Short-course, intensity-modulated radiotherapy for localized prostate cancer

The purpose of this study was to compare the toxicity, particularly rectal, between short-course, intensity-modulated radiotherapy (SCIM-RT) delivering 70 Gy in 28 fractions and three-dimensional conformal radiotherapy (3D-CRT) delivering 78 Gy in 39 fractions.

A total of 191 patients were treated with SCIM-RT. Seventy Gy was delivered using five intensity-modulated fields via a Varian dynamic multileaf collimator. The BAT transabdominal ultrasound system was used for localization. The comparison group consisted of 101 contemporary cases treated with 3D-CRT to 78.0 Gy (2.0 Gy per fraction). The study sample therefore comprised 292 cases. Seventy Gy in 28 fractions was equivalent to 78 Gy in 39 fractions for late-reacting tissues, according to the linear quadratic model. The median follow-up was 9 months. Radiation Therapy Oncology Group toxicity scores were used.

The rates of acute rectal Radiation Therapy Oncology Group toxicity scores 0, 1, 2, and 3 were 30%, 55%, 14%, and 0%, respectively, with SCIM-RT, versus 14%, 67%, 19%, and 0%, respectively, with 3D-CRT. The rates of acute urinary toxicity scores 0, 1, 2, and 3 were 17%, 62%, 20%, and 1%, respectively, with SCIM-RT, versus 22%, 58%, 20%, and 0%, respectively, with 3D-CRT. To date, only two patients who underwent SCIM-RT had grade 2 late urinarytoxicity. No grade 3 late urinary or rectal complications occurred with SCIM-RT. The actuarial late rectal grade 2 toxicity observed at 18 months was 10% after SCIM-RT, versus 12% after 3D-CRT. Only three patients had grade 3 late rectal toxicity; all of them had undergone 3D-CRT. A multivariate analysis of factors affecting grade 2-3 late rectal toxicity was performed by use of the following: age (continuous), race (black vs white), androgen deprivation (yes vs no), technique (SCIM-RT vs 3D-CRT), grade 2-3 acute rectal toxicity (yes vs no), and volume of rectum receiving the prescription dose (VrPr) (< or = 15 mL vs >15 mL). Only the VrPr was a significant independent factor predicting grade 2-3 late rectal toxicity. Only 15 SCIM-RT (7%) and 20 3D-CRT cases (20%) had a VrPr > 15 mL. With SCIM-RT, the grade 2-3 late rectal toxicity rate at 18 months with a VrPr > 15 mL was 29%, versus 5% with a VrPr < or = 15 mL. With 3D-CRT, the grade 2-3 late rectal toxicity rate at 18 months with a VrPr > 15 mL was 25%, versus 8% with a VrPr < or = 15 mL.

SCIM-RT, delivering 70.0 Gy at 2.5 Gy per fraction, had an acute and late toxicity profile up to 18 months after therapy that was similarto that of 3D-CRT delivering 78.0 Gy at 2.0 Gy per fraction. The grade 2 actuarial combined rectal toxicity rate is low (10%) at 18 months, although it increased when rectal volumes > 15 mL received 70 Gy with SCIM-RT. Only 7% of SCIM-RT cases received 70 Gy to > 15 ml of the rectum. If longer follow-up confirms the low late toxicity rates, SCIM-RT will be an alternative and more convenient method of dose-escalation in the treatment of localized prostate cancer.