الفهرس 
TitleOnly 14 pages are availabe for public view
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Abstract
Radiotherapy is a common modality for cancer treatment in which a therapeutic dose of ionizing radiation is delivered to a tumor. This radiation disrupts the malignant tissue, with the desired result being cell death or an impairment to cellular division. The dose of radiation is prescribed by an oncologist, and depends on the size, stage and location of the tumor, and the use of any other treatment modalities. Contralateral breast (CB) is the most common secondary malignancy in patients treated for first breast cancer, and it accounts for about 50% of all second primary cancers. Their incidence rates at 5, 10, 15 and 20 years are 3.5%, 7.6%, 11.3% and 15.4% respectively, with the median diagnosis time being 8.2 years. Monitoring the radiation dose delivered to a patient during a radiation therapy session has been accomplished recently by the use of metal oxide semiconductor field effect transistor (MOSFET) detectors. The system may be used to measure doses at specific patient sites such as skin dose. The detectors show good reproducibility and stability for measuring the skin dose during radiation therapy treatment. Radiotherapy for breast carcinoma inevitably results in radiation dose to the CB. Several past studies have quantified this risk. In our study 60% of the patients were below 50 years. Thus, the need for measuring CB dose becomes an important issue. Most of previous studies were phantom based studies but the present study is a clinical study conducted directly on breast cancer patients undergoing radiotherapy. In our study, five ionization chambers of MOSFET were placed, one on nipple and other on either side of nipple at a distance of 3cm along the midline.
The aim of the current study is to evaluate the dose delivered to the contralateral breast during radiotherapy of primary breast cancer.
This study included 60 female patients diagnosed with primary breast cancer. All patients were selected from those admitted to Ayadi Almostakbal Center of Clinical Oncology. Patients were treated by modified radical mastectomy (MRM) or breast conserving surgery (BCS) followed by chemotherapy, radiotherapy and hormone therapy according to their clinical condition.
For every patient, Radiotherapy treatment involves delivering a total dose of 50/42 Gy in 25/16 equal fractions using 6 MV and 15 MV photon beams. For contralateral dose measurement, five detectors were placed on the surface of the skin of contralateral breast, one at the level of nipple and four other detectors were placed 3 cm away from the nipple on four sides of the center dosimeter for each field.
The dose measurement was done using solid state MOSFET detectors because of small size, high sensitive ability to record very small doses and energy independent response. The system is also independent of relative humidity and can be used over a broad temperature range.
On the Plan the Average dose of radiation delivered to CB ranged from 87.26 to 162.6 cGy, with a mean of 107.4 ± 23.11 cGy and the median was 97.45 cGy. The Average dose of radiation delivered to CB as measured by MOSFET ranged from 103.6 to 210.0 cGy; with a mean of 139.7 ± 32.90 cGy and the median was 131.1 cGy.
Summary
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The doses delivered to CB recorded as a Dose Percent on plan ranged from 1.91 to 3.61%, with a mean of 2.44 ± 0.53% and the median was 2.27%. The lower % dose of radiation delivered to CB measured by MOSFET dosimeters ranged from 2.29 to 4.67% with a mean of 3.17 ± 0.74%and median was 2.91%.
A significant difference between plan and MOSFET doses still existed (p<0.001) when presenting date as cGy dose or a percent of total dose, with MOSFET doses being consistently higher than doses from plan. from this study, we were able to assess; for medial, lateral tangential beam and supraclavicular the doses received by five points on the CB, namely; upper, center, lower, near and distant, depicted from plan vs those measured by MOSFET dosimeters. The values of five points were significantly lower in plan than in MOSFET dosimeters measurement when measured in cGy or as % from total dose. This trend was consistent in all 5 locations of measurement, with the highest measures being near the field of treatment and the lower MOSFET measures and plan estimates being the distant. The major contribution to CB exposure, in the current work, was found to be the medial tangential beam which resulted in an exposure that ranged from 54.50cGy (2.51%) in the distant point to 135.2 cGy (6.33%) in the near position. with an average of 92.534 cGy (4.286 %). The dose to CB from lateral tangential beam ranged from 8.58cGy (0.39%) to 33.78cGy (1.52%) with an average of 18.44 cGy (0.828 %) and that from supraclavicular beam ranged from 8.72cGy (0.27%) to 47.74cGy (1.09%) with an average of 30.098 cGy (0.7 %), so the contribution of both lateral and supraclavicular beams is less than half that of the medial tangential beam. The CB dose is composed primarily of scatter dose from conventional breast radiotherapy and is influenced by wedges, blocks, gantry angle, irradiated volume, the use of half beams as well as the orientation of the field edges. In our study, both plan and MOSFET dosimetry dose decreased exponentially with increasing distance from the field edge. The scatter radiation is highest from the medial field. Here the distance from the linac head was relatively less and would give rise to an increase in dose from head scatter and transmission below the jaws. The intact CB may give rise to increased MOSFET measured scatter. The results from the current study were consistent with most of the published work, the intact CB may give rise to increased MOSFET measured scatter. but yet the dose contribution would be mainly from wedge scatter from the medial field as well as some contribution from the lateral fields. from the result of this study, we found that the following factors are likely to increase the dose to the CB: A short perpendicular distance from the contralateral breast surface to the geometric beam edge increases the dose at the surface. A short perpendicular distance can be caused by a shallow medial gantry angle or a large, protruding breast. A combination of both is the least favorable.
Using wedges will generate more scattered radiation than non-wedge. To minimize the dose to the contralateral breast, it is therefore desirable not to use wedges.
Summary
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It is observed that the contribution to contralateral breast dose due to all the fields. to the medial tangential field is almost twice as that due to lateral tangential field. The lateral tangential fields enter laterally and may be contributing to internal scatter, which is very difficult whereas the medial tangential field is close to the contralateral breast and hence the scatter and the collimator contribution is more.