Genitourinary Cancer

PV QA 1 - Poster Viewing Q&A 1

SU_23_2228 - Phase I trial of SBRT to the prostate bed following prostatectomy

Sunday, October 21
1:15 PM - 2:45 PM
Location: Innovation Hub, Exhibit Hall 3

Phase I trial of SBRT to the prostate bed following prostatectomy
L. K. Ballas1,2, C. Luo2, M. Aron2, I. Shuryak3, D. I. Quinn2, S. Jhimlee2, R. Chiu2, D. J. Brenner3, and S. Groshen2; 1University of Southern California Keck School of Medicine, Department of Radiation Oncology, Los Angeles, CA, 2University of Southern California Keck School of Medicine, Los Angeles, CA, 3Center for Radiological Research, Columbia University Medical Center, New York, NY

Purpose/Objective(s): The primary objective was to evaluate acute toxicity (within 10 weeks) associated with increasing hypofractionation to the prostate bed (PB) in post-prostatectomy prostate cancer patients (pts). We hypothesized that escalating the dose per fraction (fx) to the PB in the post-operative setting would have an acceptable toxicity profile.

Materials/Methods: Tested dose levels (DL) had a biologically effective dose (BED) equivalent to that used in a previously published hypofractionated post-prostatectomy study demonstrating no increase in acute bowel or bladder toxicity above standard fractionation and were as follows: Level 1 -3.6 Gy x 15 fx; Level 2- 4.7 Gy x 10 fx; Level 3 - 7.1 Gy x 5 fx. Escalation followed a 6+6 rules-based design: 6 pts were enrolled on each new DL; 12 pts were required at the maximum tolerated dose (MTD). Dose-limiting toxicity (DLT) was defined as Grade (G) ≥3, gastrointestinal (GI) or genitourinary (GU) toxicity by NCI CTCAE v4.03. Fiducial markers were placed in the PB prior to simulation. Treated bladder volume was assessed daily and within 10% of simulation volume; pts were given bowel prep instructions and a rectal balloon was placed for simulation and treatment. Pts completed quality-of-life questionnaires at pre-defined intervals.

Results: 24 pts with indication for adjuvant or salvage radiotherapy (RT) were enrolled (6 at DLs 1 and 2; 12 at DL 3); all evaluable. Median follow-up was 9.6 months. Median age was 66 years. Median PSA at time of enrollment was 0.06 ng/mL (range <0.03– 0.36 ng/mL). Median time from prostatectomy to enrollment was 8.5 months. 4 pts received concurrent ADT. No G ≥3 GI or GU toxicity was seen at any dose level. GI toxicities and changes in GU toxicity over baseline are summarized (Table 1). 12/24 pts experienced G2 GI toxicity: 2/6 in DL1, 3/6 in DL2 and 7/12 in DL3. All acute G2 GI toxicity resolved. 3/24 patients reported an increase to G1 and G2 GU toxicity following RT in DL1 and DL 2. No patients in DL 3 had increased GU toxicity. At week 2 post-RT, pts reported a decline in the EPIC-26 bowel domain that met criteria for minimally important difference (MID). Rectal QOL declines improved in DLs 1 and 2 by week 10. IPSS scores declined at 2 weeks post-treatment, but improved by 6-10 weeks post-treatment.

Conclusion: Dose escalation up to 7.1Gy x 5 fractions was completed without G ≥3 toxicity. There was transient G2 rectal toxicity in all dose levels and so the benefit of hypofractionated RT and SBRT should be weighed against this toxicity. Longer follow-up is required to assess late toxicity and disease control. Table 1: Maximum Toxicity by 10 weeks Attributable to RT (number of patients)

3.6Gy x 15 Fractions

(n=6)

4.7Gy x 10 Fractions

(n=6)

7.1Gy x 5 Fractions

(n=12)

Maximum Increase in Acute GU Toxicity

Grade 0 to Grade 1

0

0

0

Grade 1 to Grade 2

2

1

0

Maximum Acute GI Toxicity

1

1

2

2

2

2

3

7

Author Disclosure: L.K. Ballas: None. C. Luo: None. M. Aron: Employee; USC. I. Shuryak: None. D.I. Quinn: Employee; Kasier Permanente. R. Chiu: None. D.J. Brenner: None.

Leslie Ballas, MD

University of Southern California Keck School of Medicine

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