Field separation of adjoining therapy fields

Matching tomographic IMRT fields with static photon fields

The matching of abutting radiation fields presents a challenging problem in radiation therapy. Due to sharp penumbra of linear accelerator beams, small (1-2 mm) errors in field positioning can lead to large (>30%) hot or cold spots in the abutment region. With head and neck immobilization devices (thermoplastic mask/aquaplast) an average setup error of 3 mm has been reported. Therefore hot or cold spots approaching 50% of the prescription dose may occur along the matchline. Although abutting radiation fields have been investigated for static fields, there is no reported study regarding matching of tomographic IMRT and static fields. Compared to static fields, the matching of tomographic IMRT fields with static fields is more complicated. Since IMRT and static fields are planned on separate treatment planning computers, the dose in the abutment region is not specified. In addition, commonly used techniques for matching fields, such as feathering of junctions, are not practical. We have developed a method that substantially reduces dose inhomogeneity in the abutment region. In this method, a "buffer zone" around the matchline was created and was included as part of the target for both IMRT and static field plans. In both fields, a small dose gradient (< or =3%/mm) in the buffer zone was created. In the IMRT plan, the buffer zone was divided into three sections with dose varying from 83% to 25% of prescription dose. The static field dose profile was modified using either a specially designed physical (hard) or a dynamic (soft) wedge. When these modified fields were matched, the combined dose in the abutment region varied by < or =10% in the presence of setup errors spanning 4 mm (+/-2 mm) when the hard wedge was used and 10 mm (+/-5 mm) with the soft wedge.

The effect of beam divergence on target coverage

The well-known fact that radiation beams diverge is frequently not considered during the treatment planning process. Complacency with respect to beam divergence can, in some situations, lead to inappropriate field design. In this review, the potential problems arising from failure to adequately account for beam divergence in treatment planning are outlined, and commonly encountered clinical examples are illustrated.

Irradiation of the whole neuraxis. A method for field positioning

When the whole neuraxis is irradiated, as in medulloblastoma, the large target volume makes field arrangement complicated and 3-4 adjacent fields have to be used. A calculation method, based on geometrical matching of field edges, is presented for determining optimal separation between the opposed cranial fields and the posterior spinal field as orthogonal movements of the treatment couch top. The dosimetric verification of the method with a 4 MV photon beam indicates a dose inhomogeneity below +/- 7%.

Effect of set-up error on the dose across the junction of matching cranial-spinal fields in the treatment of medulloblastoma

PURPOSE

The effect of systematic and stochastic setup error on the dose delivered to the gap region for the three field radiation treatment of medulloblastoma is studied. The consequences of such setup error is discussed.

METHODS AND MATERIALS

The treatment of medulloblastoma is typically a 3 field technique, in which two lateral cranial fields are matched with a spine field. The x-ray dose delivered to the region between the matched fields depends upon the gap size. The choice of the gap width between the cranial and spinal fields is controversial. It is currently a compromise between minimizing the risk of dose hot spots to the spine, and the associated clinical complications, as well as the magnitude of cold spots (underdosing) across the gap, with the associated risk of disease recurrence. In this paper, we examine the effect of gap width with a moving junction, referred to as "field feathering", on the dose across the field junction for a 6MV photon beam. In addition, we have studied 129 portal films and 40 simulation films to assess the accuracy and precision of patient setup during treatment with a plan involving feathered fields. Selected landmarks observable on both portal and simulation films were identified and the variation in the distances to the field edges measured. The distribution of patient setup error was convoluted with the beam profiles for a 6MV linac. These convoluted field edges were used obtain dose profiles across the gap region as a function of gap separation. The consequences for therapy are discussed. In addition, analysis of patient setup error on an alternative treatment involving beam modifiers to broaden the beam penumbra is discussed.

RESULTS

The magnitude of the spatial stochastic and systematic setup error was determined to be approximately three and two millimeters respectively. The dosimetric consequences of patient setup error lead to over and under dosing in the spinal gap region for the three field technique. The degree of under or over dose depends on the nature and magnitude of the patient setup error.

CONCLUSIONS

The effect of patient setup error can lead to significant dosimetric errors in the dose to the gap region depending on the magnitude of the setup errors. The effective over and under dose can be compensated by the use beams modifiers such as a beam spoiler or vibrating jaws.

Border separation for adjacent orthogonal fields

Field border separations for adjacent orthogonal fields can be calculated geometrically, given the validity of some important assumptions such as beam alignment and field uniformity. Thermoluminescent dosimetry (TLD) measurements were used to investigate dose uniformity across field junctions as a function of field separation and, in particular, to review the CCSG recommendation for the treatment of medulloblastoma with separate head and spine fields.