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    Radiosurgical Irradiation of the Tumor Bed for Early Stage Breast Cancers

    Scientific Abstract:
    Radiosurgical Irradiation of the Tumor Bed for Early Stage Breast Cancers BACKGROUND: Local radiation of the breast tumor bed for breast conservation therapy (BCT) lowers the risk of ipsilateral breast tumor recurrence (IBTR) for early stage invasive breast cancers and Ductal Carcinoma in Situ (DCIS). The existing technique for irradiating the breast tumor bed primarily uses enface electrons. The concern for the treatment has been normal tissue toxicities such as telangiectasia and breast tissue fibrosis. This has been the major factor affecting the quality of the life for patients receiving BCT. The fear of toxicity and the prolonged treatment time prevent many women - particularly working women and the elderly, from receiving BCT. In this study, we propose to develop a new radiation therapy technique using magnetically collimated electrons to spare normal tissue when treating the breast tumor bed. The magnetically collimated electrons are produced by steering the electrons from a linear accelerator through an in-air axial magnetic collimator. Our preliminary studies have found that these electrons lower the surface dose by as much as 40% compared with the conventional electrons. This effect motivates us to develop a radiosurgical approach by allowing multiple fixed or arc beams to irradiate the breast tumor bed in a cross-firing manner. This approach is analogous to intracranial radiosurgery using photon beams. Since the range of the electron beam is limited, it can be placed directly above the full breast without irradiating the underlying lung and the heart. We expect that a highly conformal radiation dose can be delivered to the breast tumor bed with this approach. OBJECTIVE/HYPOTHESIS: Our objective is to develop technical tools and to demonstrate clinical applicability of the radiosurgical treatment of the breast tumor bed using magnetically collimated electron beams. We hypothesize that the dose to the skin and the normal breast tissue surrounding the tumor bed can be lowered by at least 20% as compared with the existing treatment technique. SPECIFIC AIMS: Our primary aim is to develop the treatment planning and the delivery methods for radiosurgical irradiation of the breast tumor bed using magnetically collimated electron beams. Our secondary aim is to implement these methods using phantoms that simulate actual patient shapes. In particular, we will compare the dose distributions of the radiosurgical delivery with the enface electron treatment of the breast tumor bed. STUDY DESIGN: We will construct three electron magnetic collimators of different sizes. We will measure the beam characteristics as defined by these magnetic collimators. We will develop a three-dimensional dose model to compute dose distributions for radiosurgical delivery of these beams. We will develop an inverse planning approach to optimize the dose distributions of these treatments. We will test and implement the optimized treatment plans on simulated target geometries. We will measure and compare the dose distributions for the target and for the adjacent normal breast tissue between the radiosurgical delivery and the enface electron delivery. POTENTIAL OUTCOMES AND BENEFITS OF THE RESEARCH: We will create a new radiation therapy technique for treating the breast tumor bed. The technique is non-invasive and convenient to implement with existing linear accelerators. It will spare normal breast tissue and therefore lower treatment-induced toxicities. This potentially leads to new dose fractionation schemes that reduce local recurrence and improve the quality of life for breast cancer patients receiving BCT.

    Lay Abstract:
    Radiosurgical Irradiation of the Tumor Bed for Early Stage Breast Cancers Radiation therapy of breast cancer allows many women to conserve their breasts and live longer after treatment. One objective of radiation therapy is to reduce local recurrence by irradiating the breast tumor bed. The most common technique for irradiating the breast tumor bed is using a single-field electron beam. This technique often poses a substantial risk of normal tissue complications such as skin reactions and tissue fibrosis. The fear of normal tissue toxicities and the lengthy treatment course often discourage many women from receiving breast conservation therapy (BCT). The risk of normal tissue complications also limits the amount of radiation dose that can be delivered to the tumor bed. This increases the risk of local recurrence that often deals a significant psychological blow to the patients who underwent a lengthy treatment. The goal of our study is to develop a new treatment technique using special electron beams that are collimated through an axial magnetic field. Our preliminary studies have found that these specially collimated electrons improved the normal tissue sparing by as much as 40%. The application of magnetic collimation also reduces the scattering of the electrons. These characteristics allow the magnetically collimated electron beams to target the breast tumor more precisely thus preventing unwanted radiation outside of the treatment area. As a result, it is possible to irradiate the breast tumor bed from multiple directions using a “cross-firing” approach that is analogous to radiosurgery of brain tumors. Because electron beams have a finite range of travel, they can be placed directly above the full breast to irradiate the tumor bed without affecting the underlying heart or lung. We expect that a significant improvement in normal tissue sparing can be achieved with this approach. We hypothesize that the unwanted radiation to the normal breast tissue could be reduced by at least 20% as compared with the existing treatment. We propose to pursue two specific aims in this study. Our primary aim is to implement magnetically collimated electrons for standard medical linear accelerators. We will construct three magnetic collimators and commission them for the treatment delivery. We will develop a computer model that accurately predicts the radiation dose being delivered. We will develop a patient-specific treatment planning method that allow us to best select beam orientations, collimator sizes, and beam-on time, etc. Our second aim is to perform dose calculations and measurements on a phantom that simulates actual patient shapes. We will compare the dose distributions from the radiosurgical delivery technique with those from the conventional delivery. In particular, we will compare the radiation dose to the skin, normal breast tissue, the heart, and the lung. If completed, we will create a new radiation therapy technique for treating the breast tumor bed. The technique is non-invasive and convenient to implement on the existing medical linear accelerators. We expect to significantly improve the normal breast tissue sparing with the technique. This potentially leads to a more effective treatment course with better outcome and shortened treatment time that improves the quality of life for breast cancer patients. Our long-term goal is to demonstrate these benefits in well-controlled clinical settings.