Download Coronary artery dosimetry for adjuvant left

Coronary artery dosimetry for adjuvant left-sided breast
radiotherapy: changes in reported dosimetry when the left
anterior descending artery (LAD) contour is shifted from it`s
true position
Poster No.:
R-0121
Congress:
2014 CSM
Type:
Scientific Exhibit
Authors:
S. Sampaio, P. Graham; KOGARAH/AU
Keywords:
Breast, Cardiac, CT, Radiation therapy / Oncology, Dosimetric
comparison
DOI:
10.1594/ranzcr2014/R-0121
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Page 1 of 13
Aim
Little strong dose-volume dependence data exists for cardiac structures. Undoubtedly
radiation dose to the heart results in an increased risk of late cardiac complications and
4 3 6 5
cardiac mortality
. The LAD (left anterior descending artery) is at particular risk,
given its anterior position on the heart and resultant exposure to high doses of radiation
2
in left-sided tangential radiotherapy . The inferior LAD typically receives higher doses
compared to the superior LAD which is anatomically more posteriorly located.
The LAD itself can be difficult to contour. In particular, the inferior LAD which is most
susceptible to high dose is often poorly defined on planning CT. Feng et al. demonstrated
marked variations in coronary artery contouring between radiation oncologists. Whilst
that study showed improvement in reported dosimetry with better concordance of target
delineation, it did not specifically examine the extent to which a structure such as the LAD
can be 'misplaced ' before one sees a significant difference in reported dose.
The lack of strong dose-volume response data for coronary arteries is an added barrier
to the adoption of routine coronary artery contouring into practice. The Danish Breast
Cancer Cooperative Group recommend that the volume of whole heart receiving >40Gy
be kept below 5% (www.dbcg.dk) but do not specify limitations for the coronary arteries.
Other studies have limited any part of the arch of the LAD to receiving less than 20Gy
and 10% of the whole LAD to receiving less than 20Gy but there is little evidence to justify
1
these limitations .
By simulating the potential shifts in coronary artery contours that can arise through errors
in contouring, we hope to determine to what extent a coronary artery (in this study the
LAD) and its subdivisions can be misplaced (and in which direction- medial, lateral,
superior and inferior) before one sees a significant change in reported dose.
Methods and materials
Case selection
70 patients with left-sided breast or chest wall radiotherapy were treated on the STARS
Trial (STARS, STudy of Anastrozole and Radiotherapy Sequencing, TROG 08.06) at St
George Hospital between September 2009 and March 2012. Of these, 10 patients were
Page 2 of 13
selected: 3 with low LAD mean doses (below 10Gy), 4 with medium LAD mean doses
(10-20Gy) and 3 with high LAD mean doses (>20Gy).
Target Contouring
All patients had pre-existing LAD, superior and inferior LAD contours mandated by the
STARS trial protocol and these volumes underwent regular auditing. The LAD contour
was 3mm in diameter and included the short segment of the left coronary artery before
it bifurcates. The superior LAD incorporated the proximal half of the LAD, and inferior
LAD, the distal half . Because the surface of the heart is curved, expansions of 5, 10
and 15mm were created around the existing 3mm diameter vessel contour (Figure 1.).
Identical vessel contours were then positioned 5, 10 and 15 mm medial and lateral to
the existing LAD contour (Figure 2. and 3.). As CT slices were 2mm thick, superior and
inferior 'shifts' were in 6, 12 and 18mm increments.
Dosimetry
For the purpose of this study all dosimetry was calculated without a boost and with a
prescribed total dose of 50Gy in 25 fractions.
Statistical Methods
Mean, maximum doses and V20 (volume receiving 20Gy) figures have been described.
An absolute difference in recorded mean dose of 10Gy was considered to be clinically
significant. Although there is little data to support its use, a change in whole LAD mean
dose from below to above 20Gy (and vice versa) has been reported. As limiting V20 for
the whole LAD to <10% has been used in previous studies, changes in V20 from below
to above 10% (and vice versa) have also been reported.
Images for this section:
Page 3 of 13
Fig. 1: Expansions around LAD (5,10,15mm)
Page 4 of 13
Fig. 2: Medial Contour Shifts
Page 5 of 13
Fig. 3: Lateral Contour Shifts
Page 6 of 13
Results
For the whole LAD, superior LAD and inferior LAD there was a strong trend towards a
decrease in dose when the structure was shifted medially and an increase in dose with
lateral shifts. Shifting the whole LAD inferiorly usually increased the reported dose as
did a superior shift decrease the dose. Dose to the superior LAD typically increased with
inferior shifts, whilst dose to the inferior LAD decreased. This can be explained by these
structures moving into or out of the high dose part of the radiation fields.
MEAN DOSE
Assuming a difference of 10Gy as clinically significant:
Whole LAD
There were no clinically significant differences seen in mean dose when the whole LAD
was shifted up to 15mm in any direction apart from in the medial direction. For medial
shifts clinically significant differences were seen for 2 and 6 patients for 10mm and 15mm
shifts respectively (insignificant change in dose for shifts up to 5mm). Hence for medial
shifts up to 10mm there was no significant change in dose for 8 out of 10 patients (Table1.)
