RESUMO
The aim of this work is to study the effect of the number of computed tomography [CT] slices or the inter-slice distance on the ability of treatment planning software [TPS] to reconstructs and determines three-dimensional [3D] accurate volumes from these slices. Also to study the effect of the determined volume on the prediction of the dose volume histogram [DVH] by the dose delivered to the region of interest [ROI]. This study was performed on the head phantom manufactured from wax contains on some materials have predetermined volumes. Also this study applied on twenty patients with breast cancer in the Clinical Oncology Hospital Menofiya University in the period from 15/2/2004 to 18/9/2004. These patients underwent of study the change in the irradiated volume of the lung with changing the number of CT-cuts. Patients were divided to: 10 patients with right breast cancer and 10 with left beast cancer. The phantom underwent a planning CT scan several times with slice thickness 2-mm and the inter-slice distances [ISDs] 0, 1, 2, 3, and 4 mm. All patients underwent a planning CT scan with slice thickness of 1-cm. After that patients were contoured and planned on the 3D advanced radiation therapy planning system [ARTS 3D]. Patients were planned using two tangential fields technique by 6 MV x-ray. Each patient was planned several times, where in each time; the desired number of CT-cuts selected and remains slices leave. With each selected number of CT-slices, the dose recalculated for the same treatment technique. The irradiated lung volume [ILV] and the delivered dose were determined directly using dose-volume histogram [DVH] of the lung for each plan of the patient. From studying the phantom, as the number of CT-slices increased [or the ISD decreased], the determined volume changes. For the regular shaping materials, egg and tennis table ball, as the number of CT-slices increased, the determined volume close to the actual volume. For the irregular shapes, the determined volumes are dependent on the number of CT slices. The IL V changes with changing the number of CT-cuts. The reason is that as the CT-slices increased the surface of the reconstructed volume is smoother. From the definition of DVH, which is a curve that relates the percentage of the area or volume that receives a certain dose of radiation, the received dose depends on the ROI volume The curve that relates the percentage of the lung volume received a certain dose changes with changing the number of CT-slice. When comparing the ILV from the tangential fields with c4fferent numbers of CT slices, the actual dose delivery is achieved from the full CT scan. The irradiated lung volume in the tangential fields of breast cancer is a function of the CT slices numbers or the ISDs. The dose delivered is a function of ILV