Evaluation of Optimal Technique for Left Breast Irradiation

Numerous studies have indicated that radiation therapy reduces the risk of the local recurrence of breast cancer in several cases and that it has increased the overall survival rate. Although radiation therapy is beneficial for the treatment of breast cancer, it is known to increase the risk of both radiation toxicity and secondary breast cancer. In left-sided breast cancer, radiation therapy treatment often leads to the heart and its components—such as the left ventricle and left anterior descending artery—being exposed to high doses of radiation because of the proximity of the heart to the left breast, resulting in cardiac complications several years after the treatment. Further, it is important to deliver low doses to the left lung to reduce the risk of pneumatic and lung fibrosis, particularly for patients with long survival rates. Modern 3D techniques can deliver a reduced dose to the cardiac components and lungs. However, the risk of radiation to cardiac components remains unclear, because complications are directly related to radiation dose. Treatment techniques play an important role in sparing organs at risk (OAR) without compromising the target. Specific techniques for left-sided breast cancer treatment result in higher cardiac and pulmonary toxicity, which has been shown to be related to increased risk of heart and lung diseases. In the first two studies in this dissertation, the dose-volume metrics of the OAR were calculated for different techniques for treating patients with left-sided breast cancer. In the first study, the supine free-breathing (SFB), deep inspiration breath-hold in supine (SDIBH), and prone free-breathing (PFB) techniques were evaluated to reduce the cardiac and left lung doses. Most left-sided breast cancer patients undergoing radiation treatment are treated using the SFB technique. The deep inspiration breath hold (DIBH) technique has been proven to reduce the cardiopulmonary doses for breast radiation therapy. In DIBH, a patient takes a deep breath and holds the breath during irradiation. The prone position is another technique used to reduce doses to OAR. Our first study is the only one that compares the dose-volume metrics of OAR for the same patient scanned in three different positions with respect to breast size. This study demonstrates a novel, yet simple and cost-effective, technique to implement the DIBH technique by utilizing lasers and high definition cameras. This method can be used in clinics without the need to purchase expensive breath-hold equipment to implement the DIBH technique clinically. In our second study, we included the prone deep inspiration breath-hold (PDIBH) method in addition to SFB, SDIBH, and PFB techniques to evaluate the OAR. In this study, the normal tissue complication probability (NTCP) is calculated to determine the probability of damage induced on normal tissues for given radiation doses to OAR. This study is the first to perform a biological evaluation based on radiobiological models for each OAR with specific endpoints in left-sided breast cancer treatment. The NTCP values for each OAR are compared and evaluated in addition to dose-volume histogram-based evaluations for four different techniques. In the third study, the surface dose of the prone and supine treatments was evaluated. Skin dose can be an important factor regarding the outcome and cosmesis for patients. Further, a superficial dose has a large variance that depends on the incident angle relative to the surface. Understanding surface dose dosimetry in the tangential or oblique beam is important to evaluate the skin dose, because a higher dose leads to toxicity and a lower dose can lead to recurrences. This study also evaluates superficial doses in the prone and supine positions with respect to two different grid sizes. This dissertation establishes a basis for a comprehensive evaluation to help clinicians decide on the best possible treatment techniques for left-sided breast cancer patients. Patients with healthy lungs can be recommended the DIBH technique for a reduced dose to cardiac components, whereas patients with compromised lung function can be recommended the prone technique to spare the OAR. The clinician must be careful of lower skin dose when treating patients using the prone technique, particularly for tumor bed close to the skin surface