Superior LAD
There was no clinically significant change in mean dose when the superior LAD was
shifted up to 15mm in any direction.
Inferior LAD
Shifting the inferior LAD in the medial direction resulted in a decrease of mean dose in
all ten patients (mean decrease of 8.3Gy, 15.1Gy and 19.7Gy for 5, 10 and 15mm shifts
respectively) of which 6 out of the 10 patients had a clinically significant decrease in mean
dose with shifts of only 5mm. A lateral shift in the inferior LAD caused an increase in
mean dose in all ten patients. The magnitude of change, however was small and required
a shift of 10mm or more. Hence, apart from in the medial direction, no patients had a
clinically significant change in mean dose when the inferior LAD was shifted only 5mm.
Assuming whole LAD mean dose >20Gy is considered clinically significant:
Page 7 of 13
For the seven patients who had a baseline mean whole LAD dose of <20Gy, three had
an increase in mean dose to >20Gy. This occurred with lateral shifts and inferior shifts
as small as 5mm. For the three patients who had baseline mean doses >20Gy, two had
a decrease to <20Gy with shifts in the medial and superior direction as small as 5mm.
Maximum dose
There was significant variability between patients in changes in maximum dose. As seen
with mean dose, medial shifts resulted in the largest changes for the superior, inferior
and whole LAD. The mean decrease in maximum dose for the whole LAD was 4.2Gy,
12.5Gy and 20Gy for 5mm, 10mm and 15mm shifts respectively, however the range was
large (Table 2.)
V20
Assuming for the whole LAD V20 >10% is considered clinically significant:
For the whole LAD, two patients had a baseline V20 <10%, one of which experienced a
rise of V20 to > 10% when the volume was shifted laterally and superiorly (5mm shift).
For the remaining 8 patients who had baseline V20 >10%, 6 patients had a drop in V20
to <10% when the volume was shifted medially (6 patients-15mm; 3 patients-10mm; 1
patient- 5mm)
Images for this section:
Page 8 of 13
Table 1: MEAN DOSE All movements (10 patients x 3 shifts = 30) Direction of change
(+ and - = increase or decrease in dose)
Table 2: MAXIMUM DOSE All movements (10 patients x 3 shifts = 30) Direction of change
(+ and - = increase or decrease in dose)
Page 9 of 13
Table 3: MEAN DOSE (Absolute difference as percentage)
Page 10 of 13
Conclusion
Definition of cardiac structures can be difficult for a number of reasons. Firstly it relies
on the precision of planning CT scans. Second, there is much interobserver variability
between radiation oncologists in placement of cardiac volumes. Feng et al demonstrated
percent overlap of the LAD contours by radiation oncologists with the 'gold standard'
7
ranged from 0-77% .
Cardiac and respiratory motion can also influence the uncertainty of target position.
A previous study examining displacement of the LAD with respiration found that
the displacement was greatest in the inferior direction and lowest in the left-right
direction (average of 6mm in the inferior direction at shallow breathing states but up to
8
2.8cm between expiration and deep inspiration) . Wang et al demonstrated the mean
displacement of the LAD due to cardiac motion was 2.3mm towards and away from the
posterior edge of the treatment fields. The degree of displacement was however very
variable and ranged from 3 to 10.9mm and 0 to 4.6mm in the left-right direction.
When considering the whole LAD, provided shifts in contouring are within 10mm, there
does not appear according to this study to be a clinically significant change in recorded
mean dose in 8 out 10 patients. This allows for fairly significant errors in contour
placement before one sees a significant change in recorded dose. This also falls within
the range of the LAD displacement due to cardiac motion and at least shallow respiration
reported in previous studies.
However when considering only the inferior LAD, this study demonstrates that whilst
there was much variability between patients, the potential margins for error in contour
positioning are smaller and are significant for shifts of only 5mm.
As expected marked shifts (up to 15mm) of the superior LAD did not result in significant
changes in mean dose. This can be explained by posterior location of the superior LAD
and thus greater distance from the radiation fields.
In this study, patients who had mean whole LAD doses that were 10Gy or above
(corresponding to a MMD of 10 or 15mm) were more likely to experience significant
changes in dose with shifts, suggesting that such patients should be targeted for review
of accurate target delineation.
Changes in maximum dose and V20 were variable between patients and can be
explained by variability of anatomical relationships between the anterior surface of the
Page 11 of 13
heart and the posterior edge of the radiation beams. As a result fairly small shifts can
dramatically change maximum dose and V20 values as the volume is shifted in and out
of the fields and hence changes in these values may be less predictable.
The relevance of establishing the degree to which accurate definition of target volumes
influences reported dose is important for future studies of dose-volume effects for cardiac
structures.
Personal information
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