University Hospital Rijeka, Radiotherapy Department, Physics Division, Rijeka, Croatia.
Radiol Oncol. 2011 Dec;45(4):310-4. doi: 10.2478/v10019-011-0020-9. Epub 2011 Jul 20.
External photon beam modulation using compensators in order to achieve a desired dose distribution when brachytherapy treatment is followed by external beam radiation is a well-established technique. A compensator modulates the central part of the beam, and the dose beneath the thickest part of the compensator is delivered mostly by scattered, low energy photons. A two-dimensional detector with a good spatial resolution is needed for the verification of those beams. In this work, the influence of different types of detectors on the measured modulated dose distributions was examined.
Dosimetric verification was performed using X-Omat V, Eastman Kodak radiographic films at different depths in a solid water phantom. The film measurements were compared with those made by ionization chambers. Photon beams were also modelled using EGSnrc Monte Carlo algorithm to explain the measured results.
Monte Carlo calculated over-response of the film under the thickest part of the compensator was over 15%, which was confirmed by measurements. The magnitude of over-response could be associated with changes in the spectra of photon energy in the beam.
The radiographic film can be used for the dosimetry of compensated high energy photon beams, with limitations in volumes where photon spectra are hardly degraded.
在近距离放射治疗后采用外照射放射治疗时，使用补偿器对外照射光束进行调制以实现期望的剂量分布是一种成熟的技术。补偿器调制光束的中心部分，而在补偿器最厚部分下方的剂量主要由散射的低能光子提供。对于这些光束的验证，需要具有良好空间分辨率的二维探测器。在这项工作中，研究了不同类型的探测器对测量调制剂量分布的影响。
在固体水模体中不同深度处使用 X-Omat V、Eastman Kodak 射线照相胶片对剂量进行验证。将胶片测量值与电离室测量值进行了比较。还使用 EGSnrc 蒙特卡罗算法对光子束进行建模，以解释测量结果。
蒙特卡罗计算出补偿器最厚部分下胶片的过响应超过 15%，这一结果通过测量得到了证实。过响应的幅度可能与光束中光子能量谱的变化有关。
射线照相胶片可用于补偿高能光子束的剂量测定，但在光子谱几乎没有退化的体积内存在限制。
