Hypofractionation with VMAT versus 3DCRT in post-operative patients with prostate cancer

Aims: To retrospectively evaluate and compare the incidence of acute genitourinary (aGU), upper gastrointestinal (uGI) and rectal (lGI) injuries after radiotherapy with hypo-fractionation by volumetric modulation arc therapy (VMAT, the Hypo-RapidArc group) and conventional fractionation by three-dimensional conformal radiotherapy (3DCRT) in patients with localized prostate cancer treated, after radical prostatectomy, with prostatic bed irradiation.

Patients and methods: Between 2007 and 2012, 84 consecutive patients with clinically localized prostate cancer submitted to radical prostatectomy were also treated with irradiation to the prostate bed. Forty-five received 3DCRT and 39 Hypo-RapidArc. The median age was 67 and 69 years for 3DCRT and Hypo-RapidArc groups respectively. The median dose to the prostatic bed was 70 Gy in both groups: 2 Gy/fraction in the 3DCRT group and 2.5 Gy/fraction in the Hypo-RapidArc group. After radical prostatectomy, the median time-to-RT was 15 and 16 months respectively. Acute and late toxicities were scored according to the Radiation Therapy and Oncology Group/European Organization for Research and Treatment of Cancer system.

Results: Grade 2aGU was recorded in 16% of cases in the 3DRCT group and in 10% in the Hypo-RapidArc group. No acute grade 2 upper gastrointestinal (uGI) toxicities were found in the 3DCRT versus 5% in the Hypo-RapidArc group. The incidence of grade 2 lower gastrointestinal (lGI) toxicities was 22% in the 3DCRT group versus 15% in the Hypo-RapidArc group. No grade 3 or greater toxicities were found in either group. In both groups, good planning target volume coverage was achieved: V95% was recorded as 96.3 ± 3.6% (mean ± standard deviation) and 95.7 ± 8.9 for the 3DRCT and the Hypo-RapidArc groups, respectively. The mean rectal volume dose receiving at least 70 Gy was 9.1 ± 10.8% and 0.1 ± 0.6% respectively. The mean dose to the bladder was 49.5 ± 12.3 Gy and 37.2 ± 5.2 Gy respectively. Significant correlation between late rectal toxicity and the maximum dose to the rectum, V70Gy, was found in the 3DCRT group, while no significant correlations were found for acute toxicity.

Conclusion: The results presented in this study demonstrate the feasibility of a moderate hypo-fractionation regimen with RapidArc in the postoperative setting. Longer-term data are needed to confirm late toxicity profiles.