Calculating Contra-Lateral Breast’s Absorption Dose in Different Radiotherapy Methods of Large Breasts Cancer

Introduction: Breast cancer is one of the most common known cancers in women all around the world. It is the most common cause of death in women after lung cancer. Radiotherapy is a method for therapy of cancer in which certain dose of high energy radiations is used to destruct cancer cells. Radiotherapy is designed so that radiation of eartain does to the target area in body is allowed. In this therapy more radiation dose should be used in tumor and the least dose is used in normal tissues. In this design, Contra-lateral breast’s absorption dose in different radiotherapy methods in large breasts is calculated. In large breasts, normal tissues and more sensitive organs exist in the field due to its anatomical structure. Materials and Methods: Measurement is performed by 3D Aculips therapy design system and EBT3 Gap chromic film and TLD. This plan was an experimental study. 60 time radiations were used by X-Varian linear medical accelerator device with 6MV energy that 30 times were performed by Gap chromic film and 30 times were performed by TLD. All radiations were performed on the phantom due to protective problems. In this work, CT-simulator phantom was used and then, it was transferred into therapy design computer. TLD and Gap chromic film were installed on Contra-lateral breast, and in all radiation steps, 100 manitor unit (Mu) were used. Results: After calibrating TLD, first, absorbtion does in breast CT scan is calculated by TLD. This dose is 1.6 cGy. We are inevitable to remove this dose. In radiotherapy planing, three methods are studied. In the first method, common two tangents method, results indicate that a part of the contra-lateral breast tissue and internal memolary lymph nodes are in the radiation field, therefore a part of the breast is placed because the breasts are large. In this method, dose maximum is 84.1 cGy. In the second method two photon-radiation field are placed in the medial area and a part of breast is removed and dose used for the breast is 15cGy. In the method of combination of photon and electron, a part of the main breast is removed from the radiation field. The removed area is in internal area, which is covered by electron tangent with internal field. In this method, the maximum dose reachs to 6 cGy. After calculating by radiotherapy planing, we perform exprimental calculation by TLD in certain points. In usual therapy method, a part of the contra-lateral breast is in radiation field, and it receives a full dose. In edge of field-therapy, absorpthion dose is 83.2% of perspective dose. In this method, tivo beside photon fields reach to 14.9% of perspective dose; and two electron and photon fields reach to 6% in another method for calculating absorpthion dose, after calibration of Gap chromic film of absorption dose, three used methods are calculated, and by putting film on contra-lateral breast, dose phantom is evaluated. From performed measurement, dose level in usual method, two photon fields were 84.1% of perspective dose. In method of two photon fields together percentage of the perspective dose was 15.1%, and for photon-electron fields this value was 6.4%. Conclusion: With respect to this design, it can be concluded that if two beside photon- electron fields method is used, dose entered into the contra-lateral breast will be significantly reduced. However, in this design, dose for other sensitive organs must be considered