Hsi Wen C, Moyers Michael F, Nichiporov Dmitri, Anferov Vladimir, Wolanski Mark, Allgower Chris E, Farr Jonathan B, Mascia Anthony E, Schreuder Andries N
Midwest Proton Radiotherapy Institute, Bloomington, Indiana 47408 and University Florida Proton Therapy Institute, Jacksonville, Florida 32206, USA.
Med Phys. 2009 Jun;36(6):2297-308. doi: 10.1118/1.3132422.
In proton therapy delivered with range modulated beams, the energy spectrum of protons entering the delivery nozzle can affect the dose uniformity within the target region and the dose gradient around its periphery. For a cyclotron with a fixed extraction energy, a rangeshifter is used to change the energy but this produces increasing energy spreads for decreasing energies. This study investigated the magnitude of the effects of different energy spreads on dose uniformity and distal edge dose gradient and determined the limits for controlling the incident spectrum. A multilayer Faraday cup (MLFC) was calibrated against depth dose curves measured in water for nonmodulated beams with various incident spectra. Depth dose curves were measured in a water phantom and in a multilayer ionization chamber detector for modulated beams using different incident energy spreads. Some nozzle entrance energy spectra can produce unacceptable dose nonuniformities of up to +/-21% over the modulated region. For modulated beams and small beam ranges, the width of the distal penumbra can vary by a factor of 2.5. When the energy spread was controlled within the defined limits, the dose nonuniformity was less than +/-3%. To facilitate understanding of the results, the data were compared to the measured and Monte Carlo calculated data from a variable extraction energy synchrotron which has a narrow spectrum for all energies. Dose uniformity is only maintained within prescription limits when the energy spread is controlled. At low energies, a large spread can be beneficial for extending the energy range at which a single range modulator device can be used. An MLFC can be used as part of a feedback to provide specified energy spreads for different energies.
在使用射程调制束进行质子治疗时，进入传输喷嘴的质子能谱会影响靶区内的剂量均匀性及其周边的剂量梯度。对于具有固定引出能量的回旋加速器，使用射程移位器来改变能量，但这会导致能量越低能量展宽越大。本研究调查了不同能量展宽对剂量均匀性和远端边缘剂量梯度的影响程度，并确定了控制入射能谱的限度。针对具有各种入射能谱的非调制束，在水中测量的深度剂量曲线对多层法拉第杯（MLFC）进行了校准。在水模体和多层电离室探测器中，针对使用不同入射能量展宽的调制束测量了深度剂量曲线。一些喷嘴入口能谱在调制区域可产生高达±21%的不可接受的剂量不均匀性。对于调制束和小射野范围，远端半影宽度可相差2.5倍。当能量展宽控制在规定限度内时，剂量不均匀性小于±3%。为便于理解结果，将这些数据与来自可变引出能量同步加速器的测量数据和蒙特卡罗计算数据进行了比较，该同步加速器在所有能量下都具有窄能谱。只有当能量展宽得到控制时，剂量均匀性才能维持在处方限度内。在低能量时，较大的展宽对于扩展可使用单个射程调制装置的能量范围可能是有益的。MLFC可作为反馈的一部分，为不同能量提供特定的能量展宽。
