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Patients are seeking less invasive therapies when diagnosed with localized prostate cancer. Determination of a successful ablation of the clinical treatment volume (CTV) relies on knowledge of the new or nadir PSA (nPSA) following the ablation. We propose a novel method of determining this value.

The nPSA can be derived from the contribution of PSA from the prostatic epithelium (benign and cancer) of the treated (CTV) and the untreated volume. We calculated the contribution of PSA from the CTV by analyzing a cohort of 2861 men treated with prostate brachytherapy (PB) of which 2064 (72%) were without Phoenix failure Phoenix failure and were followed > 5 years. Delivered doses was converted to the biologically effective dose (BED) using the post-implant D90 and any external beam boost with an α/β of 2. BED levels of < 150, > 150-200 and > 200 Gy were compared to a last PSA <0.2 ng/ml by ANOVA from the PB database. Secondly, the PSA was determined for untreated non-cancerous prostate volume by analyzing another cohort of patients. 123 (55.1%) of 243 men with clinical suspicion of prostate cancer who underwent transperineal mapping biopsy and were found to have only benign pathology (no cance).  PSA values for the men with benign disease were analyzed in 4 percentile groups for their PSAD (PSA density) values. Regression analysis with a linear model was used to determine which group had the highest R² value. An equation was constructed to determine the nPSA for a patient treated with partial gland ablation.

For the first cohort, with a mean follow-up of 11.3 years (range 5-24) the last mean PSA for the non-failures were 0.14, 0.08 and 0.05 ng/ml (p<0.001) for the 3 BED groups. The percent of men with a PSA < 0.2 was 82.8, 93.1 and 96.8%, respectively. For the second cohort of patients with a benign biopsy mean age, PSA, PSAD, prostate volume (PV) and number of biopsy specimens taken were: 64.8 years, 7.6 ng/ml, 0.15, 54.6 cc and 45. Mean PSAD for the 4 percentiles ranged from 0.117-0.12 (p<0.001). The optimal PSAD (highest R²) for  men without prostate cancer was 0.12 ng/ml² (figure 1). For men receiving a partial implant to the CTV where the delivered dose had a BED of > 200 Gy (I-125 post-implant D90 > 189 Gy), the PSA contribution of the treated volume would be < 0.2 ng/ml, and the PSA contribution of the remaining benign prostate could be calculated as:  pretreatment PV (cc) – CTV (cc) x 0.12. For example, the nPSA of a recent patient with a PV of 42 cc implanted with I-125 to the CTV of 18 cc which received a D90 of 205 Gy should reach (42-18) x 0.12 = 2.88 ng/ml (figure 2).

We proposed a simple method for determining the new nadir PSA following partial gland ablation. This formula assumes the treated portion of the gland would be not produce any PSA after receiving an ablative dose of radiation. The remaining benign gland should generate a nPSA 0.12 times the untreated prostate volume.  Further validation of this concept is warranted.