Two-dimensional ionization chamber arrays for IMRT plan verification.

In this paper we describe a concept for dosimetric treatment plan verification using two-dimensional ionization chamber arrays. Two different versions of the 2D-ARRAY (PTW-Freiburg, Germany) will be presented, a matrix of 16 x 16 chambers (chamber cross section 8 mm x 8 mm; the distance between chamber centers, 16 mm) and a matrix of 27 x 27 chambers (chamber cross section 5 mm x 5 mm; the distance between chamber centers is 10 mm). The two-dimensional response function of a single chamber is experimentally determined by scanning it with a slit beam. For dosimetric plan verification, the expected two-dimensional distribution of the array signals is calculated via convolution of the planned dose distribution, obtained from the treatment planning system, with the two-dimensional response function of a single chamber. By comparing the measured two-dimensional distribution of the array signals with the expected one, a distribution of deviations is obtained that can be subjected to verification criteria, such as the gamma index criterion. As an example, this verification method is discussed for one sequence of an IMRT plan. The error detection capability is demonstrated in a case study. Both versions of two-dimensional ionization chamber arrays, together with the developed treatment plan verification strategy, have been found to provide a suitable and easy-to-handle quality assurance instrument for IMRT.

[Resolution and sensitivity of a two-dimensional ionisation chamber array (PTW type 10024)]. / Uber das Auflösungsvermögen und die Empfindlichkeit eines zweidimensionalen Ionisationskammer-Arrays (PTW: Typ 10024).

The two-dimensional verification of intensity-modulated radiation plans is one of the major requirements for the safe application of this technique. The present study examines the resolution and sensitivity of a two-dimensional ionisation-chamber array (PTW2D-Array, type 10024), which can be used for plan verification instead of films. According to the Shannon-Nyquist theorem, the resolution of the 2D-Array is sufficient for dose distributions with a minimal field size of 2 cm x 2 cm. The minimal field size can be reduced to 1 cm x 1 cm by shifting the array 5 mm in the direction of the MLC movement and by repeating the measurements. The high sensitivity against a monitor decalibration for a single field of a sequence is demonstrated on the basis of an individual case. The minimal threshold for MLC misalignment detected by a chamber of the array is less than 1 mm. Therefore, the resolution capabilities of the 2D-Array are sufficient for most intensity-modulated radiation therapy (IMRT)fields.

[Use of a two-dimensional ionization chamber array for quality assurance in medical linear accelerators]. / Einsatz eines zweidimensionalen Ionisationskammer-Arrays zur Qualitätssicherung an medizinischen Linearbeschleunigern.

Two-dimensional dosimetry measurements are an important tool of Quality Assurance in modern radiotherapy techniques, such as the Intensity Modulated Radiation Therapy (IMRT). Common procedures are usually based on the use of films or semiconductor arrays as dosimeters. This paper presents our experience with a two-dimensional ionization-chamber array. The methods presented here allow the daily use of the array for a constancy check of the accelerator. It is also shown that the position of the individual leafs of the multi-leaf collimator (MLC) can be verified to within +/- 1 mm of their calibration. A procedure for the measurements is described and discussed.