Screen: 8
Patrick Courtney, MD, MAS
UCLA David Geffen School of Medicine/UCLA Medical Center
Los Angeles, CA
Dosimetric Comparison of Margin-Reduced CT-Guided with MRI-Guided Stereotactic Body Radiotherapy for Localized Prostate Cancer
Purpose/Objective(s): The MIRAGE trial (NCT 04384770) reported an acute toxicity benefit for patients with localized prostate cancer randomized to MRI-guided (MRgRT) compared with CT-guided (CTgRT) stereotactic body radiotherapy. This was attributed to PTV margin reduction from 4mm to 2mm enabled by improved soft tissue resolution during planning and real-time target motion tracking with MRgRT. However, there are clinical and planning limitations with MRgRT, and smaller CTgRT PTV margins may be feasible with triggered imaging and auto beam hold. We evaluated the dosimetric impact of reducing PTV margins in CTgRT compared with MRgRT patients in MIRAGE.
Materials/
Methods: 16 CTgRT patients were retrospectively re-planned with 2mm and 3mm prostate PTV margins from 4mm per trial protocol. In patients receiving elective pelvic nodal irradiation (EPNI), nodal PTV margins were 4mm in both groups. We measured target, rectum, bladder, and small bowel dosimetric parameters and compared them in 16 similar MRgRT patients with Wilcoxon rank sum tests.
Results: In each group, 10 patients received 40 Gy in 5 fractions to the prostate only, and 6 patients received 25 Gy in 5 fractions EPNI with a simultaneous integrated boost (SIB) to 40 Gy to the prostate; 5 and 3 of these patients, respectively, received a SIB to 42 Gy gross prostate tumor. 7 and 9 patients had spacing gel and 2 and 1 patients had unilateral hip implants in the CTgRT and MRgRT arms, respectively. Dosimetric analysis is reported in the table.
Conclusion: Reducing prostate PTV margins from 4mm in CTgRT resulted in similar to superior target and organs-at-risk dosimetry compared with 2mm MRgRT prostate PTV margins, even with larger target volumes. Prospective evaluation is required to validate the safety and efficacy of prostate PTV margin reduction for patients undergoing CTgRT for localized prostate cancer.
Abstract 3152 – Table 1
1Median (IQR).
2Among EPNI patients.
*P-value <.05 by Wilcoxon rank sum test compared with MRgRT.
| Parameter1 | Trial-specified CTgRT 4mm PTV | Re-planned CTgRT 3mm PTV | Re-planned CTgRT 2mm PTV | Trial-specified MRgRT 2mm PTV |
| PTV |
|
|
|
|
| 40 Gy Volume (mL) | 99.4 (83 - 121)* | 86.1 (72 - 107)* | 75.2 (61 - 95) | 68.8 (54 - 76) |
| 25 Gy Volume (mL)2 | 632 (563 - 667) | 632 (563 - 667) | 632 (563 - 667) | 501 (491 - 544) |
| V40Gy (%) | 95.0 (95 - 95) | 95.0 (95 - 95) | 95.0 (95 - 95) | 95.0 (94.5 - 95) |
| V25Gy (%)2 | 97.9 (97 - 98)* | 97.0 (96 - 98)* | 96.1 (95 - 97)* | 87.7 (82 - 90) |
| Rectum |
|
|
|
|
| V40Gy (%) | 0.1 (0 - 0.1) | 0.04 (0 - 0.2) | 0 (0 - 0.1) | 0.1 (0 - 0.7) |
| V38Gy (mL) | 1.2 (0.4 - 2) | 0.7 (0.3 - 2) | 0.1 (0 - 0.8) | 0.45 (0 – 1.5) |
| V36Gy (%) | 3.0 (1 - 5) | 1.9 (0.6 - 3) | 0.7 (0 - 2) | 1.0 (0.1 - 3) |
| V32Gy (%) | 5.2 (3 - 8) | 3.9 (1 - 5) | 1.5 (0.4 - 4) | 2.7 (0.7 - 6) |
| V20Gy (%) | 14.7 (9 - 20) | 15.6 (9 - 23) | 10.2 (4 - 16) | 16.8 (9 - 26) |
| Bladder |
|
|
|
|
| V40Gy (%) | 0.9 (0.4 - 2) | 0.5 (0.2 - 0.7) | 0.2 (0 - 0.4) | 0.3 (0.1 - 0.4) |
| V39Gy (mL) | 4.9 (3 - 7) | 2.9 (2 - 5) | 1.5 (0.6 - 2) | 2.2 (1 - 3) |
| V20Gy (%) | 15.3 (7 - 29) | 11.9 (6 - 17) | 9.5 (5 - 16) | 15.4 (7 - 24) |
| Small Bowel2 |
|
|
|
|
| V20cc (mL) | 24.7 (8 - 29) | 20.8 (8 - 27) | 18.1 (15 - 26) | 26.0 (24 - 29) |