Potential Impact of 68Ga-PSMA-11 PET/CT on the Planning of Definitive Radiation Therapy for Prostate Cancer

Abstract

Standard-of-care imaging for initial staging of prostate cancer (PCa) underestimates disease burden. Prostate-specific membrane antigen (PSMA) PET/CT detects PCa metastasis with superior accuracy, having a potential impact on the planning of definitive radiation therapy (RT) for nonmetastatic PCa. Our objectives were to determine how often definitive RT planning based on standard target volumes covers ⁶⁸Ga-PSMA-11 PET/CT-defined disease and to assess the potential impact of ⁶⁸Ga-PSMA-11 PET/CT on definitive RT planning. Methods: This was a post hoc analysis of an intention-to-treat population of 73 patients with localized PCa without prior local therapy who underwent ⁶⁸Ga-PSMA PET/CT for initial staging as part of an investigational new drug trial. Eleven of the 73 were intermediate-risk (15%), 33 were high-risk (45%), 22 were very-high-risk (30%), and 7 were N1 (9.5%). Clinical target volumes (CTVs), which included the prostate, seminal vesicles, and (in accord with the Radiation Therapy Oncology Group consensus guidelines) pelvic lymph nodes (LNs), were contoured on the CT portion of the PET/CT images by a radiation oncologist masked to the PET findings. ⁶⁸Ga-PSMA-11 PET/CT images were analyzed by a nuclear medicine physician. ⁶⁸Ga-PSMA-11-positive lesions not covered by planning volumes based on the CTVs were considered to have a major potential impact on treatment planning. Results: All patients had one or more ⁶⁸Ga-PSMA-11-positive primary prostate lesions. Twenty-five (34%) and 7 (9.5%) of the 73 patients had ⁶⁸Ga-PSMA-11-positive pelvic LN and distant metastases, respectively. The sites of LN metastases in decreasing order of frequency were external iliac (20.5%), common iliac (13.5%), internal iliac (12.5%) obturator (12.5%), perirectal (4%), abdominal (4%), upper diaphragm (4%), and presacral (1.5%). The median size of the LN lesions was 6 mm (range, 4-24 mm). RT planning based on the CTVs covered 69 (94.5%) of the 73 primary lesions and 20 (80%) of the 25 pelvic LN lesions, on a per-patient analysis. Conclusion:⁶⁸Ga-PSMA-11 PET/CT had a major impact on intended definitive RT planning for PCa in 12 (16.5%) of the 73 patients whose RT fields covered the prostate, seminal vesicles, and pelvic LNs and in 25 (37%) of the 66 patients whose RT fields covered the prostate and seminal vesicles but not the pelvic LNs.

Keywords: PET/CT; PSMA; definitive radiotherapy; initial staging; prostate cancer.

FIGURE 1.

Axial CT views of prostate CTV (yellow) and of pelvic LN and seminal vesicle CTV (green). CTVs were contoured on CT dataset of PET/CT for all 73 patients by experienced radiation oncologist who was masked to ⁶⁸Ga-PSMA-11 PET findings. Pelvic LN CTV included presacral, distal common iliac, internal iliac, external iliac, and obturator LNs (upper limit, L4/L5).

FIGURE 2.

(Left) Three-dimensional rendering of all ⁶⁸Ga-PSMA-11–positive lesions (yellow) in patients with extraprostatic metastasis: 20 N1M0 lesions (5 with out-of-field positive lesions), 3 N1M1a lesions, 2 N0M1b lesions, 1 N1M1aM1b lesion, and 1 N1M1bM1c lesion. (Right) Three-dimensional rendering of targeted volumes for prostate (yellow) and for pelvic LN plus seminal vesicles (green).

FIGURE 3.

Examples of ⁶⁸Ga-PSMA-11–positive disease within radiation fields on axial CT (top), PET (middle), and PET/CT (bottom). Once positive lesions were identified on PET, contours of prostate CTV (yellow) and pelvic LN CTV (green) were drawn on the basis of CT. (A) Primary prostate tumor (MTV, 4 cm³; SUV_max, 34.6). (B) Invaded seminal vesicles (SUV_max, 18.0). (C) Right obturator LN (short axis, 6 mm; SUV_max, 4.6). (D) Left external iliac LN (short axis, 7 mm; SUV_max, 22.3). (E) Left common iliac LN (short axis, 5 mm; SUV_max, 4.1).

FIGURE 4.

Examples of ⁶⁸Ga-PSMA-11–positive disease outside radiation fields on CT (top), PET (middle), and PET/CT (bottom). Once positive lesions were identified on PET, contours of prostate CTV (yellow) and pelvic LN CTV (green) were drawn on the basis of CT. (A) Primary prostate tumor (MTV, 3 cm³; SUV_max, 12) without CT correlate, located more than 1 cm below CTV. (B) Right perirectal LN (short axis, 8 mm; SUV_max, 6.1). (C) Multiple abdominal LNs (short axis, 4–7 mm; SUV_max, 4.7–17.2). (D) Multiple left subclavicular LNs (short axis, 3–4 mm; SUV_max, 3.0–9.1). (E) Sacral bone metastasis without CT correlate (SUV_max, 8.4). (F) Left lung nodule (short axis, 7 mm; SUV_max, 1.5).