders of treatment fields is not well calculated. Now days using modern radiotherapy techniques such as IMRT because of many advantages is most common, while the increase time delivery and also higher number of monitor units (MU), results to increase peripheral dose which can lead to rise potentially risk of secondary cancers after IMRT. Based on documents, increase in collimator scatter and head leakage has been estimated to increase carcinogenic Risk. Because of lack of the study on modern technique(IMRT) and any type of TPSs, the aim of this project is the evaluation of accuracy of out-of-field dose calculations by Isogary and Eclipse  TPSs, through the comparison of phantom measurements.
Materials and Methods: In this study, For mesurements 2 phantoms were used in 2 steps, at first for evaluated the Eclipse treatment planning system, and at second step  ISogary . The dose value was calculated by the Eclips and also ISogary in 3 different depths e.g., 10,15,20 cm and also in 3 different simple quadrate field size, 10×10, 20×20 and 30×30 cm2   at the 1 up to 8 cm from filed edge in 1cm gap interval. Next this values in the same condition were measured by the water phantom and ion chamber dosimeter (Farmer 0.6 cc PTW) and the reading of each point was convert to the Dose in Gy according to the TRS 398 protocol. In the 2th step the dose values were compared by the Delta4+  phantopm in IMRT technique for 10 patients and in the same condition the dose of Rectum and femoral head and bladder was calculated by the Eclipse TPS.
 Result: Based on the results of the current study, the underestimation in average was demonstrated 38.80 % in Eclipse TPS. And also in the Isogary TPSs the results showed that the overestimation in the 1 to 8 cm distance from field edge was in average 34.65%. Also in the 10 cm depth the average Error was Significant (P-value<0.05).  In IMRT the rectum and bladder and also femoral head dose is underestimated by TPS in comparison with the measured values.
 Conclusion: In conclusion based on this results the calculation of dose contributions by the TPSs was poor in out-of-field area. In IMRT technique underestimated of the dose calculations by TPS in the OARs was proved. While the magnitude of the underestimation was different and was more in the organs near the field edge. The underestimation of TPS is increase with increase distance from field edge.

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