IMRT for postoperative treatment of gastric cancer: covering large target volumes in the upper abdomen: a comparison of a step-and-shoot and an arc therapy approach.
Data from the randomized Intergroup Trial 116 suggest effectiveness of adjuvant radiochemotherapy in patients with advanced gastric cancer. Late toxicity, however, especially with respect to the kidneys, may pose significant longtime problems. Intensity-modulated radiotherapy (IMRT) may reduce toxicity to organs at risk. To evaluate the relative merits of different IMRT approaches, we performed a plan comparison between a step-and-shoot class solution and an AP-PA setup, a conventional box technique and the Peacock tomotherapy approach. Computed tomographies and structure data from 15 patients who had been treated postoperatively for advanced (T3/T4/N+) gastric cancer at our department formed the basis of our plan comparison study. For each patient data set, 5 plans or plan combinations (conventional 3D plan, AP-PA plan, step-and-shoot IMRT, tomotherapy with 1-cm or 2-cm collimation) were chosen, and evaluation was performed for a total dose of 45 Gy delivered as the median dose to the target volume for each plan or plan combination. Median kidney dose generated from the IMRT plans is reduced individually by >50% for the kidney with the highest exposure (usually the left kidney) from 20 to 30 Gy with conventional 3D planning down to values between 8 and 10 Gy for IMRT. On average, median dose to the right kidney is the same for the conventional box technique and IMRT (between 8 and 10 Gy) but lower for the AP-PA technique. In 3 patients, kidney dose might have been ablative for both kidneys with both the AP-PA technique and the box technique, whereas it was acceptable with IMRT. Median dose to the liver was subcritical with all modalities but lowest with AP-PA fields. Differences between step-and-shoot IMRT and tomotherapy plans are small when compared to the differences between IMRT plans and conventional conformal 3D plans. For some patients, however, their body and target diameters obviate treatment with tomotherapy. Treatment time for the step-and-shoot approach and for tomotherapy with 2-cm collimation can be kept <20 min. For postoperative radiotherapy of advanced gastric cancer, step-and-shoot IMRT as well as tomotherapy can deliver efficient doses to target volumes while delivering dose to the kidneys in a fashion that is different from a conventional technique and is clearly advantageous in a small number of patients. An advantage for the majority of patients is likely with the normal tissue complication probability data presented in this series, but, given the uncertainty of the reaction of the kidney to inhomogeneous dose distributions, cannot be considered unequivocal at the moment. Different technical limitations apply to the different IMRT techniques. The choice of approach is therefore determined by departmental circumstances.