Automatic Prediction of Metastasis-Free Survival (MFS) Using Prostate-Specific Membrane Antigen (PSMA) PET for Oligometastatic Castration-Sensitive Prostate Cancer (omCSPC)

PURPOSE/OBJECTIVE(S)

Recent data have shown the promise of metastasis-directed therapy (MDT) to improve outcomes in omCSPC patients. Response biomarkers are still needed to identify patients at an early time point who will not respond to the treatment. We have shown that PSMA PET-CT SUVmax changes over time may be a useful response biomarker that correlates with MFS, an important endpoint in these omCSPC patients. This study investigated radiomic imaging biomarkers derived from PSMA-PET acquired pre- and post-MDT for MFS prediction, which may provide better features to discriminate response in the future to improve the outcome of these patients.

MATERIALS/METHODS

We accrued an international multi-institutional cohort of omCSPC patients treated with stereotactic ablative radiation therapy (SABR) MDT. The cohort includes 32 patients from Institution 1 (USA) and 38 patients from Institution 2 (Europe). Both pre- and 6-month post-treatment PSMA-PET/CT were acquired. Combat was used for data harmonization in the image domain to minimize imaging variations across institutions. We defined the GTV volume as zone 1 and a 3-5 mm expansion ring area outside the GTV as zone 2 for radiomics analysis. 874 radiomics features (214 original and 660 wavelet filtered features) were extracted from both zones using open-source software and used together for MFS prediction. Function Chi2 was used to select the most significant five features. Several machine learning models (Random Forest, Logistic regression, Support Vector Machine, Naïve Bayesian) were implemented to predict MFS. The models were tested using both a leave-one-out strategy and cross-validation across the two institutions.

RESULTS

In the leave-one-out biomarker using 70 patients, random forest achieved the best accuracy, with MFS predicted correctly for 56 (80% of 70) patients. The five radiomic features identified based on their ability to predict MFS included Entropy, Skewness, and Compactness from zone 1, Skewness, and Mesh volume from zone 2. In the cross-institution tests, random forest predicted MFS correctly for 24 (75% of 32) patients when being trained using 38 Institution 2 patients and validated against 32 Institution 1 patients. Vice versa, the model predicted MFS correctly for 28 (74% of 38) patients when being trained using Institution 1 patients and validation using Institution 2 patients. The five features identified for prediction included Entropy and Skewness from both zones and Flatness from zone 1.

CONCLUSION

Our study demonstrated the promise of using pre- and post-MDT PSMA-PET-based imaging radiomic biomarkers for MFS prediction for omCSPC patients. Imaging biomarkers predictive of MFS were identified in both GTV and the ring area outside GTV. Over 74% prediction accuracy was achieved in the cross-institution validation test. The model provides a valuable tool for prognosis prediction early following MDT, which opens up a unique opportunity for monitoring or treatment interventions for patients identified with poor prognoses to improve outcomes.

Treatment Outcomes of Simultaneous Integrated Boost to Intraprostatic Lesions with External Beam Radiotherapy in Localized Prostate Cancer Patients

PURPOSE/OBJECTIVE(S)

To evaluate the treatment outcomes and toxicity of definitive radiotherapy (RT) for prostate cancer (PC) patients using the simultaneous integrated boost (SIB) technique to intraprostatic lesion (IPL).

MATERIALS/METHODS

All the patients received 78 Gy for their prostate ± seminal vesicles and 86 Gy for the IPL using the SIB technique. High-risk patients received pelvic nodal irradiation. The pelvic lymphatic field received 46-54 Gy of radiation. Univariable and multivariable analyses were conducted of the prognostic factors for freedom from biochemical failure (FFBF), progression-free survival (PFS), and PC-specific survival (PCSS) of 619 PC patients who received definitive RT between September 2012 and August 2021. Predictors of late Grade ≥ 2 genitourinary (GU) and gastrointestinal (GI) toxicities were also identified using logistic regression.

