DOSIMETRIC STUDY OF FIELD JUNCTION IN ADJACENT BEAMS USING ASYMMETRIC COLLIMATORS AND MULTI LEAF COLLIMATOR (MLC)د

Abstract

Adjacent treatment fields are commonly employed in external beam radiation therapy, such as the mantle and inverted-Y fields for the treatment of Hodgkin's disease. In some cases, the adjacent fields are orthogonal, as in the case of craniospinal fields used in the treatment of medulloblastoma. Another example is the irradiation of head and neck tumors when the lateral neck fields are placed adjacent to the anterior supraclavicular field. In each of these situations, there is a possibility of introducing very large dosage errors across the junction. Consequently, this region is at risk for tumor recurrence if it is underdosed or severe complications if it is overdosed. One of the important factors that affect treatment planning is the Divergent nature of radian. Because of this nature it is so hard to align two radiation beams without overlapping. This overlapping is the reason to appear unfavorable hot and cold spots. Also the shape of the iso-dose curves has a great effect on treatment planning. When two beams adjoined together and the overlap occurred, iso-dose curves overlapped together. Also this will give hot and cold spots. According to the distance between the two adjoined fields the hot spots and cold spots appear at different depths. Suitable field separation has to be measured to obtain the homogenous dose distribution in treatment planning. The aim of the present work is assessment the required gap separations between adjacent fields which make homogenous dose distribution through overlap regions. This aim has been carried out with Eclipse (TPS) to calculate the dose distribution correctly in overlap regions, and make the error in the dose distribution through overlap region and through the treatment plan is about +/- 5.0 % for the most treatment conditions available in linear accelerator. The suitable separation gap between adjacent treatment fields can be determined geometrically or dosimetrically. Dosimetric method basically depends on matching between the isodose curves of adjacent beams. The separation of thefields can be determined by matching isodose curve for each field till dose distribution is uniform at the desired depth and the hot and cold spots are acceptable. The accuracy of this procedure depends on the accuracy of the individual field isodose curves especially in the penumbra region. Geometric method based on matching boundaries and border of the adjacent fields to reduce the effect of beam divergent on dose distribution. The dose distribution laterally across the junction between adjacent fields incident from one side is more or less uniform, depending on the inter-field scatter contribution and the penumbra characteristics of the beam. Hot spots appear underneath the junction and cold spots appear above it. Dose distribution across the junction will be more uniform when the two fields parallel opposing to each other but there is still cold spots around the junction. Two Geometric solutions have been obtained to get uniform dose distribution. First solution is to align the divergent edges of the beam, hot and cold spots generated by the penumbra of each beam will 'cancel out' the other. second solution is half beam blocking. The half beam block solution is similar to the method of aligning of divergent beams, but is easier to set up (less movements the couch/gantry) and means that the beam is not oblique on the skin. In both geometric and dosimetric method, Different films were irradiated gradually with radiation energy from 20 to 100 motor units (20 to 100 cGray). These films received radiation dose similar to the dose received by patient in real treatment. Unirradiated film has been used as standard reference film. Processing the film and draw and drawing iso-dose curve through which we can determine the absorbed dose for each irradiated film. Also optical density value can be determined. The measured data obtained from films could be compared with calculated data obtained from TPS. Dose profile for one radiation photon beam had been studied. Also Dose profiles for two radiation photon beams with different separation gaps (1mm to 5mm) were studied with a 20x20 cm² opened field size. Separation gap value greatly affects the homogeneity of dose distribution through overlapped region. This effect no longer exists when gap separation becomes larger than 0.5 cm. The different separation gaps applied between adjacent treatment fields show obvious variations in dose distribution and dose homogeneity through overlap regions. Hot and cold spots locations change by changing the magnitude of separation gap between treatment fields. The greater the separation between adjacent fields, the deeper the appearance of hot spots underneath the junction. A 3-mm gap is approximately very well gap between photon fields at 1.0, 3.0 and 5.0 cm depths, respectively, for divergent photon beams. A 4-mm abut and gap resulted in an unacceptable dose inhomogeneity in the junction. The magnitudes of either the hot-and cold-spots were proved to be clinically acceptable for the 0.2 cm width, which in turn, would be a suitable selection for abutment region width. This is because it possesses a shape that appears to be “transition” between the smaller and larger abutment region widths. Dose profile for different treatment techniques such as SSD techniques and 3D techniques were studied. The study reveals that 3D asymmetric technique could result in good dose homogeneity in match region than other techniques. But it still has the disadvantage of the setup difficult which leads to affect the tumor control. The assessment of separation gap between treatment adjacent fields is an important treatment parameter. The overlapping between adjacent fields can cause a disturbance in dose distribution which leads to reduce the tumor control. If the effect of separation gap has been neglected, serious hazards will occur during treatment. All treatment parameters included separation gap should be combined and synchronized together to obtain the optimum result for cancer treatment. Furthermore, good results depend mainly on the good cooperation between treatment team.