In radiotherapy of head and neck cancer patients portal imaging to achieve accurate positioning is of major importance. Van Herk et al.3 and Stroom et al.8 studied the relationship between set-up variations and target margins. They concluded that the margin between clinical target volume (CTV) and planning target volume (PTV) can be decreased most effectively by decreasing the standard deviation of the overall systematic error.
This overall systematic error is predominantly determined by the difference in patient position between planning CT and linear accelerator (Linac) and can be decreased by using an off-line verification protocol. In our department we use the Shrinking Action Level (SAL) verification protocol by Bel et al.1. In the SAL protocol the patient position at the Linac is only corrected if the average set-up error exceeds a certain action level. The action level decreases with the number of measurements N according to the relation: aN = a0/ÖN, where a0 is the initial action level, aN the action level after N measurements and N £ Nmax. After Nmax fractions, the second stage of the protocol starts in which only once a week portal images are collected and the action level stays equal to aNmax.
The set-up error has to be determined in a coordinate system that corresponds to the patient/couch directions: ventrodorsal/height, craniocaudal/longitudinal and late-ral to be able to correct the patient position by means of table movements at the Linac. For patients that are treated with an irradiation technique that contains at least one lateral and one anterior posterior (or a posterior anterior) beam the set-up errors can be determined from the portal images of these treatment beams.
At our department there are two groups of patients treated with techniques with oblique beams only. The first group includes patients with double-sided lymph node metastases from squamous cell head and neck cancer, that are treated with a technique designed to spare the spinal cord. This technique is based on the technique described by Fogliata et al.5 and will be discussed in more detail in the Materials and methods section. The main radiation beams have oblique gantry angles. In all other beams the spinal cord is blocked out and therefore, no vertebrae can be recognized on the portal images of these beams, which make these beams unsuitable for position verification. This technique is referred to as ‘Bellinzona technique’.
The second group includes patients with pituitary adenomas that are treated with a technique in which the four radiation beams form a tetrahedron shape. In this technique, referred to as ‘Tetrahedron technique’, all radiation beams have oblique gantry angles and two beams also have a table rotation. For the position verification of these two patient groups additional anterior posterior (AP) and lateral verification beams are used.
We have calculated that the verification beams result in an extra dose to the spinal cord of about 1.0 Gy for the Bellinzona technique when the SAL verification protocol is used and in case one position correction is necessary (14 fractions with two verification beams of 4 MU). Because both patient groups are treated with advanced techniques in order to spare the normal tissue as much as possible, it was considered inappropriate to add a dose of about 1 Gy to a relative large area, just for verification. However, set-up errors can result in much larger dose effects, especially in the Bellinzona technique with a high dose gradient close to the spinal cord in ventrodorsal direction. Therefore, the main purpose of this study was to investigate the use of portal images of the oblique radiation beams of the Bellinzona and the Tetrahedron technique for verification as well. In case of a comparable accuracy, the anterior posterior and lateral portal images can then be abandoned.