RESULTS

The median age of the entire cohort was 69 years (range, 47-89 years). Most of the patients (82.7%) had tumors with a GS of 6 or 7, 134 (21.6%) had PC with a GS of 3 + 4, 126 (20.4%) had PC with a GS of 4 + 3, and only 9% had clinical nodal metastasis. The median follow-up time for the entire cohort was 68.5 months (95% CI, 64.4-72.6 months). The 5-year FFBF, PFS, and PCSS rates were 93.2%, 83.2%, and 98.6%, respectively. They were predicted by the serum prostate specific antigen (PSA), Gleason score (GS), clinical nodal stage, and D'Amico risk group. Only 45 patients (7.3%) developed disease recurrence 41.9 months after RT. Of these patients, 12 (1.9% of the total) had BF only, 31 (5.3%) had both BF and distant metastasis, and 2 (0.3%) had distant metastasis only. The 5-year FFBF rates for low-, intermediate-, and high-risk disease were 98.0%, 93.1%, and 88.5%, respectively (p < 0.001). The 5-year PFS and PCSS rates according to risk groups were 91.0%, 82.1%, and 77.4% (p < 0.001), and 99.2%, 96.4%, and 95.9% (p = 0.03), and respectively. GS > 7 and lymph node metastasis negatively predicted FFBF and PCSS in multivariable analysis. Lymph node metastasis was associated with poor PFS, but only marginally (p = 0.06). Ninety (14.6%) and 44 (7.1%) patients had acute Grade ≥ 2 GU and GI toxicities, respectively, and 42 (6.8%) and 27 (4.4%) patients had late Grade ≥ 2 GU and GI toxicities, respectively. Diabetes (OR = 2.61; 95% CI = 1.29-5.26; p = 0.007) and transurethral resection of prostate (OR = 6.75; 95% CI = 3.37-13.54; p = 0.001) independently predicted late Grade ≥ 2 GU toxicity, but no significant predictor of late Grade ≥ 2 GI toxicity was found.

CONCLUSION

Localized PC was effectively and safely treated with definitive RT using the SIB technique to deliver 86 Gy to the IPL in 39 fractions without severe late toxicity. This finding must be validated with long-term results.

The impact of air pockets around the vaginal cylinder on vaginal vault brachytherapy

OBJECTIVE

To evaluate the incidence, size and predisposing factors for air pockets around the vaginal cylinder and their dosimetric effect on the vaginal mucosa.

METHODS

We investigated 174 patients with endometrial carcinoma treated with external radiotherapy (RT) and brachytherapy (BRT) (101 patients, 58%) or BRT alone (73 patients, 42%). The quantity, volume and dosimetric impact of the air pockets surrounding the vaginal cylinder were quantified. The proportions of patients with or without air pockets during application were stratified according to menopausal status, treatment modality and interval between surgery and RT.

RESULTS

Air pockets around the vaginal cylinder were seen in 75 patients (43%), while 99 patients (57%) had no air pockets. Only 11 patients (6.3%) received less than the prescribed dose (average 93.9% of prescribed dose; range, 79.0-99.2%). Air pockets were significantly fewer in pre-menopausal patients or in patients treated with the combination of external RT and BRT than in post-menopausal patients or patients treated with BRT alone. A significant correlation existed between the mucosal displacement of the air gap and the ratio of the measured dose at the surface of the air gap and prescribed dose (Pearson r = -0.775; p < 0.001).

CONCLUSION

Air pockets were still a frequent problem during vaginal vault BRT, especially in post-menopausal patients or in patients treated with BRT alone, which may potentially cause dose reductions at the vaginal mucosa.

ADVANCES IN KNOWLEDGE

Air pockets around the vaginal cylinder remain a significant problem, which may potentially cause dose reduction in the target volume.

Simultaneous integrated boost to intraprostatic lesions using different energy levels of intensity-modulated radiotherapy and volumetric-arc therapy

OBJECTIVE

This study compared the dosimetry of volumetric-arc therapy (VMAT) and intensity-modulated radiotherapy (IMRT) with a dynamic multileaf collimator using the Monte Carlo algorithm in the treatment of prostate cancer with and without simultaneous integrated boost (SIB) at different energy levels.

METHODS

The data of 15 biopsy-proven prostate cancer patients were evaluated. The prescribed dose was 78 Gy to the planning target volume (PTV78) including the prostate and seminal vesicles and 86 Gy (PTV86) in 39 fractions to the intraprostatic lesion, which was delineated by MRI or MR-spectroscopy.

RESULTS

PTV dose homogeneity was better for IMRT than VMAT at all energy levels for both PTV78 and PTV86. Lower rectum doses (V30-V50) were significantly higher with SIB compared with PTV78 plans in both IMRT and VMAT plans at all energy levels. The bladder doses at high dose level (V60-V80) were significantly higher in IMRT plans with SIB at all energy levels compared with PTV78 plans, but no significant difference was observed in VMAT plans. VMAT plans resulted in a significant decrease in the mean monitor units (MUs) for 6, 10, and 15 MV energy levels both in plans with and those without SIB.

CONCLUSION

Dose escalation to intraprostatic lesions with 86 Gy is safe without causing serious increase in organs at risk (OARs) doses. VMAT is advantageous in sparing OARs and requiring less MU than IMRT.

ADVANCES IN KNOWLEDGE

VMAT with SIB to intraprostatic lesion is a feasible method in treating prostate cancer. Additionally, no dosimetric advantage of higher energy is observed.