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Authors: Haywood, JR (corresponding author); Meineke, M; Grzetic, S; Smith, J

Abstract

Purpose: We provide heterogeneity and geometry correction factors for use in electron monitor unit verification calculations. Method: We use the unrestricted collisional stopping power for tissues encountered in electron beam treatments to make the heterogeneity correction factors table. We create the geometric correction factors table by taking the ratio of the doses in spherical phantoms to the dose in a flat phantom. We then added the correction factors to the TG-71 monitor unit verification equation. Results: The heterogeneity and geometry correction factors range from (0.9-1.01) and (0.8-1.0), respectively, for the energies presented. The differences between the treatment planning system and the TG-71 calculations drop from (3-14)% to (0-3)% using our modified equation. Conclusion: Monitor units calculated with the correction factors typically increase for patients with a convex curvature, which matches the behavior of Monte Carlo based planning algorithms. An increase in monitor units lowers the percent difference between the second check and the treatment planning system to under the TG-114 recommended 5% actionable level.

Introduction

The current guidance for calculating dose distributions and monitor units (MU) for clinical electron beams is to use CT datasets and 3D heterogeneity corrections¹. The recommendation assumes the treatment planning system (TPS) is properly commissioned following the guidelines in TG-53 ².  However, anecdotal evidence suggests that there are a large number of centers that utilize their TPS for the calculation of electron isodose lines, yet choose to treat the planned aperture and geometry with hand or second check calculated MU rather than the MU from the TPS³.

At our institutions, when modeling dose in the breast, chest wall, and scalp, the MU calculated  by the treatment planning system (Eclipse eMC, Varian Medical Systems, Palo Alto, CA) are generally greater than the MU calculated using the TG-71⁴ formalism.  TG-114 and TG-71 indicate that patient heterogeneity and complex geometries (obliquity and patient curvature) are the most likely causes ^{5, 4}.  Khan has discussed obliquity factors for electron beams⁶, and those treatment types are not considered here. Our focus in this study is en face treatment beams.  We denote the corrections associated with patient curvature as geometry corrections.  Below we present tables of heterogeneity and geometry correction factors. These factors are sufficient to correct hand calculations in several anatomic locations using en face treatment beams. 

Continue reading “Correction factors for monitor unit verification of clinical electron beams”

Introduction

The introduction of electron Monte Carlo (eMC) into clinical practice was over a decade ago (~2006). Many clinics have adopted eMC as the standard calculation algorithm over less accurate algorithms like Gaussian Pencil Beam. Surprisingly, however, not all clinics that use eMC use the MU that it provides. A 2016 survey of the MedPhys listserve showed that nearly half of the respondents used the MU generated from hand calculations for delivering the patient’s treatment. Yet, the physician was presented the isodose curves based on the MU generated by eMC in ~75% of those clinics.

In my clinic, we were using the hand calculated MU for treatments and were not generating isodose plans at all. EMC usually gave MU that were 5-15% higher than the hand calculated MU. We were reluctant to use the eMC MU. One day, the radiation oncologist noted that the electron patients were not getting “red enough” and we had an “aha” moment. We figured if she wanted them redder, we should use higher MU. EMC was giving higher MU and we thought let’s just use the higher MU from eMC.

Her comments prompted the following study to show that eMC was delivering the correct dose with the higher MU.

Continue reading “Verification of Varian eMC”