A comparative planning study of step-and-shoot IMRT versus helical tomotherapy in a model patient

In order to assess the potential benefit of being able to use non-coplanar beams in a step-andshoot intensity-modulated radiotherapy (ssIMRT) differently from helical tomotherapy (HT), we performed a comparative planning evaluation in a model patient whose clinical target volumes enclosed critical normal tissue lying in the same plane. A patient with extensively relapsed malignant lymphoma in the circumferential chest wall was selected for comparison. Multiple, separated gross tumors were delineated, and the clinical target volume included the circumferential chest wall containing gross tumor volumes so that the critical organ, the right lung was fully enclosed. Three types of plans were analyzed; the first one was ssIMRT using only coplanar beams, the second was ssIMRT using non-coplanar beams, and the third was HT. The obtained results showed that dose homogeneity inside the gross tumor volume was better in the case of HT than noncoplanar ssIMRT and coplanar ssIMRT. However, the volume fraction of the right lung receiving a high dose was much smaller in the noncoplanar ssIMRT. As far as the dose to the clinical target volume (CTV) fully enclosing the right lung is concerned, not only the dose coverage of the CTV but also the dose homogeneity and conformity in the noncoplanar ssIMRT was better than in both of HT and the coplanar ssIMRT. We can conclude that appropriate isodose coverage for a complex target, which fully encloses a critical organ lying in the same plane, can be achieved by using a non-coplanar beam arrangement with no enforced penalty of higher dose to a critical organ.