Focal dose

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Focus dose (HD) is the dose of ionizing radiation ( radiation dose ) that reaches the target tissue ( focus ) as part of a therapy ( radiation therapy or nuclear medicine therapy) . Since the focal dose usually cannot be measured in the target tissue, calculations are made to determine the dose before the therapy .

The focus dose is given as the focus-related absorbed dose in gray (Gy). Typical target doses for radiation therapy for benign diseases are in the order of 0.5 to 20 Gy; for malignant diseases in the range from 45 to 80 Gy. In the radioiodine therapy of benign thyroid diseases, focus doses of 120 to 400 Gy are aimed for; in the treatment of thyroid cancer , over 1000 Gy are also achieved.

The focal dose should not be confused with the equivalent dose (measured in Sievert ), which - weighted according to the type of radiation and the respective organ - mostly relates to the entire body of an individual. Here, whole-body doses of just a few Sieverts can lead to death.

Radiation planning in radiation therapy

→ Main article radiation therapy
Planning using PET data

For radiation therapy, it is determined which type of radiation is to act on the target tissue in which intensity and from which direction (radiation planning). A specific focal dose in gray is prescribed by the radiation therapist . Since the dose in the body fluctuates significantly, the point or volume for which this dose should apply must also be specified. It is not a trivial task. In practice, the isocenter of the field arrangement is often used. The isodose surrounding the tumor can also serve as a reference. However, this information can be inaccurate because it is often difficult to assess which tissue in the CT images is to be attributed to the tumor and which is not. In scientific studies that require the highest level of accuracy, the definitions of the ICRU (ICRU Report No. 50, 1993) are used. For example, it is stipulated there that the dose reference point should lie within the visible tumor (GTV, large tumor volume ) or that the dose within the entire planning volume must not be below 95% or above 107% of the specified reference dose anywhere. In addition to the CT and MR images, modern radiation therapy plans are increasingly integrating functional data from positron emission tomography in order to be able to more precisely delimit what is considered to be a "focus" and has to be treated with the focus dose.

When using atypical fractionation schemes, instead of the real physical focal dose, the biological equivalent dose (BED) can also be specified, i.e. the equally effective dose in conventional fractionation (2 Gy daily 5x / week), calculated according to the linear-square model . This radiation therapeutic parameter must not be confused with the dose equivalent from radiation protection.

Dose determination in nuclear medicine

In the field of nuclear medicine therapy , the dose is determined in advance taking into account the MIRD (Medical Internal Radiation Dose) scheme . In particular, the distribution of the radiopharmaceutical in the body ( uptake in the target organ) and the effective half-life are taken into account. Estimates are available for all routinely used forms of therapy. For radio-iodine therapy , the standardized dose is determined as part of a radio-iodine test .

Since the actual effective half-life and the actually achieved uptake in the target organ during the therapy can deviate from the previously determined parameters, in nuclear medicine therapy - in contrast to radiation therapy - the focal dose ultimately achieved can in some cases differ significantly from the targeted focal dose.

See also

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  1. ^ S. Mutic: PET and PET / CT for Radiotherapy Planning. In: W. Schlegel, T. Bortfeld, A. Grosu (Eds.): New Technologies in Radiation Oncology. Springer, Berlin / Heidelberg 2006, ISBN 3-540-00321-5 , pp. 133-149.
  2. ↑ B. Jones et al: Biological equivalent dose assessment of the consequences of hypofractionated radiotherapy. In: Int J Radiat Oncol Biol Phys . (47), 2000, pp. 1379-1384. PMID 10889393
  • A. Schaefer, D. Hellwig: Dosimetry. In: T. Kuwert, F. Grünwald, U. Haberkorn, T. Krause: Nuclear medicine. Stuttgart / New York 2008, ISBN 978-3-13-118504-4 .