Influence of interfraction motion on margins for radiotherapy of gastric cancer.

Objective: The aim of this study was to evaluate interfraction uncertainties using kilovoltage (kV) radiographs for patients with gastric cancer during chemoradiotherapy and to calculate the planning target volume (PTV) margins.

Methods: 1284 measurements of set-up errors were analysed for treated patients. The measurements of craniocaudal (axis y), laterolateral (axis x) and anteroposterior (axis z) shifts in kV radiographs were performed. Interfraction clinical target volume (CTV)-PTV margins for all directions were calculated using the van Herk formula.

Results: The main shift for the y-axis was 0.7 mm [standard deviation (SD), ±7.6], for the x-axis was 0.4 mm (SD, ±3.7) and for the z-axis was 0.6 mm (SD, ±3.5). The CTV-PTV margin in the x, y and z directions was 8.3, 15.5 and 8.0 mm, respectively. We observed that the interfractional motion for patients increased approximately 0.0034 cm along the x direction with each subsequent fraction, whereas a 0.0058-cm reduction in length along the y-axis was observed. No time effects for the z direction were noticed.

Conclusion: According to our experience, a PTV margin of 9 mm along the x-axis, 16 mm along the y-axis and 8 mm along the z direction should be considered in the absence of image-guided radiotherapy.

Advances In Knowledge: This knowledge concerning PTV set-up margins could be particularly useful for centres without a kV on-board imaging system.