[Resolutions of high superficial dose in intensity-modulated radiation therapy when the target area is close to the surface].

OBJECTIVE

To study the impact of different planning target volume (PTV) margin settings on surface and superficial dose distribution and explore the resolution of high superficial dose when the target area is close to the surface during head and neck intensity-modulated radiotherapy (IMRT).

METHODS

A typical superficial target volume was designed in an circular neck phantom. Two experimental inverse IMRT plans were conducted with 8MV X ray, and in plan A, the superficial side of PTV margin ranged from 0 to 5 mm, while other side margins were 5 mm; in plan B, an suppositional machine dosimetry data for IMRT optimization was established in which the build-up dose was eliminated, and this machine data was used to optimize the inverse IMRT plan followed by recalculation of the planned dose distribution with the actual clinical machine dosimetry data. The variation of the surface and superficial dose resulting from set-up error and the dose distribution to CTV were compared. The adequate PTV margin was analyzed when the CTV approached the surface.

RESULTS

In plan A, the high dose greater than the prescribed dose was found in the surface and superficial region when the superficial sparing distance was between the surface and PTV d<3 mm, and the 3 mm set-up margin increased this superficial dose remarkably. Reducing the superficial side of PTV margin lowered the high superficial dose effectively and allowed maintenance of the prescribed dose to the CTV. To avoid reduction of the dose to the CTV to below 95% of the prescribed dose, the superficial side of PTV margin should be greater than 1 mm. Plan B effectively lowered the surface doses and maintained the prescribed dose to the CTV.

CONCLUSIONS

With appropriate techniques for optimizing inverse IMRT, more homogeneous superficial dose can be achieved.