Beam angle manipulation to reduce cardiac dose during breast radiotherapy

Abstract

Objective: Standard tangential radiotherapy techniques after breast conservative surgery (BCS) often results in the irradiation of the tip of the left ventricle and the left anterior descending coronary artery (LAD), potentially increasing cardiovascular morbidity. The importance of minimising radiation dose to these structures has attracted increased interest in recent years. We tested a hypothesis that in some cases, by manipulating beam angles and accepting lower-than-prescribed doses of radiation in small parts of the breast distant from the surgical excision site, significant cardiac sparing can be achieved compared with more standard plans.

Methods: A sample of 12 consecutive patients undergoing radiotherapy after left-sided BCS was studied. All patients were planned with a 6 MV tangential beam, beam angles were manipulated carefully and if necessary lower doses were given to small parts of the breast distant from the surgical excision site to minimise cardiac irradiation ("institutional" plan). Separate "hypothetical standard" plans were generated for seven patients using set field margins that met published guidelines.

Results: In seven patients, the institutional plans resulted in lower doses to the LAD and myocardium than the hypothetical standard plans. In the other five patients, LAD and myocardial doses were deemed minimal using the hypothetical standard plan, which in these patients corresponded to the institutional plan (the patients were actually treated using the institutional plans).

Conclusion: Much attention has been devoted to ways of minimising cardiac radiation dose. This small sample demonstrates that careful manipulation of beam angles can often be a simple, but effective technique to achieve this.

Figure 1

The hypothetical standard plan: beam set-up with planned target volume (PTV) (red line), myocardium (yellow line) and left anterior descending artery plus 1 cm margin (blue circle indicated with an arrow) marked. This set-up accords with published guidelines to fully encompass the PTV.

Figure 2

The institutional plan: beam set-up with planned target volume (PTV) (red line), myocardium (yellow line) and left anterior descending artery plus 1 cm margin (blue circle indicated with an arrow) marked. This set-up minimises cardiac volume irradiated by accepting poor coverage of PTV at sites distant from the tumour bed (the tumour was in upper breast so no seroma is seen on this CT section). Geometrically, the main difference between this and the hypothetical standard plan is the more horizontal angulation of these beams, with a more anterior lateral entrance point.

Figure 3

Comparative dose–volume histograms (DVHs) (in dose percentages) for planned target volume (PTV) using the hypothetical standard (upper, yellow line) and institutional (lower, green line) set-ups. By conventional criteria, the distribution in the latter is poor, but the low-dose areas are well away from the tumour bed, so not clinically important.

Figure 4

Comparative dose–volume histograms (in dose percentages) for myocardium using the hypothetical standard (upper, yellow line) and institutional (lower, green line) set-ups. With the former, small areas receive over 75% of prescribed dose whereas the latter receives no more than 25%.

Figure 5

Comparative dose–volume histograms (in dose percentages) for left anterior descending coronary artery (LAD) using the hypothetical standard (upper, yellow line) and the institutional (lower, green line) set-ups. Note that an arbitrary 1 cm margin was drawn around the LAD, so precise figures should be regarded with caution, but the relationship between the two lines would be similar regardless of the size of the margin used.