Design and manufacture of a head and neck phantom for the assessment of IMRT delivery with gel dosimetry

The complexity of intensity-modulated radiation therapy (IMRT) treatments has increased considerably in comparison to conventional radiotherapy (RT). This bears the risk of more potential sources of error and, therefore, appropriate measures have to be taken to ensure that the dose delivered corresponds to the dose that has been prescribed. For conventional RT, the routine quality assurance (QA) within a radiotherapy department may be sufficient to ensure accurate beam delivery [1]. For IMRT, the verification of the delivery of each fraction of a course of radiotherapy treatment requires careful verification [2]. Before routine IMRT treatment can be provided, it is also necessary to assess the selected beam intensity modulation method, such as multi leaf collimators (MLC) or compensators, to test both the geometric and dosimetric accuracy of the delivery method. Point measurements in a three-dimensional (3D) space can be obtained by placing one-dimensional dosimeters, such as TLDs or an ion chamber [3], within the 3D space. More spatial information can be obtained by placing films in consecutive layers in a 3D phantom, which can be a difficult and time-consuming task [4]. A more elegant alternative is the use of radiosensitive gels [5,6], e.g. BANG™ gel, which enables measuring the radiation dose at any given spatial point. It has been demonstrated by e.g. De Deene et al. [7] that polymer gel is adequate for verification of 3D conformal radiotherapy.