A 4D CT Analysis of Techniques for Gastric Irradiation

Purpose/Objective: Gastric irradiation is indicated in patients with gastric lymphoma, and is currently being investigated as part of induction chemo-radiotherapy in operable gastric cancer RTOG 99–04; Ajani 2005. Organs in the upper abdomen can exhibit considerable respiration-induced mobility, thereby resulting in inadequate target coverage and/or unreliable estimates of radiation exposure to normal tissues. Respiration-correlated (or 4D) CT scans allow for the individualized evaluation of intra-fractional mobility. The aims of this study were to (1) assess the adequacy of standard fields recommended for gastric irradiation, (2) determine the renal doses when adequate target coverage is achieved, and (3) evaluate the role of respiration-gated radiotherapy.Materials/Methods: A retrospective analysis was performed on 4DCT datasets of 5 patients who underwent abdominal radiotherapy. The entire stomach, duodenal C-loop (to ensure coverage of regional nodal stations) and both kidneys were contoured in all 10 phases (bins) of the 4DCT in Eclipse version 6.5 (Varian Medical Systems). Subsequently two gating windows were identified, each comprising 3 consecutive bins in end-expiration and end-inspiration. The separate gastro-duodenal structures of each gating window were encompassed by an internal target volume (ITV); resulting in an expiratory ITVexp, an inspiratory ITVinsp and an ITVall phases, representing the position in all 4D phases. Renal positions were likewise contoured to generate composite renal volumes for the corresponding respiratory phases. A symmetric margin of 1 cm was added to all ITVs in order to generate planning target volumes (PTVs). A virtual simulation, in which the PTV and renal volumes were not visualized, was performed using digitally reconstructed radiographs to set up an anterior-posterior field arrangement as specified in the RTOG 99—04 protocol. The superior and inferior field borders were positioned at the Th8—9 and L3—4 interspaces, the right border 4 cm lateral to the vertebral body and the left border was to include ¾ of the left hemidiaphragm. Then, the PTVall phases and corresponding renal volumes were visualized in order to assess the adequacy of the RTOG fields. Next, 3D planning was performed in order to optimize target coverage and to minimize the renal volume receiving 20 Gy. The planned dose was 45 Gy to the isocenter in 1.8 Gy fractions, and plans were calculated for all three PTVs.Results: The standard fields recommended in RTOG 99—04 did not fully cover the PTV in any case. CT planning for PTVall phases optimized target coverage, but resulted in a mean dose of 19.9 Gy to the left renal volume (range 9.8—34 Gy) and 29.0 Gy to the right renal volume (range 23.8—42.8 Gy). Gated radiotherapy would have resulted in clinically insignificant dose reductions in comparison to non-gated radiotherapy, a finding accounted for by parallel movements of the stomach and kidneys during quiet respiration. Mean renal doses were similar for irradiation at either end-inspiration or end-expiration. However, the gating PTVs were up to 11% smaller, which could decrease the volume of irradiated bowel. Results of respiration-gated IMRT planning for these patients will be available at the time of the meeting.Conclusions: This 4DCT analysis indicates that individualized field margins are necessary for gastric irradiation. Conventional 3D planning can result in significant renal irradiation, and 4D gated radiotherapy reduces mean renal doses by only 10%. Respiration-gated IMRT may further reduce renal doses. Purpose/Objective: Gastric irradiation is indicated in patients with gastric lymphoma, and is currently being investigated as part of induction chemo-radiotherapy in operable gastric cancer RTOG 99–04; Ajani 2005. Organs in the upper abdomen can exhibit considerable respiration-induced mobility, thereby resulting in inadequate target coverage and/or unreliable estimates of radiation exposure to normal tissues. Respiration-correlated (or 4D) CT scans allow for the individualized evaluation of intra-fractional mobility. The aims of this study were to (1) assess the adequacy of standard fields recommended for gastric irradiation, (2) determine the renal doses when adequate target coverage is achieved, and (3) evaluate the role of respiration-gated radiotherapy. Materials/Methods: A retrospective analysis was performed on 4DCT datasets of 5 patients who underwent abdominal radiotherapy. The entire stomach, duodenal C-loop (to ensure coverage of regional nodal stations) and both kidneys were contoured in all 10 phases (bins) of the 4DCT in Eclipse version 6.5 (Varian Medical Systems). Subsequently two gating windows were identified, each comprising 3 consecutive bins in end-expiration and end-inspiration. The separate gastro-duodenal structures of each gating window were encompassed by an internal target volume (ITV); resulting in an expiratory ITVexp, an inspiratory ITVinsp and an ITVall phases, representing the position in all 4D phases. Renal positions were likewise contoured to generate composite renal volumes for the corresponding respiratory phases. A symmetric margin of 1 cm was added to all ITVs in order to generate planning target volumes (PTVs). A virtual simulation, in which the PTV and renal volumes were not visualized, was performed using digitally reconstructed radiographs to set up an anterior-posterior field arrangement as specified in the RTOG 99—04 protocol. The superior and inferior field borders were positioned at the Th8—9 and L3—4 interspaces, the right border 4 cm lateral to the vertebral body and the left border was to include ¾ of the left hemidiaphragm. Then, the PTVall phases and corresponding renal volumes were visualized in order to assess the adequacy of the RTOG fields. Next, 3D planning was performed in order to optimize target coverage and to minimize the renal volume receiving 20 Gy. The planned dose was 45 Gy to the isocenter in 1.8 Gy fractions, and plans were calculated for all three PTVs. Results: The standard fields recommended in RTOG 99—04 did not fully cover the PTV in any case. CT planning for PTVall phases optimized target coverage, but resulted in a mean dose of 19.9 Gy to the left renal volume (range 9.8—34 Gy) and 29.0 Gy to the right renal volume (range 23.8—42.8 Gy). Gated radiotherapy would have resulted in clinically insignificant dose reductions in comparison to non-gated radiotherapy, a finding accounted for by parallel movements of the stomach and kidneys during quiet respiration. Mean renal doses were similar for irradiation at either end-inspiration or end-expiration. However, the gating PTVs were up to 11% smaller, which could decrease the volume of irradiated bowel. Results of respiration-gated IMRT planning for these patients will be available at the time of the meeting. Conclusions: This 4DCT analysis indicates that individualized field margins are necessary for gastric irradiation. Conventional 3D planning can result in significant renal irradiation, and 4D gated radiotherapy reduces mean renal doses by only 10%. Respiration-gated IMRT may further reduce renal doses.