A Dosimetric Analysis of the Overlapping and Gap Areas Produced by Simulated Set-Up Errors on a Treatment Planning System in a Case of the Cranio-Spinal Irradiation of an Adult Patient

The purpose of this study is to make a dosimetric analysis of the overlapping and gap areas in the case of a cranio-spinal irradiation. A simulation of the overlaps and gaps was performed on a treatment planning system. The overlaps and gaps were simulated for the 2, 4, 5, 6 and 8 mm overlaps or gaps. The data recorded: global and local maximum of doses, depth of the 95, 100 and 125% dose, the depth of spine location in the overlapping/gap areas. The local maximum values were increased up to 7.03 Gy in the case of the cranial-upper spinal field overlaps and up to 5.45 Gy in the case of the upper-lower spinal fields. The gaps cause the decreases of the local maximum values up to 6.65 Gy in the case of cranial-upper spinal field. An increase of the depth of 125% isodose line of 1.04 cm in the case of the 5 mm upper-lower spine fields overlapping was recorded. The total coverage of the target volume by 125% isodose line, in the case of the 5 mm + 5 mm overlapping between the cranial and the upper spinal field may be a source of a damage of spinal cord. The partially covered treatment volume by 95% isodose line in the case of 4 mm + 4 mm and 5 mm + 5 mm gap area between the cranial and the upper spinal field, may cause a relapse of the tumor.