Spinal cord doses in palliative lung radiotherapy schedules

We aim to check the safety of the standard palliative radiotherapy techniques by using the Linear quadratic model for a careful estimation of the doses received by the spinal cord, in all standard palliative lung radiotherapy fields and fractionation.

All patients surveyed at this prospective audit were treated with palliative chest radiotherapy for lung cancer over a period from January to June 2005 by different clinical oncology specialists within the department. Radiotherapy field criteria were recorded and compared with the recommended limits of the MRC trial protocols for the dose and fractionation prescribed. Doses delivered to structures off the field central axis were estimated using a standard CT scan of the chest. Dose estimates were made using an SLPLAN planning system. As unexpected spinal cord toxicity has been reported after hypofractionated chest radiotherapy, a sagittal view was used to calculate the isodoses along the length of the spinal cord that could lie within the RT field. Equivalent dose estimates are made using the Linear- Quadratic Equivalent Dose formula (LQED). The relative radiation sensitivity of spinal cord for myelopathy (the a/b dose) cord has been estimated as a/b = 1 Gy.

17 Gy in 2 fraction and 39 Gy in 13 fraction protocols have spinal cord equivalent doses (using the linear-quadratic model) that lie within the conventional safe limits of 50 Gy in 25 fractions for the 100% isodose. However when the dosimetry is modelled for a 6 MV 100 cm isocentric linnac in 3 dimensions, and altered separations and air space inhomogeneity are considered, the D-Max doses consistently fall above this limit on our 3 model patients.

The 17 Gy in 2 fraction and 39 Gy in 13 fraction protocol would risk spinal cord damage if the radiotherapist was unaware of the potential spinal cord doses. Alterative doses are suggested below 15.5 Gy/ 2 fractions (7 days apart) would be most acceptable.