Development of whole-body representation and dose calculation in a commercial treatment planning system

For the epidemiological evaluation of long-term side effects of radiotherapy patients, it is important to know the dosesto organs and tissues everywhere in the patient. Computed tomography (CT) images of the patients which contain theanatomical information are sometimes available for each treated patient. However, the available CT scans usually coveronly the treated volume of the patient including the target and surrounding anatomy. To overcome this limitation, in thiswork we describe the development of a software tool using the Varian Eclipse Scripting API for extending a partial-bodyCT to a whole-body representation in the treatment planning system for dose calculation. The whole-body representationis created by fusing the partial-body CT with a similarly sized whole-body computational phantom selected from a librarycontaining 64 phantoms of different heights, weights, and genders. The out-of-field dose is calculated with analytical modelsfrom the literature and merged with the treatment planning system-calculated dose. To test the method, the out-of-field dosedistributions on the computational phantoms were compared to dose calculations on whole-body patient CTs. The meandoses, D2% and D98% were compared in 26 organs and tissues for 14 different treatment plans in 5 patients using 3D-CRT,IMRT, VMAT, coplanar and non-coplanar techniques. From these comparisons we found that mean relative differencesbetween organ doses ranged from −10% and +20% with standard deviations of up to 40%. The developed method will helpepidemiologists and researchers estimate organ doses outside the treated volume when only limited treatment planning CTinformation is available.