Zabatis Ch, Koligliatis Th, Xenofos St, Pistevou K, Psarakos K, Haritanti A, Beroukas K
1st Department of Radiation Oncology, St. Savvas Anticancer Hospital, Athens, Greece.
J BUON. 2008 Apr-Jun;13(2):253-62.
To describe and evaluate a method that uses a 3-dimensional (3D) treatment planning system (TPS) to determine the relative dose to the lung, and to study the beam filtration required for lung sparing in translation total body irradiation (TBI). Special dosimetric problems related to moving couch were also considered.
The irradiation technique employed in our hospital is that of patient translation. The patient is positioned on a moving couch passing under a stationary Co-60 beam so that his/her entire body is irradiated. Measurements of basic data at source-skin distance (SSD)=150 cm were used to implement the Co-60 TBI unit to TPS (THERAPLAN plus), which was then used in dose computations. Two stationary, opposed anterior-posterior (40 x 40 cm) fields were employed to irradiate the Alderson phantom. The midline dose to either lung was computed and correction factors (CFs) were obtained that depend on the anatomy and densities of the tissues involved. These factors give the midline lung dose increase relative to the midline dose at the level of the mediastinum. Once the required lung dose was decided, the computed CF was used to estimate the filtration required from the measured broad beam attenuation data. The shielded lung dose distribution could be obtained from the TPS using a transmission corresponding to narrow beam geometry. To verify the TPS computations, measurements using a dosimeter and a diode system were carried out, employing solid water phantoms and the Alderson phantom.
For the TPS employed, the computed midline CFs were lower than those measured in simple geometry phantoms for lung densities of 0.2-0.35 g/cm(3), by no more than 2%. For the Alderson phantom studied (lung density of 0.32 g/cm(3)), the computed CF was 1.11, which was 2% higher than the measured value.
The advantages of a 3D TPS (dose distribution inside the lung, lung dose volume histograms [DVH], accurate attenuator shape from patient's anatomy etc.) allowed to study the lung dose in the Alderson phantom and to estimate the beam filtration required for lung sparing in TBI. The accuracy in lung dose computations, excluding the soft-tissue/lung interface was < or = 5%, which is within the clinical dose requirements. This procedure has been applied to a number of patients prior to their irradiation. Computations and in vivo measurements were in good agreement.
描述和评估一种使用三维（3D）治疗计划系统（TPS）来确定肺部相对剂量的方法，并研究在平移全身照射（TBI）中实现肺部保护所需的线束过滤。还考虑了与移动治疗床相关的特殊剂量学问题。
我院采用的照射技术是患者平移技术。患者置于在固定的钴-60线束下移动的治疗床上，以便其全身受到照射。在源皮距（SSD）=150 cm处测量的基础数据用于将钴-60 TBI单元与TPS（THERAPLAN plus）进行匹配，然后用于剂量计算。采用两个固定的、相对的前后野（40×40 cm）照射Alderson体模。计算任一肺的中线剂量，并获得取决于所涉及组织的解剖结构和密度的校正因子（CF）。这些因子给出相对于纵隔水平中线剂量的中线肺剂量增加量。一旦确定了所需的肺剂量，就使用计算出的CF根据测量的宽束衰减数据估算所需的过滤。使用对应于窄束几何形状的透射率，可从TPS获得屏蔽后的肺剂量分布。为验证TPS计算结果，使用剂量仪和二极管系统，采用固体水模体和Alderson体模进行测量。
对于所使用的TPS，对于肺密度为0.2 - 0.35 g/cm³ 的情况，计算出的中线CF低于在简单几何形状体模中测量的值，相差不超过2%。对于所研究的Alderson体模（肺密度为0.32 g/cm³），计算出的CF为1.11，比测量值高2%。
3D TPS的优势（肺内剂量分布、肺剂量体积直方图[DVH]、根据患者解剖结构精确的衰减器形状等）使得能够研究Alderson体模中的肺剂量，并估算TBI中实现肺部保护所需的线束过滤。排除软组织/肺界面后，肺剂量计算的准确性≤5%，这在临床剂量要求范围内。该程序已应用于一些患者照射前的情况。计算结果与体内测量结果吻合良好。
