Use of a matchline dosimetry analysis tool (MDAT) to quantify dose homogeneity in the region between abutting tangential and supraclavicular radiation fields

Kenneth L Homann,Steven M Kirsner,B Earl Gates,Thomas A Buchholz,R Allen White,David S Followill,Mohammad Salehpour,Karl L Prado

doi:10.1120/jacmp.v11i4.3294

Abstract

In this work, we develop and test a matchline dosimetry analysis tool (MDAT) to examine the dose distribution within the abutment region of two or more adjoining radiotherapy fields that employ different blocking mechanisms and geometries in forming a match. This objective and quantitative tool uses calibrated radiographic film to measure the dose in the abutment region, and uses a frequency distribution of area versus dose (a dose‐area histogram) to visualize the spatial dose distribution. We tested the MDAT's clinical applicability and parameters by evaluating the dose between adjacent photon fields incident on a flat phantom using field‐matching techniques employing collimator‐jaw and multileaf collimator (MLC) configurations. Additionally, we evaluated the dose in the abutment regions of four different clinical tangential‐breast and supraclavicular matching techniques using various combinations of collimator and MLC matches. Using the MDAT tool, it was determined that a 1 cm abutment region width (centered about the theoretical matchline between fields) is the most appropriate width to determine dose homogeneity in a field matching region. Using the MDAT, both subtle and large differences were seen between fields that used MLCs to form the match, compared to flat edge devices such as collimators and external cerrobend blocks. We conclude that the MDAT facilitates a more precise evaluation of the distribution of dose within the region of abutment of radiotherapy fields.PACS number: 87.55.dk

Highlights

207 Homann et al.: Matchline dosimetry analysis fundamental goal of field matching is to produce a dose within the abutment region that is equivalent to the dose within the abutting fields
The first program was DoseLab, which consists of MATLAB (The Math Works, Inc., Natick, MA) code designed to convert film optical density to a pixel matrix of absorbed-dose values; the second program was the matchline dosimetry analysis tool (MDAT), which consists of a Microsoft Excel spreadsheet that “bins” the absorbed-dose pixels within a field abutment region into discrete dose ranges, and presents them in a histogram distribution
For both the XP profiles and the percent depth dose (PDD), the relative dose measurements showed that the results obtained with the film dosimetry system agreed within 3% in low-dose gradient regions and within 2 mm distance-to-agreement (DTA) in high-dose gradient regions with the results obtained with the ion chamber

Summary

Introduction

207 Homann et al.: Matchline dosimetry analysis fundamental goal of field matching is to produce a dose within the abutment region that is equivalent to the dose within the abutting fields.The process of field abutment entails matching the edges of the treatment fields in such a manner as to create a seamless transition both dosimetrically (homogenous dose distributions) and geometrically (nondivergent beams) from one field to the forming a “perfect match”. In order to form our matchline between the tangents and supraclavicular field, we employ the “Rod and Chain” technique.[2] In this technique, an external block is used to shape the superior border of the tangential fields in order to achieve a geometric match with the inferior border of the anterior supraclavicular (SC) field. The use of these blocks became impractical, when we began treating our breast patients on treatment units with tertiary multileaf collimators because of the reduced clearance between the patient and the block tray. A principal motivation of this project was development of an objective means of evaluating dose between these adjacent fields

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Conclusion