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

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 deter-mined 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.

Figure 1

Schematic representation of the experimental setup used to determine the position of the central matchline and the location of the abutment region. The pinpricks located outside of the field (6 cm on either side of the isocenter) are placed on the central matchline. The abutment region encompasses an equal area extending superiorly and inferiorly relative to the central matchline. The regions of interest (ROI) are used to determine the dose that will be used to normalize the pixels' dose values within the abutment region.

Figure 2

Screen captures of Kodak EDR2 film‐density images depicting the three matching techniques: (a) Jaw–Jaw, (b) Jaw–MLC–0°, and (c) Jaw–MLC–45°. Note the effects of the MLC on the abutment region in (b) and (c), compared to the straight collimator edge in (a).

Figure 3

Screen captures from Pinnacle³ TPS depicting the four matching configurations tested for the clinical breast fields: a)External Block – an external cerrobend block is placed on the tangent fields to form the match between these fields and the anterior supraclavicular field; b) Jaw–Jaw – the collimator jaw now forms the match and MLC leaves are used to block the lung fields (Note how this is the only instance where the MLC is used to block the lung; this is required to compensate for the portion of lung that is unshielded in order to use the rotation of the collimator to form the “perfect” match); c)MLC‐Perpendicular – the leaves are brought in a closely approximated “perpendicular” orientation relative to the matchline; d) MLC‐Parallel – leaves are brought in a closely approximated “parallel” orientation relative to the matchline. Notice the orientation of the leaves for b), c) and d). Additionally, recall that the MLC‐Parallel technique is used for $<\hspace{0.17em}10\hspace{0.17em}\%$ of the MLC‐Perpendicular treatment in order to orient the leaves to perform “field‐in‐field” reduction of hot spots in the tangent fields.

Figure 4

Cartoon depicting the clinical breast field setup and evaluation methods, highlighting the coronal plane at a depth of 3.5 cm.

Figure 5

Crossplane (XP) profiles (a) and percent depth doses (PDDs) (b) obtained using the Wellhofer CC04 ion chamber in water and the Kodak EDR2 film in plastic water.

Figure 6

Cumulative DAHs of the Jaw–MLC–0° matching technique calculated using four dose bin sizes. Although the bin size is commonly defined by the inherent systematic error, we found that bin size has little effect on the shape of the DAH.

Figure 7

Abutment region widths for the three matching techniques. All graphs show a change in structure between the 0.75 cm and 1.0 cm abutment region widths. Regions wider than 1.0 cm appear to have a similar curve structure, whereas those narrower than 0.75 cm seem to illustrate a different curve structure.

Figure 8

An integral dose‐area histogram (DAH) of the matchline dose analysis tool (MDAT) used to examine dose differences between the three beam‐matching techniques. Of particular interest are the shoulder and tail regions of the DAHs.

Figure 9

An integral dose‐area histogram (DAH) of the matchline dose analysis tool (MDAT) used to examine dose differences between the four clinical breast techniques.