ASTRO 2022: PSMA PET/CT–Based Atlas for Prostatic Bed Recurrence of Prostate Cancer after Radical Prostatectomy: Clinical Implications for Salvage Radiation Therapy

(UroToday.com) The 2022 ASTRO annual meeting featured a session on new insights into post-prostatectomy radiotherapy, including a presentation by Dr. Alan Dal Pra discussing PSMA PET/CT–based atlas for prostatic bed recurrence of prostate cancer after radical prostatectomy. Postoperative radiotherapy to the prostate bed is a potentially curative treatment after radical prostatectomy. The prostate bed target definition follows contouring guidelines mostly based on expert consensus and does not account for anatomical patterns of recurrence on more contemporary imaging. At ASTRO 2022, Dr. Dal Pra and colleagues presented their analysis of the patterns of recurrence in the prostate bed of prostate cancer patients with biochemical recurrence after radical prostatectomy using 68Ga-PSMA-11 PET/CT (PSMA PET). They also evaluated patterns of PSMA PET recurrence in relation to the RTOG-based clinical target volumes definition.

 Patients with biochemically recurrent prostate cancer after radical prostatectomy, without previous history of prostate bed radiotherapy, were retrospectively included if their PSMA PET showed evidence of recurrence in the prostate bed. Two nuclear medicine physicians manually delineated the prostate bed lesions on the CT images of the PSMA PET/CT. Four radiation oncologists, masked to the PSMA PET component of the PET/CT study and to the PSMA-based delineations, contoured the RTOG-based clinical target volumes on the same CT. Three-dimensional heat maps of the PSMA PET-positive prostate bed lesions were generated to visually depict patterns of PSMA PET recurrences. The coverage of the PSMA PET recurrence by the RTOG-based clinical target volumes was categorized in consensus by a nuclear medicine physician and a radiation oncologist as follows: completely covered by clinical target volume, partly covered by clinical target volume, or not covered by clinical target volume:

 

ASTRO 2022_Alan Dal Pra_0 

 

 There were 2,451 PSMA PET scans performed at UCLA between November 2016 and November 2020 screened for inclusion criteria. Ultimately, there were 226/2,451 (9%) scans of patients with prostate bed recurrence on PSMA PET included; 127 (56%) patients had miTrN0M0, 30 (13%) miTrN1M0, 32 (14%) miTrN0M1, and 37 (16%) miTrN1M:

 

ASTRO 2022_Alan Dal Pra_1 

 

The patient population stratified by disease stage is as follows:

 

ASTRO 2022_Alan Dal Pra_2 

 

In the miTrN0M0 cohort, PSMA-positive recurrences were fully covered by the clinical target volume in 68/127 (54%) of the patients, partly covered in 43/127 (33%), and fully outside in 16/127 (13%). Lesions with partial coverage or fully outside the clinical target volume were located at the posterior border in 51%, postero-lateral in 24%, posteroinferior in 5%, anterior in 1%, anteroinferior in 1%, superior in 2%, and inferior in 17% of patients:

 

ASTRO 2022_Alan Dal Pra_3 

 

The median PSA levels were 1.11 ng/mL (IQR 1.7), 1.10 ng/mL (IQR 2.17) and 0.84 ng/mL (IQR 0.49), and the volumes of PSMA PET recurrences were 0.57 cc (IQR 0.77), 1.01 cc (IQR 1.71) and 0.68 cc (IQR 0.59) in patients with recurrences fully within, partially covered and fully outside the clinical target volume, respectively. The clinical target volume coverage was associated with tumor volume (p = 0.04), rectal/bladder wall involvement on imaging (p = 0.001), and miTNM (p = 0.04). The clinical target volume coverage status was not associated with serum PSA levels (p = 0.98).

While 13% of the PSMA PET recurrences in the fossa were fully outside the RTOG-based clinical target volume, 87% were completely or partially covered. Several potential limitations of this study were (i) potential volume overestimation of the recurrent lesion on PSMA-PET (spillover effect), (ii) possible inaccuracy in image registration, (iii) lack of MRI correlation, and (iv) limited pathology data. This data suggests that new contouring guidelines should consider possible clinical target volume expansion at the postero-lateral (recto prostatic angles) and inferior borders, and reduction of the clinical target volume antero-superiorly (next to the pubic bone).

 

Dr. Dal Pra concluded this presentation discussing PSMA PET/CT–based atlas for prostatic bed recurrence of prostate cancer after radical prostatectomy with the following take home messages:

  • In biochemically recurrent patients with disease limited to the prostatic fossa on PSMA PET (miTrN0M0), the RTOG contouring guidelines fully or partially covered the extent of disease in 87% of the cases, while 13% were entirely outside the clinical target volume
  • PSMA PET can be a valuable tool for salvage radiotherapy planning and should be incorporated into a redefinition of current contouring guidelines

 

Presented by: Alan Dal Pra, MD, Department of Radiation Oncology, University of Miami/Sylvester Comprehensive Cancer Center, Miami, FL

Co-Authors: I. Sonni,1,2 D. O'Connell4, M. Benz2, K. Nguyen2, S. Yoon5, J. Deng5, C. P. Smith6, N. G. Nickols7, M. Cao5, A. U. Kishan5, and J. Calais21Department of Radiological Sciences, Los Angeles, CA, 2Ahmanson Translational Theranostics Division, UCLA Nuclear Medicine, Los Angeles, CA, 3Department of Radiation Oncology, University of Miami/Sylvester Comprehensive Cancer Center, Miami, FL, 4University of California, Los Angeles, Los Angeles, CA, 5Department of Radiation Oncology, University of California, Los Angeles, Los Angeles, CA, 6NIH/NCI, Bethesda, MD, 7University of California Los Angeles, Department of Radiation Oncology, Los Angeles, CA