Clinical effect of multileaf collimator width on the incidence of late rectal bleeding after high-dose intensity-modulated radiotherapy for localized prostate carcinoma.

Several studies have confirmed a dosimetric advantage associated with use of a smaller leaf in intensity-modulated radiation therapy (IMRT). However, no studies have identified any clinical benefits.

We investigated the effect of a smaller multileaf collimator (MLC) width on the onset of late rectal bleeding after high-dose prostate IMRT.

Two hundred and five prostate cancer patients were treated with a total dose of 78 Gy in 39 fractions by use of a dynamic MLC technique; however, two different MLC were used: a 10-mm-wide device and a 5-mm-wide device. Gastrointestinal toxicity and several clinical factors were assessed.

The 5-year actuarial risk of grade 2 or higher rectal bleeding was 6. 9 % for the 10-mm-wide group (n = 132) and 1. 8 % for the 5-mm-wide group (n = 73) (p = 0. 04). The median estimated rectal doses for the two groups were 55. 1 and 50. 6 Gy (p < 0. 001), respectively. Univariate analysis showed that acute toxicity, rectal V30-60, median rectal dose, normal tissue complication probability (NTCP), and MLC type were significant predictive factors for late rectal toxicity. In multivariate analysis, acute toxicity and NTCP remained significant.

In our planning approach for prostate IMRT, a decrease in MLC width from 10 to 5 mm contributed to further rectal dose reduction, which was the most important predictor of late rectal toxicity.

International journal of clinical oncology. 2015 Jul 12 [Epub ahead of print]

Haruo Inokuchi, Takashi Mizowaki, Yoshiki Norihisa, Kenji Takayama, Itaru Ikeda, Kiyonao Nakamura, Mitsuhiro Nakamura, Masahiro Hiraoka

Department of Radiation Oncology and Image-Applied Therapy, Kyoto University Graduate School of Medicine, 54 Shogoin Kawahara-cho, Sakyo-ku, Kyoto, 606-8507, Japan.