A physics-based analytical model of absorbed dose from primary, leakage, and scattered photons from megavoltage radiotherapy with MLCs

Abstract

A burgeoning population of cancer survivors is at risk of late health effects following radiation therapy including second cancers, with the majority of these cancers occurring outside of the treatment volume of the primary cancer. Commercial radiotherapy treatment planning systems underestimate the out-of-field dose. Previous analytical models of out-of-field dose have assumed radial symmetry and/or approximated the dimensions of collimators as semi-infinite planes. The purpose of this work was to develop a physics-based analytical model of total absorbed dose from primary, scattered, and leakage radiation for square fields from a 6 MV beam at any arbitrary point in a phantom, including in-plane, cross-plane, and out-of-plane locations. The model includes the essential physics of radiation transport through beam-limiting-devices including rounded edges of MLC leaves. The model agreed well with measurements and Monte Carlo simulations of absorbed dose in a water-box phantom and was validated for field-sizes ranging from 2 × 2 to 20 × 20 cm 2 . The signed and unsigned average percent differences were −3.0% and 15.9%, respectively, for all points and field-sizes considered. An extended gamma index analysis reveals a 92% pass rate with criteria of 3 mm distance-to-agreement, 3% relative dose difference in-field, and 3 mGy Gy −1 absolute dose difference out-of-field. The average wall-clock time to calculate dose to one million points was 3.3 min. These results suggest that it is feasible to calculate absorbed dose from both therapeutic and stray radiation using physics-based analytical models with good accuracy, thus overcoming a major obstacle to routinely assessing exposures. Additionally, this work demonstrates the importance of relaxing certain simplifications such as assuming a radially symmetric stray-dose distribution. Because the model is physics-based and may be reconfigured according to the dimensions of beamlimiting-devices, adapting it to other treatment units should be straight forward. Uses for such a model include clinical and research applications, such as clinical trials and epidemiological studies.

Method to quickly and accurately calculate absorbed dose from therapeutic and stray photon exposures throughout the entire body in individual patients

“…The relatively lower signal in the far‐from‐field region necessitated a more sensitive detector, and therefore, we used a farmer‐type chamber (IBA FC65‐G SN: 3068, Louvain‐la‐Neuve) for measurements in this region. We corrected the measurements made with the diamond detector for dose‐rate effects following the methods of Laub et al We corrected the farmer chamber measurements for variations in photon spectral fluence among all measurement locations (i.e., in field, near field, and far from field) following the methods of Chofor et al…”

“…The relatively lower signal in the far-from-field region necessitated a more sensitive detector, and therefore, we used a farmer-type chamber (IBA FC65-G SN: 3068, Louvain-la-Neuve) for measurements in this region. We corrected the measurements made with the diamond detector for dose-rate effects following the methods of Laub et al 35 We corrected the farmer chamber measurements for variations in photon spectral fluence among all measurement locations 32 (i.e., in field, near field, and far from field) following the methods of Chofor et al 36,37 We performed two tests to characterize the dosimetric accuracy compared to the measurements used to configure the model (see Table I). For the first, we calculated the relative local dose differences (dD R ) in the in-and near-field regions and the absolute local dose differences (dD A ) in the far-from-field region for both CERR and CERR-LSU.…”

“…Analytical models, on the other hand, are comparatively simple, fast, and free of statistical fluctuations. There is also promising evidence suggesting they can be dosimetrically accurate in the out‐of‐field region . Further, a recently published analytical model is readily generalizable to different treatment machines and irradiation conditions without the need for proprietary information due to its physics‐based framework .…”

“…There is also promising evidence suggesting they can be dosimetrically accurate in the out-of-field region. 13,17,24,[29][30][31][32] Further, a recently published analytical model is readily generalizable to different treatment machines and irradiation conditions without the need for proprietary information due to its physics-based framework. 17,29 It is not known, however, whether such algorithms could be implemented into a TPS to quickly and accurately predict the otherwise missing out-of-field dosimetric information.…”

A Publicly Available Dataset of Out-of-Field Dose Profiles of a 6 MV Linear Accelerator

“…Methods of estimating out-of-field doses include consulting simple reference data in the literature [6,[9][10][11], Monte Carlo simulations [12][13][14], and increasingly refined analytical models [15][16][17][18]. These models may need to consider the physical geometry of clinical linacs, including jaws, primary collimator, MLC leaves and carriage, and the arrangement of additional head shielding.…”