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Characterization of Breast Motion and its Dosimetric Consequences
Lumpectomy followed by six weeks of daily radiation encompassing the entire breast tissue is an established method of treating early stage breast cancer. To reduce the treatment volume, partial breast irradiation (PBI) is becoming more common. One challenge for external beam PBI is potential shift of the target volume during treatment. Another problem is accurate definition of the clinical target volume (CTV) and planning target volume (PTV). The CTV is currently defined by uniformly expanding the excision cavity volume by 10-15 mm and PTV is assigned by an additional extension of at least 10 mm around the CTV to compensate for the variability of treatment setup and breathing motion. Since PBI is applied to small tumors any excessive expansion can lead to a significant increase in treatment volume. Increasing the volume of high dose has been correlated with higher late severe toxicities (fat necrosis). If we were able to reduce a PTV margin alone, we could reduce the treatment volume 2-4 times, depending on the tumor size.
Our main aims are to use advanced imaging technology for:
1) Quantification of breast motion in supine and prone position
2) Improved target localization
3) Dosimetric evaluation of the reduced margins for various treatment modalities
This research is motivated by our preliminary data, which indicates that the breathing motion in the supine and prone position direction rarely exceeds 3 mm and 1 mm, respectively.
We will quantify breast motion with dynamic MRI imaging (sub-second temporal and sub-millimeter spatial resolution). Patients with different breast size, age, menopausal status and breathing types will be considered. Intra-breathing variation, free-breathing periodicity and breath-holding will be evaluated. Correlation with representative chest-wall markers will be determined. Dynamic MRI and CT imaging will be compared. Target (lumpectomy cavity) and the change of its size and shape will be evaluated with additional scanning during the treatment. Dosimetric evaluation will be performed by comparing standard PTV expansions and the specific PTV from our study. Different treatment modalities (brachytherapy and external beam radiotherapy) and both treatment orientations (supine and prone) will be investigated.
We expect that our work will provide guidelines for more accurate non-invasive external beam PBI thus allowing more patients to be treated with breast conservation therapy.
Breast tumor excision followed by the radiation therapy is an established method of treating early stage breast cancer. Conventionally the whole breast is irradiated, even though treating only a part of the breast is likely adequate. Irradiating less breast causes less damage to the healthy breast tissue, thus reducing the risk of side effects (i.e. fat necrosis). In partial breast irradiation the treatment volume is currently expended for approximately 10-15 mm to account for possible microscopic (undetected) disease, and additional 10 mm to account to breast position during treatment (mainly due to breathing). Since typical tumors are small, such margins enormously increase the treatment volume. At the moment, we are unable to detect the microscopic disease. On the other hand, if we were able to characterize the actual breast motion and set-up variations, we could reduce the treatment volume 2-4 times, depending on the tumor size. We have recently developed a methodology based on dynamic magnetic resonance imaging (MRI) to accurately address breast motion. Our preliminary data indicates that breathing motion rarely exceeds 3 mm, when patients lay on the back and is almost negligible when the patients lay on the stomach. This indicates that current margins are considerably overestimated resulting in excessive treatment volumes.
Our main aims are to use advanced imaging technology to (1) better quantify breast motion (2) develop methods for improved localization of the treatment volume and (3) evaluate received dose for different treatment techniques. In order to achieve these goals we will perform dynamic MRI and CT imaging and consider effects of breast size, age, menopausal status and breathing type on the breast motion. We will investigate change in the breast structure during the course of treatment and evaluate dosimetric benefits if the treatment volume is reduced according to our findings.
We anticipate that our work will provide guidelines for more accurate partial breast irradiation thus allowing more patients to be treated with breast conservation therapy.