Hossain Murshed, Rhoades Jeffrey
Fox Chase Cancer Center, 333 Cottman Avenue, Philadelphia, PA, 19111, USA.
UnityPoint Health Trinity Cancer Center, 500 John Deere Road, Moline, IL, 61265, USA.
Australas Phys Eng Sci Med. 2016 Mar;39(1):135-45. doi: 10.1007/s13246-015-0408-8. Epub 2015 Dec 3.
Ratio of percentage depth dose (PDD) at two depths, PDD at a depth of 10 cm (PDD10), and beam flatness are monitored regularly for radiotherapy beams for quality assurance. The purpose of this study is to understand the effects of changes in one of these parameters on the other. Is it possible to monitor only the beam flatness and not PDD? The investigation has two components. Naturally occurring i.e., unintended changes in PDD ratio and in-plane flatness for 6 and 10 MV photon beams for one particular Siemens Artiste Linac are monitored for a period of about 4 years. Secondly, deliberate changes in the beam parameters are induced by changing the bending magnet current (BMI). Relationships between various beam parameters for unintended changes as well as deliberate changes are characterized. Long term unintentional changes of PDD ratio are found to have no systematic trend. The flatness in the inplane direction for 6 and 10 MV beams show slow increase of 0.43 and 0.75% respectively in about 4 years while the changes in the PDD ratio show no such trend. Over 10% changes in BMI are required to induce changes in the beam quality indices at 2% level. PDD ratio for the 10 MV beam is found to be less sensitive, while the depth of maximum dose, d(max), is more sensitive to the changes in BMI compared to the 6 MV beam. Tolerances are more stringent for PDD10 than PDD ratio for the 10 MV beam. PDD ratio, PDD10, and flatness must be monitored independently. Furthermore, off axis ratio alone cannot be used to monitor flatness. The effect of beam quality change in the absolute dose is clinically insignificant.
为确保放射治疗射束的质量，需定期监测两个深度处的百分深度剂量（PDD）比值、10 cm深度处的PDD（PDD10）以及射束平整度。本研究的目的是了解这些参数之一的变化对另一个参数的影响。是否有可能仅监测射束平整度而不监测PDD？该研究有两个部分。对某一特定西门子Artiste直线加速器的6和10 MV光子束，监测其PDD比值和平面内平整度的自然发生（即非故意的）变化，为期约4年。其次，通过改变弯曲磁铁电流（BMI）来有意改变射束参数。对非故意变化以及有意变化的各种射束参数之间的关系进行了表征。发现PDD比值的长期非故意变化没有系统趋势。6和10 MV射束在平面内方向的平整度在约4年内分别缓慢增加了0.43%和0.75%，而PDD比值的变化没有这种趋势。需要超过10%的BMI变化才能在2%的水平上引起射束质量指标的变化。发现10 MV射束的PDD比值较不敏感，而与6 MV射束相比，最大剂量深度d(max)对BMI变化更敏感。对于10 MV射束，PDD10的公差比PDD比值的公差更严格。PDD比值、PDD10和平整度必须独立监测。此外，仅离轴比不能用于监测平整度。射束质量变化对绝对剂量的影响在临床上不显著。
