Wen Ning, Guan Huaiqun, Hammoud Rabih, Pradhan Deepak, Nurushev T, Li Shidong, Movsas Benjamin
Henry Ford Health System, Detroit, MI, USA.
Phys Med Biol. 2007 Apr 21;52(8):2267-76. doi: 10.1088/0031-9155/52/8/015. Epub 2007 Apr 2.
With the increased use of cone beam CT (CBCT) for daily patient setup, the accumulated dose from CBCT may be significantly higher than that from simulation CT or portal imaging. The objective of this work is to measure the dose from daily pelvic scans with fixed technical settings and collimations. CBCT scans were acquired in half-fan mode using a half bowtie and x-rays were delivered in pulsed-fluoro mode. The skin doses for seven prostate patients were measured on an IRB-approved protocol. TLD capsules were placed on the patient's skin at the central axis of three beams: AP, left lateral (Lt Lat) and right lateral (Rt Lat). To avoid the ring artefacts centred in the prostate, the treatment couch was dropped 3 cm from the patient's tattoo (central axis). The measured AP skin doses ranged 3-6 cGy for 20-33 cm separation. The larger the patient size the less the AP skin dose. Lateral doses did not change much with patient size. The Lt Lat dose was approximately 4.0 cGy, which was approximately 40% higher than the Rt Lat dose of approximately 2.6 cGy. To verify this dose asymmetry, surface doses on an IMRT QA phantom (oval shaped, 30 cm x 20 cm) were measured at the same three sites using TLD capsules with 3 cm table-drop. The dose asymmetry was due to: (1) kV source rotation which always starts from the patient's Lt Lat and ends at Lt Lat. Gantry rotation gets much slower near the end of rotation but dose rate stays constant and (2) 370 degrees scan rotation (10 degrees scan overlap on the Lt Lat side). In vivo doses were measured inside a Rando pelvic heterogeneous phantom using TLDs. The left hip (femoral head and neck) received the highest doses of approximately 10-11 cGy while the right hip received approximately 6-7 cGy. The surface and in vivo doses were also measured for phantoms at the central-axis setup. The difference was less than approximately 12% to the table-drop setup.
随着锥形束CT（CBCT）在日常患者摆位中的使用增加，CBCT累积的剂量可能显著高于模拟CT或射野成像的剂量。本研究的目的是在固定的技术参数和准直条件下测量每日盆腔扫描的剂量。CBCT扫描采用半扇区模式、半束光器进行采集，X射线以脉冲荧光模式发射。根据机构审查委员会批准的方案，测量了7例前列腺癌患者的皮肤剂量。热释光剂量仪（TLD）置于患者皮肤的三个射束中心轴上：前后位（AP）、左侧位（Lt Lat）和右侧位（Rt Lat）。为避免前列腺区域出现环形伪影，治疗床从患者体表标记（中心轴）下降3 cm。在20 - 33 cm的间距下，测量的AP皮肤剂量范围为3 - 6 cGy。患者体型越大，AP皮肤剂量越低。侧位剂量随患者体型变化不大。左侧位剂量约为4.0 cGy，比右侧位剂量约2.6 cGy高约40%。为验证这种剂量不对称性，在相同的三个部位，使用TLD并将治疗床下降3 cm，测量了调强放射治疗质量保证模体（椭圆形，30 cm×20 cm）表面剂量。剂量不对称的原因是：（1）千伏源旋转，总是从患者左侧位开始并在左侧位结束。机架旋转在接近旋转结束时变得慢得多，但剂量率保持恒定；（2）370度扫描旋转（左侧位有10度扫描重叠）。使用TLD在Rando盆腔非均匀模体内测量体内剂量。左髋部（股骨头和颈部）接受的最高剂量约为10 - 11 cGy，而右髋部接受的剂量约为6 - 7 cGy。还测量了模体在中心轴摆位时的表面剂量和体内剂量。与治疗床下降摆位相比，差异小于约12%。
