Cardiac avoidance in breast radiotherapy: a comparison of simple shielding techniques with intensity-modulated radiotherapy
Introduction
Radiotherapy (RT) to the breast is an integral part of the management of early breast cancer. In combination with wide local excision of the primary tumour, local control rates can be achieved that are equivalent to mastectomy [6]. There is also mounting evidence that this local effect translates into an overall survival advantage for some groups of women [7]. At the same time it is clear that certain women are put at higher risk of fatal cardiac events as a result of undergoing breast RT [4]. The latest Early Breast Cancer Trialists’ Collaborative Group overview highlighted this problem. They showed that whilst an absolute benefit of 0.8–4.1% might be achieved in terms of reduced risk of breast cancer-related deaths (depending on age and major prognostic factors) there was, for certain groups of patients, an increased risk of cardiac mortality of 0.8–5.6% [7]. From their data on proportional mortality changes it appears that patients with the lowest risk of dying from breast cancer (because of good prognosis tumours) are at the highest risk of cardiac mortality due to their longer relapse-free survival. In the United Kingdom, 20% of screen-detected cancers are now pre-invasive, intraductal carcinomas [24]. Many of these patients are recommended to receive breast RT for its beneficial effect on local control but cannot expect a significant survival advantage. They are exposed to similar doses and techniques of breast RT as patients with invasive cancer and are therefore potentially at risk of having significantly reduced overall long-term survivals when treated with breast RT. It is clearly essential to identify those patients at high risk of cardiac toxicity and develop techniques that allow them to maintain improved local disease control with RT without the possibility of serious side effects.
Retrospective cohort studies show that the patients who are most at risk of cardiac mortality are those requiring left breast RT [26], [27]. A detailed examination of the dose to the individual coronary arteries has shown that whilst the right coronary and circumflex arteries received a far lower dose with megavoltage compared to orthovoltage RT, the left coronary artery still receives the full prescribed dose [10]. This is because the left coronary artery lies on the anterior wall of the heart, which is within the high-dose volume for a left-sided treatment. Gagliardi et al. investigated the relationship between risk of cardiac mortality and the volume of heart irradiated [11], [12]. With a predictive model based on retrospective analysis of cardiac mortality data for patients in RT vs. no RT studies they found that individual patients might have an absolute risk of cardiac death as high as 9% depending on the volume of heart treated. Hurkmans et al. applied the same model to tangential irradiation and found that increasing maximum heart depth predicted for increased risk of excess cardiac mortality [20].
The aim of modern breast conservation therapy should be to optimize the therapeutic ratio, preserving high tumour control rates with minimal late cardiac toxicity. In the absence of a large prospective clinical series with sufficiently long follow-up, it is unclear what is an acceptable level of cardiac irradiation. It is therefore our current clinical policy to attempt to reduce any high-dose cardiac irradiation, whilst maintaining target coverage. We have investigated how this might be achieved using computed tomography (CT)-based three-dimensional treatment planning, comparing standard tangential techniques, with or without conformal lead shielding, with more complex beam arrangements incorporating intensity-modulated radiotherapy (IMRT).
Section snippets
Patients and methods
A planning study was carried out on a group of patients with early breast cancer, managed by wide local excision prior to radiotherapy. These patients had been CT scanned as part of a previous study investigating inhomogeneity in the breast [23]. Difficulties in CT-scanning patients in the routine treatment position, along with the use of a CT simulator, are well documented [35]. For the current study, CT scans were acquired with the patients in a supine position with arms fully abducted to
Results
Table 1 shows various parameters relating to PTV coverage with the different treatment plans. PTV coverage by the 95% isodose was comparable between standard tangents and the four-field IMRT technique for all patients. The partial shielding and two-field IMRT techniques showed slightly worse coverage by the 95% isodose. Both the six-field IMRT technique and complete cardiac shielding significantly reduced the coverage by the 95% isodose, with the latter also giving an average volume of 1.8%
Discussion
High-dose cardiac irradiation is a concern when using whole-breast radiotherapy to treat patients with early breast cancer. The aim of this study was in line with our current clinical policy of trying to reduce any high-dose cardiac irradiation without clinically significant compromise of PTV coverage. We therefore included all available patients in whom the heart was seen to be within the posterior border of the tangential beams, giving a range of MHD from 1.0 to 2.6 cm. The techniques
Conclusions
We have presented a number of alternative methods by which to achieve cardiac avoidance during breast RT without compromising coverage of the PTV. This will improve the therapeutic ratio with regard to breast RT and cardiac irradiation for patients in whom a significant amount of heart is included within standard tangential fields. The introduction of blocks to partially shield the heart, without shielding the PTV, resulted in a significant reduction in heart dose, as did the use of a
Acknowledgements
The authors would like to thank Ms E. Donovan and Dr P. Evans for their helpful comments on the manuscript. This work was supported in part by MDS Nordion Therapy Systems.
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