Radiation dose to rectum in high-dose-rate brachytherapy with a single implant and two fractions for prostate cancer, and its prediction by prostate volume, "Beyond the Abstract," by Morio Sato, MD

BERKELEY, CA (UroToday.com) - High-dose-rate (HDR) brachytherapy for prostate cancer is supplied by two or three implants with a single radiation fraction or by a single implant with one to eight fractions. The incidence of urethral stricture is higher in patients with multiple implants with a single fraction than in those with a single implant with multiple fractions. A greater number of implants seems to be correlated with a higher incidence of urethral stricture. However, the incidence of rectal bleeding is higher after a single implant with multiple fractions than after multiple implantations with a single fraction.

Rectal bleeding is caused by vascular enlargement in the mucous membrane, erosion, or ulceration of the anterior wall of the rectum. It is unclear whether allowing the applicator needle to remain increases the radiation dose to the rectum. We measured the radiation dose to the rectum each time patients underwent HDR brachytherapy Namely, we measured the maximum radiation dose to the rectum during the first and second fractions of HDR brachytherapy. However, evaluating the dose to the rectum with a rectal dosimeter is difficult, with a risk of rectal perforation.

We attempted to clarify the differences between the estimated rectal dose (ERD) and the first measured dose (FMD) and second measured dose (SMD) to the rectum during high dose-rate (HDR) brachytherapy, and to predict FMD from the prostate volume (PV) or the rectal dose–volume parameters (RDVPs). ERD, FMD, and SMD were assessed with a rectal dosimeter during HDR brachytherapy of 18 Gy given in two fractions (5 hours interval) to 110 patients (48 hormone recipients, 62 hormone-naïve patients) with prostate cancer. The correlations between FMD and PV, and between FMD and RDVP (D2ml–D5ml) were investigated.

ERD (mean ± SD) was 219 ± 44 cGy, FMD was 255 ± 52 cGy, and SMD was 298 ± 63 cGy, which differed significantly (p < 0.001). The correlation coefficients between ERD and FMD, and between FMD and SMD, were 0.82 and 0.78, respectively. SMD was equivalent to 118 ± 16% FMD. The measured doses were significantly greater in the hormone recipients than in the hormone-naïve patients (p < 0.001). The increase in FMD correlated with the increases in PV and in RDVPs. The correlation coefficients between PV and FMD in all of the patients, in the hormone recipients, and in the hormone-naïve patients, were 0.61, 0.64, and 0.64, respectively, whereas that between RDVPs and FMD was < 0.53.

In the present study, significant increases were observed in the first and second measured doses to the rectum compared with the estimated rectal dose. Ishikawa et al. reported that the thickness of the anterior rectal wall before the second HDR brachytherapy session was reduced significantly compared with that before the first because of edema and/or hematoma of the prostate after it was pierced by the applicator needle. Taking into consideration the recovery of the normal prostate gland and enhancement of the anti-tumor effect, the interval between the first and the second HDR brachytherapy was at least 5 h. The interval between the completion of implantation and the first HDR brachytherapy was 1 h. This means that, as time passed, the applicator needles piercing the prostate moved closer to the rectal dosimeter, thus increasing the second measured rectal dose. The measured dose to the rectum increased more in the hormone-naïve patients than in the hormone-recipient patients, and the increase in the measured dose to the rectum was correlated with the increase in the prostate volume. These results also imply that larger prostates approach the anterior rectal wall more closely, causing an increase in the dose to the rectum. From the point of view of lessening the radiation exposure to the rectum, a lesser time with the pierced needle is more favorable.

The first and the second measured dose to the rectum were equivalent to 117% and 138% of the estimated dose, respectively, and the second measured rectal dose was equivalent to 118% of the first measured rectal dose. Furthermore, the prostate volume was correlated positively with the increase in the first measured rectal dose in all of the patients, irrespective of whether they were hormone-naïve or were hormone recipients. Therefore, the first and the second dose to the rectum can be predicted from the prostate volume, without the insertion of a rectal dosimeter.

In conclusion, the dose to the rectum increased with time and was correlated with the increases in PV and RDVPs. The correlation coefficient between FMD and PV was greater than that between FMD and RDVPs.

Written by:
Morio Sato, MD as part of Beyond the Abstract on UroToday.com. This initiative offers a method of publishing for the professional urology community. Authors are given an opportunity to expand on the circumstances, limitations etc... of their research by referencing the published abstract.

Department of Radiology, Wakayama Medical University, 811-1 Kimiidera, Wakayama Shi, Wakayama, 641-8510, Japan

Radiation dose to rectum in high-dose-rate brachytherapy with a single implant and two fractions for prostate cancer, and its prediction by prostate volume - Abstract

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