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In-vivo dosimetry with alanine/ESR

30.03.2009

Dosimetry plays a decisive role in radiotherapy. In modern forms of radiation therapy with a dose escalation in the planned target volume, a precise determination of the dose is becoming ever more important, especially in areas surrounding the tumour which must be protected (tissue at risk).

One of the most wide-spread type of cancers is the prostate carcinoma. Tissues which are at risk in this case are in particular the rectum, the urethra and the bladder septum. Although in-vivo dosimetry, i.e. dose measurement inside the patient, is not a typical field of application of secondary standard measuring systems, PTB’s measuring equipment, which is based on alanine and electron spin resonance (ESR), offers a unique opportunity to test directly the reliability of irradiation plannings and their application. Therefore, in collaboration with the University Hospital of Göttingen, investigations have already been made in the rectum in the case of prostate irradiation [1]. Current work is aimed at measuring the dose directly in the urethra while the prostate is being irradiated with 192Ir which is injected into the target tissue via hollow needles by means of a so-called afterloader. For the irradiation planning, i.e. the initially virtual positioning of the hollow needles, particular attention is paid to the protection of the urethra. By means of the alanine/ESR measuring system, the actually applied dose is to be compared with the planned dose.

In a bladder catheter with an inner diameter of only 2.7 mm, alanine tablets with an outer diameter of 5 mm cannot be used. Therefore, a new form of detector was developed. Alanine powder mixed with binding agent is impermeably wrapped with shrinkable tubing and fixed to a synthetic stick having an outer diameter of 2 mm so that it can be introduced into the catheter. The detector powder is pressed into tablets only after irradiation has taken place.

This procedure implied unknown influence quantities – for instance, the influence of the pressing after the irradiation was not known. It turned out that the detectors that had been generated this way showed much stronger fading (loss of dose information with time) than the alanine tablets pressed before the irradiation which are normally used. Fortunately, as could be shown in a measurement series, in this case, the fading was not dependent on the use of 192Ir or 60Co for irradiation. Therefore, the dose is now determined by means of tube detectors in such a way that the patient is irradiated and, at the same time, an identical tube detector is irradiated with a known dose in a reference field. The dose to be measured in the urethra is then determined as a function of the known dose; the ratio is thereby independent of the fading, i.e. the influence of the fading can be ruled out.

Furthermore, the response of alanine/ESR is not known for 192Ir as a function of 60Co. In cooperation with the Working Group "Brachytherapy", a provisional value could be determined. The response for iridium lies around 4.3% below the response for cobalt.

As a preparation for the in-vivo measurement planned for the first quarter of 2009, the measurement procedure was successfully tested on a special prostate phantom. The measured and planned doses were – within the estimated uncertainty of approx. 4% – in good agreement. The positioning of the detectors inside the urethra contributed essentially to the uncertainty budget [2]. This problem can be expected to worsen in patient dosimetry. However, measurement results are expected to shed light on irradiation planning reliability, especially with regard to the protection of tissue at risk. Furthermore, from the upcoming transition of the primary standard for 192Ir from air kerma to absorbed dose to water, it is expected that the uncertainty of the response can be reduced.

Literature

  1. Daniela Wagner, Mathias Anton, Hilke Vorwerk, Tammo Gsänger, Hans Christiansen, Bjoern Poppe, Clemens Friedrich Hess, Robert Michael Hermann:
    In vivo alanine/electron spin resonance (ESR) dosimetry in radiotherapy of prostate cancer: a feasibility study
    Radiotherapy and Oncology 88 (2008), pp 140-147
  2. Mathias Anton, Daniela Wagner, Hans-Joachim Selbach, Thomas Hackel, Robert Michael Hermann, Clemens Friedrich Hess, Hilke Vorwerk:
    In vivo dosimetry in the urethra using alanine/ESR during 192Ir HDR brachytherapy of prostate cancer - a phantom study
    Physics in Medicine and Biology 54 (2009), pp 2915-2931