DOE OSTI2016
Cancer is one of the most significant health care problems today. It is estimated that 1 in 2 men and women will be diagnosed with some form of cancer in their lifetime. Nearly two thirds of cancer patients will receive radiation therapy at some point in their care and the vast majority of these radiotherapy treatments will be performed with megavoltage photon therapy. Photon treatment techniques typically devote considerable attention to limiting the exposure of healthy tissues outside of the intended treatment volume. Nonetheless, small levels of unwanted stray radiation are incident upon the patient's whole body, which can increase the risk for numerous late effects, including second malignancies. When one considers that second cancers account for approximately 15% of all cancers and that it is projected that by the year 2020, there will be almost 20 million cancer survivors in the United States and four million in the UK, the need for improved accuracy in stray dose calculations appears likely to increase dramatically. Despite the obvious benefits this capability would afford, contemporary commercial treatment planning systems (TPSs) lack the ability to accurately predict stray dose outside the treatment field, e.g., beyond the 5% isodose surface. Thus, there is a large, variable, and systematic underestimation of stray radiation exposures by commonly used clinical TPSs for photon therapy. To overcome this problem, in a companion paper we proposed a simple, physics-based analytical model that calculates the total therapeutic and stray dose from photon treatment fields. The purpose of this work was to evaluate the accuracy of the basic physics in the analytical absorbed dose model for several simple treatment fields. We assessed the performance of the basic physics in an analytical model that calculates the total absorbed dose deposited by primary, leakage, and scattered photons both inside and outside of square treatment fields of 6 MV photon conventional radiotherapy. The assessment was accomplished by comparing model predictions of total dose with measured dose profiles in a water-phantom. The major finding of this study is that the physics-based model performed well, predicting dose in water within 11.5% dose difference or 5 mm distance to agreement with measured values in the therapeutic region and within 1.1 mGy/Gy or 0.5 mm distance to agreement with measured values in the out-of-field region at 90% of the locations considered. Contemporary external-beam photon TPSs do not include the capability to accurately predict the absorbed dose beyond a few centimeters outside of the treatment field. Our work suggests that it is feasible to improve dosimetric accuracy in the out-of-field region using a simple and computationally fast, physics-based analytical algorithm. This is demonstrated by the measured absorbed dose as well as the corresponding values calculated by two contemporary commercial TPSs (Pinnacle, Philips Health care, Andover, MA and Eclipse, Varian Medical Systems, Inc., Palo Alto, CA) and the analytical model considered in this work. It is apparent that both TPSs underestimated the absorbed dose by greater than 3 mGy/Gy at locations outside of the 0.5% relative dose level. In contrast, the analytical model evaluated in this work calculated absorbed dose in the same region within 0.4 mGy/Gy of measured values. The physics-based analytical dose model could prove useful in a number of different applications. The accurate knowledge of absorbed dose deposited outside of the treatment field is especially important for predicting radiation late effects, such as second cancer, fertility complications, and cardiac toxicity. It may also facilitate predictions of the effects of in utero exposure. We conclude that it is possible to accurately calculate the therapeutic and stray absorbed dose from a conventional radiotherapy treatment field of 6 MV photons using a physics-based analytical model. This was demonstrated by the agreement between measured and calculated doses in water within 11.5% relative dose difference or 5 mm distance to agreement in the therapeutic region, and within 1.1 mGy/Gy absolute dose difference or 0.5 mm distance to agreement in the out-of-field region at 90% of positions for all of the fields considered in this work. This is also illustrated by Figure 4 which plots dose values from analytical model predictions and measurements at five representative irradiation conditions and shows that the analytical model agreement is good in all cases. Furthermore, the model validated in this work evidently can predict dose values with better accuracy at out-of-field locations than contemporary TPSs. This and previous works, taken together, suggest that analytical physics-based models will offer advantages over TPSs or Monte Carlo models for whole-body dose calculations for several radiotherapy applications.
61 RADIATION PROTECTION AND DOSIMETRY↗