Comparison of Volumetric Modulated Arc Therapy (VMAT) and Standard IMRT: GI and GU Toxicity in the Pelvic Nodal Treatment of High-Risk Prostate Cancer

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The irradiation of the pelvic nodes in prostate cancer has been discussed controversially for many years. Since the frequency of the gastrointestinal toxicity in patients receiving 4-field conventional or conformal whole pelvis radiation therapy is relevant, the indication for pelvic nodal irradiation is to be considered with caution. The aim of this study was to compare the toxicity in patients with high risk prostate cancer treated with an optimized pelvic Sliding window (SW) to the VMAT technique. Between September 2005 and September 2012, pelvic nodal IMRT to 50.4 Gy had been applied in 229 high-risk prostate cancer patients (pts). In 24% of the pts, an additional boost of 6 to 16 Gy was administered in cases of MRI-staged lymph node metastases. While 123 pts received a 7 or 8-field-IMRT sliding-window technique up to a median total dose to the prostate of 79.2 Gy/1.8 Gy, 108 pts were treated with VMAT technique up to a median total dose to the prostate of 81.0 Gy/1.8 Gy. Acute and late toxicities were prospectively scored by the RTOG/LENT-SOMA morbidity grading scales (until 2009) and a modified CTCAEv3.0 score (since 2009), respectively. The median follow- up time was 48 months (range, 3-87 months). The IMRT dose distribution provided excellent PTV coverage and satisfying protection of all the organs at risk, with less than 2% of all pts experiencing grade (G)3 toxicities but no G4 toxicities were observed. In total 41.8%/4.2%/0.5% of the pts developed acute G1/G2/G3 genitourinary (GU) toxicities. A total of 42.7%/3.7% of pts experienced acute G1/G2 gastrointestinal side effects, no patient developed acute >G2 gastrointestinal symptoms. Maximum late GU toxicities were G1/G2/G3 for 2.4%/0%/0.9% of the pts, respectively. Maximum late gastrointestinal toxicities were G1 for 4.9% of the pts, no patient experienced acute >G1 gastrointestinal symptoms. For acute GU and gastrointestinal toxicity, no significant differences could be seen between pts treated with SW vs VMAT: acute G1/G2/G3 GU toxicities have occurred in 40.4%/5.5%/0% vs 43.4%/2.8%/0.9% of the pts. A total of 44.3%/4.7% of the SW patients developed acute G1/G2 gastrointestinal toxicities compared to 42.4%/4.7% VMAT pts. Maximum late GU toxicities were G1/G2 for 3.2%/0% of SW pts and 4.8%/0% of RA pts, respectively. A total of 1.9% of the SW pts developed late G3 GU toxicities compared to no pts in the VMAT group. Maximum late gastrointestinal toxicities were G1 for 7.6% of SW and VMAT pts, no late >G1 GU symptoms could be observed in either group so far. These data demonstrate that pelvic nodal IMRT is a well-tolerated technique in patients with high-risk prostate cancer. The overall low frequency and severity of side effects do not seem to differ significantly in SW and VMAT technique despite a further dose escalation in the VMAT group by 3.5%.