لایه نیم جذب برای اندازه گیری کیفیت پرتو X در انرژی های سطحی و اورتوولتاژ امکان پذیر است (HVL آلومینیم برای سطحی و HVL مس برای اورتوولتاژ)؛ ولی در مورد انرژی های مگاولتاژ امکان پذیر نیست زیرا در این دامنه انرژی، تغییرات ضریب تضعیف برحسب انرژی بسیار ناچیز است.
Half - Value Layer (HVL)
رفرنس: فیزیک آنکولوژی تابش - پادگورساک
به منظور طراحی درمان و رسیدن به حداکثر راندمان رادیوتراپی یعنی جذب حداکثر دز در تومور و حداقل دز در بافتهای سالم احتیاج به چگونگی توزیع دز در بافتهای بدن و یا معادل بافتهای بدن یعنی آب است. یکی از پارامترهای مورد نیاز طراحی درمان اطلاع از توزیع درصد دز عمقی (PDD) بافت برای پرتوها ایکس و یا الکترونی است. طبیعی است با افزایش انرژی پرتو عمق نفوذ و درصد دز در عمق افزایش مییابد به همین دلیل است که برای تومورهایی در عمق بدن، استفاده از پرتوهای پرانرژی توصیه می شود...
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SARs are used for the purpose of calculating scattered dose in the medium. The computation of the primary and the scattered dose separately is particularly useful in the dosimetry of irregular fields.
SAR may be defied as the ratio of the scattered dose at a given point in the phantom to the dose in free space at the same point. The SAR, like the TAR, is independent of the SSD but depends on the beam energy, depth, and field size.
Because the scattered dose at a point in the phantom is equal to the total dose minus the primary dose at that point, SAR is mathematically given by the difference between the TAR for the given field and the TAR for the 0 × 0 field
SAR (d, rd) = TAR (d, rd) - TAR (d,0)
Here TAR (d,0) represents the primary component of the beam.
Khan's The Physics of Radiation Therapy Fifth Edition
Tissue-Air Ratio
Tissue-air ratio
(TAR) was first introduced by Johns in 1953 and was
originally called the “tumor-air ratio.” At that time, this quantity was
intended specifically for rotation therapy calculations. In rotation therapy,
the radiation source moves in a circle around the axis of rotation, which is
usually placed in the tumor. Although the SSD may vary depending on the shape of
the surface contour, the source-axis distance remains constant.
Since the percent
depth dose depends on the SSD , the SSD correction to
the percent depth dose will have to be applied to correct for the varying SSD—a procedure that becomes cumbersome to apply
routinely in clinical practice. A simpler quantity—namely TAR—has been defined to remove the SSD dependence. Since the time of its
introduction, the concept of TAR has been refined to facilitate calculations not only for rotation therapy, but also for stationary isocentric
techniques as well as irregular fields.
Tissue-air ratio may
be defined as the ratio of the dose (Dd) at a given point in the phantom to the dose in free
space (Dfs) at the same point.
This is illustrated in Figure 2 For a given quality
beam, TAR depends on depth d and field size
rd at that depth:
Fig 2: Illustration of the definition of tissue-air ratio (TAR). TAR(d,rd) = Dd/Dfs
Physics of Radiation Therapy, The, 5th Edition
Faiz M. Khan PhD
Professor Emeritus
