In Vivo Dose Measurements For Radiation Therapy Of Breast Cancer Patients

Background and purpose: Breast cancer is one of common malignant tumors of female. Radiation therapy has been the mainstay treatment for breast cancer. the patient's chest contour and normal tissue constraints may require more than one beam configuration .This could cause asymmetric dose distributions in the region of a field junction and chest wall. A split beams technique have been developed to achieve dose uniformity in the area of the field junction. In theory, the split beams should match perfectly. In the clinical situation, however, there is a possibility of introducing dose deviations across the junction. These can originate from factors related to mechanical properties and quality assurance (QA) procedures of the linear accelerator's jaws and to the treatment set-up, e.g. patient movements.This region is at risk for tumour recurrence if it is underdosed or for complications if it is overdosed. Consequently, in vivo dose measurements is necessarily for radiation therapy of breast cancer patients. The purpose of the study was to observe and analysis dose distributions in the region of a field junction in a split beam technique, both phantom and patien for breast cancer. In vivo dose measurements were performed for breast cancer patients to investigate the skin dose of the field junction of supraclavicular-axillary field and tangential field, the actual skin dosage of tangential region for breast cancer patients.Materials and methods: The dose variation at the junction was measured with the metal oxide semiconductors field-effect transistors (MOSFET)dosimetry system produced by Thompson and Nielsen Electronics Ltd.(Ottawa, Canada). First , Phantom measurements were performed to investigate dose distribution in the area of the field junction in a half-field matching method and the influence of factors related to the accelerator. In vivo dose measurements were performed for breast cancer patien to investigate the skin dose of the field junction of supraclavicular-axillary field and tangential field, the skin dose of supraclavicular-axillary region and tangential region in a half-field matching method in 6 MV x-ray beams.Results: Phantom measurements showed dose distributions in the region of a field junction , depending on the matching field direction (X or Y). In vivo measurement of tangential region for patients showed that, the maximum dose deviation between measurement and calculation was -30.39%, the minimum deviation was -18.85%,the average dose deviation was -24.76%. One individual measurement showed significant dose variation in the field junction. The average values of 5 irradiation sessions showed a continuous, almost linear, variation of dose value across the junction.Conclusions: 1.The MOSFET detector , with its advantages of being in small size, ease of use, immediate readout after irradiation , many detectors can be placed together in a relatively small area, can be used for dose measurement of a field junction in irradiation loco-regional treatment for breast cancer.2.The QA procedure to check the dose at the junction line should simulate the clinical situation(the field size, collimator and gantry angles et al) as accurately as possible in the phantom measurement. 3.For individual fractions of a treatment, significant dose deviations may be present at the junction area ,because random errors affect the measurement. These deviations average out over repeated treatments to give relatively smooth dose distributions over the junction area.4.The actual skin dosage was lower than prescribed dose in the patients undergoing tangential chest wall radiotherapy.The actual skin dosage was insufficient for breast cancer patients undergoing tangential chest wall radiotherapy only.