A method to improve target dose homogeneity of craniospinal irradiation using dynamic split field IMRT

Craniospinal irradiation (CSI) is technically very challenging and field edge matching is needed because of the mechanical limitations of standard linear accelerators. We assessed the feasibility of intensity-modulated radiotherapy (IMRT) in CSI to overcome the standard feathering and dose inhomogeneities associated with the standard feathering technique in the junction areas.

The use of IMRT in CSI was studied with five patients CT scanned in the supine position. Isocentric treatment plans of three dimensional conventional radiotherapy (3D-CRT) and split field IMRT (sfIMRT) with dynamic intrafractional feathering were created with the same field setup and the resulted dose distributions were compared. The effect of treatment inaccuracy was simulated with an intentional shift of +/-3mm with both treatment plans. Dosimetric verification of the sfIMRT treatment plan was performed with radiographic films placed in a phantom.

The sfIMRT treatment plans resulted in a better dose coverage and uniformity in the target volume. The +/-3mm shift had only a minor effect on the dose distribution of the sfIMRT treatment plan whereas with the 3D-CRT the shift resulted in an error of +/-38% of the calculated dose in the spinal cord. The measured dose distribution of the sfIMRT treatment plan correlated well with the calculations.

Improved dose homogeneity in the target volume was achieved with the sfIMRT compared to the conventional 3D-CRT treatment plan. With the sfIMRT technique only a single treatment plan is required to deliver the total treatment dose and the resulting dose distribution is also less volatile for technical uncertainties of the treatment.