Comparison of dosimetric parameters and acute toxicity after whole-pelvic vs prostate-only volumetric-modulated arc therapy with daily image guidance for prostate cancer

Objective: To compare dosimetric parameters and acute toxicity rates between whole-pelvic (WP) and prostate-only (PO) volumetric-modulated arc therapy (VMAT) in patients with localized prostate cancer. Methods: A total of 224 consecutive patients treated with definitive VMAT to 78 Gy in 39 fractions were enrolled. Of these, 119 patients received initial WP VMAT at 46.8 Gy in 26 fractions using a simultaneous integrated boost technique, and 105 patients received PO VMAT. Image-guided radiotherapy was practised with daily cone beam CT. Results: The mean rectal dose, the rectal volume receiving >= 30 Gy (V-30Gy), rectal V-50Gy, the mean bladder dose, bladder V-30Gy and bladder V-50Gy were significantly increased in the WP group (p < 0.05 each); however, the rectal V-70Gy did not differ between groups (p=0.101), and the bladder V-70Gy was significantly lower in the WP group (p=0.029). The WP group experienced a significantly increased frequency of acute grade 2 diarrhoea relative to the PO group (5.9% vs 0%; p=0.015). No differences were seen between the WP and PO groups in terms of acute grade 2 proctitis (10.1% vs 6.7%; p=0.360) and genitourinary (GU) toxicity (12.6% vs 10.5%; p=0.620). Conclusion: Despite larger rectum and bladder volumes at low-and medium-dose levels, WP VMAT resulted in no significant increase in acute proctitis or GU toxicity when compared with PO VMAT. Advances in knowledge: This study demonstrates that whole-pelvic radiotherapy has comparable acute toxicity to those observed with prostate-only radiotherapy when VMAT with daily image guidance is used.