Lonski P, Taylor M L, Franich R D, Kron T
Physical Sciences Department, Peter MacCallum Cancer Centre, St Andrews Place, East Melbourne, VIC, 3002, Australia,
Australas Phys Eng Sci Med. 2014 Mar;37(1):15-23. doi: 10.1007/s13246-013-0235-8. Epub 2013 Dec 8.
Leakage radiation from linear accelerators can make a significant contribution to healthy tissue dose in patients undergoing radiotherapy. In this work thermoluminescent dosimeters (LiF:Mg,Cu,P TLD chips) were used in a focused lead cone loaded with TLD chips for the purpose of evaluating leakage dose at the patient plane. By placing the TLDs at one end of a stereotactic cone, a focused measurement device is created; this was tested both in and out of the primary beam of a Varian 21-iX linac using 6 MV photons. Acrylic build up material of 1.2 cm thickness was used inside the cone and measurements made with either one or three TLD chips at a given distance from the target. Comparing the readings of three dosimeters in one plane inside the cone offered information regarding the orientation of the cone relative to a radiation source. Measurements in the patient plane with the linac gantry at various angles demonstrated that leakage dose was approximately 0.01% of the primary beam out of field when the cone was pointed directly towards the target and 0.0025% elsewhere (due to scatter within the gantry). No specific 'hot spots' (e.g., insufficient shielding or gaps at abutments) were observed. Focused cone measurements facilitate leakage dose measurements from the linac head directly at the patient plane and allow one to infer the fraction of leakage due to 'direct' photons (along the ray-path from the bremsstrahlung target) and that due to scattered photons.
直线加速器的泄漏辐射会对接受放射治疗的患者的健康组织剂量产生重大影响。在这项工作中，热释光剂量计（LiF:Mg,Cu,P TLD芯片）被用于一个装有TLD芯片的聚焦铅锥中，目的是评估患者平面处的泄漏剂量。通过将TLD放置在立体定向锥的一端，创建了一个聚焦测量装置；使用6 MV光子在Varian 21-iX直线加速器的主射束内外对其进行了测试。在锥体内使用了厚度为1.2 cm的丙烯酸等效建成材料，并在距靶标给定距离处使用一个或三个TLD芯片进行测量。比较锥体内一个平面上三个剂量计的读数可提供有关锥体相对于辐射源方向的信息。在直线加速器机架处于不同角度时在患者平面进行的测量表明，当锥体直接指向靶标时，泄漏剂量约为射野之外主射束剂量的0.01%，在其他位置为0.0025%（由于机架内的散射）。未观察到特定的“热点”（例如，屏蔽不足或对接处有间隙）。聚焦锥测量有助于直接在患者平面测量直线加速器机头的泄漏剂量，并能推断出由于“直接”光子（沿韧致辐射靶的射线路径）和散射光子导致的泄漏比例。
