Surveillance on interfacility differences in dose-prescription policy of intensity-modulated radiation therapy plans for prostate cancer

Intensity-modulated radiation therapy (IMRT) has recently become popular in Japan. Prostate cancer is indisputably one of the main targets of IMRT. However, the current status and interfacility differences in dose-prescription policies for prostate IMRT are unknown. Therefore, a nationwide survey of 43 institutions that had implemented prostate IMRT was conducted by sending a questionnaire regarding the above-mentioned issues. Thirty-three institutions (77%) had responded to the questionnaire by the end of October 2010. A total of 5245 patients with localized prostate cancer had been treated with IMRT by the end of 2009. Regular multileaf collimator-based techniques were the most common beam delivery method. Dose-prescription policies were divided into four major categories: isocenter-based (@isocenter), dose delivered to 95% of the planning target volume (PTV) (D95)-based (D95@PTV), mean dose to the PTV-based (Mean@PTV), and mean dose to the clinical target volume (CTV)-based (@CTV). The mean doses of the CTV and PTV, and the volume of the PTV receiving 95% of the dose (V95) were significantly higher with the D95@PTV policy than with the other prescription policies. Low-dose areas and hot spots were observed within the PTV in plans with @isocenter and @CTV policies. In conclusion, there are currently considerable differences among institutions in Japan regarding target doses for prostate IMRT. The D95@PTV prescription policy resulted in significant dose escalation compared with the other policies. These differences should be taken into consideration when interpreting treatment outcomes and creating multi-institutional protocols in the future.