Butson Martin, Pope Dane, Haque Mamoon, Chen Tom, Song Guangli, Whitaker May
Department of Radiation Oncology, Chris O'Brien Lifehouse, Chris O'Brien Lifehouse Centre, NSW, Australia; Institute of Medical Physics, University of Sydney, Camperdown, NSW, Australia.
Department of Radiation Oncology, Chris O'Brien Lifehouse, Chris O'Brien Lifehouse Centre, NSW, Australia.
J Med Phys. 2016 Apr-Jun;41(2):149-52. doi: 10.4103/0971-6203.181632.
Total body irradiation (TBI) treatments are mainly used in a preparative regimen for hematopoietic stem cell (or bone marrow) transplantation. Our standard clinical regimen is a 12 Gy/6 fraction bi-daily technique using 6MV X-rays at a large extended source to surface distance (SSD). This work investigates and quantifies the dose build-up characteristics and thus the requirements for bolus used for in vivo dosimetry for TBI applications. Percentage dose build-up characteristics of photon beams have been investigated at large extended SSDs using ionization chambers and Gafchromic film. Open field measurements at different field sizes and with differing scatter conditions such as the introduction of standard Perspex scattering plates at different distances to the measurement point were made in an effort to determine the required bolus/build-up material required for accurate determination of applied dose. Percentage surface dose values measured for open fields at 300 cm SSD were found to range from 20% up to 65.5% for fields 5 cm × 5 cm to 40 cm × 40 cm, respectively. With the introduction of 1 cm Perspex scattering plates used in TBI treatments, the surface dose values increased up to 83-90% (93-97% at 1 mm depth), depending on the position of the Perspex scattering plate compared to the measurement point. Our work showed that at least 5 mm water equivalent bolus/scatter material should be placed over the EBT3 film for accurate dose assessment for TBI treatments. Results also show that a small but measurable decrease in measured dose occurred with 5 mm water equivalent thick bolus material of areas '3 cm(2). As such, we recommend that 3 cm × 3 cm × 5 mm bolus build-up is the smallest size that should be placed over EBT3 Gafchromic film when used for accurate in vivo dosimetry for TBI applications.
全身照射（TBI）治疗主要用于造血干细胞（或骨髓）移植的预处理方案。我们的标准临床方案是一种12 Gy分6次、每两天一次的技术，使用6MV X射线，源皮距（SSD）较大。这项工作研究并量化了剂量建成特性，从而确定了TBI应用中体内剂量测定所需的 bolus（组织等效物）要求。使用电离室和Gafchromic薄膜在较大的扩展SSD下研究了光子束的百分比剂量建成特性。在不同野大小和不同散射条件下进行了开放野测量，例如在距测量点不同距离处引入标准有机玻璃散射板，以确定准确测定应用剂量所需的 bolus/建成材料。发现在300 cm SSD下开放野测量的百分比表面剂量值，对于5 cm×5 cm到40 cm×40 cm的野，分别为20%至65.5%。在TBI治疗中引入1 cm有机玻璃散射板后，表面剂量值增加到83 - 90%（在1 mm深度处为93 - 97%），这取决于有机玻璃散射板相对于测量点的位置。我们的工作表明，为了对TBI治疗进行准确的剂量评估，应在EBT3薄膜上放置至少5 mm水等效的bolus/散射材料。结果还表明，对于面积为“3 cm²”的5 mm水等效厚bolus材料，测量剂量出现了小但可测量的下降。因此，我们建议，当用于TBI应用的准确体内剂量测定时，3 cm×3 cm×5 mm的bolus建成材料是应放置在EBT3 Gafchromic薄膜上的最小尺寸。
