Delineation of Margins for the Planning Target Volume (PTV) for Image-Guided Radiotherapy (IGRT) of Gastric Cancer Based on Intrafraction Motion

Background: Application of the image-guided radiotherapy (IGRT) system for gastric cancer involving daily verification of patient positioning on the treatment machine allows minimisation of geometrical errors as a consequence of intra- and inter-fraction motion. The purpose of this study was to define the intrafraction motion in gastric cancer patients during a treatment session based on the IGRT system and designation of margins around the clinical target volume CTV (internal target volume ITV) necessary to delineate the planning target volume (PTV). Methods: Twenty gastric cancer patients were analysed. The total radiation dose for each was 45Gy in 25 fractions within 5 weeks. The margins for the PTV were calculated according to van Herk (2004), Stroom and Heijmen (2002) and the International Commission on Radiation Units and Measurements (ICRU) Report 62 formulas based on craniocaudal (Y axis), laterolateral (X axis) and anteroposterior (Z axis) shifts. Results: Delineated margins for the PTV in gastric cancer with the three formulas applied were respectively 0.2, 0.2, and 0.2cm in the lateral plane, 0.3, 0.3, and 0.3cm in the craniocaudal plane and 0.3, 0.3, and 0.2cm in the anteroposterior plane. Conclusions: Recommended margins for the PTV in gastric cancer calculated in this study based on intrafraction motion are 0.3cm, 0.2cm and 0.3cm in the craniocaudal, lateral and anterioposterior directions, respectively. Use of the IGRT system corrects for the motions between factions and allows reduction in ITV-PTV margins. The main advantage of the smaller margins in comparison to the non-IGRT radiotherapy is a reduction in the probability of radiation complications.