Fonseca Gabriel P, Viana Rodrigo S S, Podesta Mark, Rubo Rodrigo A, de Sales Camila P, Reniers Brigitte, Yoriyaz Hélio, Verhaegen Frank
Instituto de Pesquisas Energéticas e Nucleares-IPEN-CNEN/SP, São Paulo 05508-000, Brazil and Department of Radiation Oncology (MAASTRO), GROW School for Oncology and Developmental Biology, Maastricht University Medical Center, Maastricht 6201 BN, The Netherlands.
Instituto de Pesquisas Energéticas e Nucleares-IPEN-CNEN/SP, São Paulo 05508-000, Brazil.
Med Phys. 2015 Jan;42(1):412-5. doi: 10.1118/1.4903286.
The dose delivered with a HDR (192)Ir afterloader can be separated into a dwell component, and a transit component resulting from the source movement. The transit component is directly dependent on the source speed profile and it is the goal of this study to measure accurate source speed profiles.
A high speed video camera was used to record the movement of a (192)Ir source (Nucletron, an Elekta company, Stockholm, Sweden) for interdwell distances of 0.25-5 cm with dwell times of 0.1, 1, and 2 s. Transit dose distributions were calculated using a Monte Carlo code simulating the source movement.
The source stops at each dwell position oscillating around the desired position for a duration up to (0.026 ± 0.005) s. The source speed profile shows variations between 0 and 81 cm/s with average speed of ∼ 33 cm/s for most of the interdwell distances. The source stops for up to (0.005 ± 0.001) s at nonprogrammed positions in between two programmed dwell positions. The dwell time correction applied by the manufacturer compensates the transit dose between the dwell positions leading to a maximum overdose of 41 mGy for the considered cases and assuming an air-kerma strength of 48 000 U. The transit dose component is not uniformly distributed leading to over and underdoses, which is within 1.4% for commonly prescribed doses (3-10 Gy).
The source maintains its speed even for the short interdwell distances. Dose variations due to the transit dose component are much lower than the prescribed treatment doses for brachytherapy, although transit dose component should be evaluated individually for clinical cases.
高剂量率（HDR）铱-192后装治疗机输送的剂量可分为驻留分量和源运动产生的传输分量。传输分量直接取决于源速度分布，本研究的目的是测量准确的源速度分布。
使用高速摄像机记录铱-192源（瑞典斯德哥尔摩的医科达公司生产的Nucletron）在驻留间距为0.25 - 5 cm、驻留时间分别为0.1、1和2 s时的运动。使用模拟源运动的蒙特卡罗代码计算传输剂量分布。
源在每个驻留位置停止时，会在期望位置附近振荡，持续时间长达（0.026 ± 0.005）s。对于大多数驻留间距，源速度分布在0至81 cm/s之间变化，平均速度约为33 cm/s。源在两个编程驻留位置之间的非编程位置会停止长达（0.005 ± 0.001）s。制造商应用的驻留时间校正补偿了驻留位置之间的传输剂量，在所考虑的情况下，假设空气比释动能强度为48000 U时，最大过量剂量为41 mGy。传输剂量分量分布不均匀，会导致剂量过高和过低，对于常用处方剂量（3 - 10 Gy），这种情况在1.4%以内。
即使在较短的驻留间距下，源也能保持其速度。尽管对于临床病例应单独评估传输剂量分量，但由于传输剂量分量导致的剂量变化远低于近距离放射治疗的处方治疗剂量。
