Real-World Toxicity of Conventional Versus Hypofractionated Definitive Prostate Radiotherapy across a Large, Diverse, Academic and Community-Based Enterprise

PURPOSE/OBJECTIVE(S)

Over the past 10-15 years there has been increasing adoption of moderate hypofractionation (HF) for definitive prostate radiotherapy as compared to conventional fractionation (CF). Based on several randomized trials hypofractionation results in equivalent treatment efficacy with similar rates of long-term toxicity. However, some studies suggest higher acute GI toxicity with moderate hypofractionation. We sought to compare the rates of toxicity between these two groups across our enterprise including 16 community-based practices and one academic NCI-designated comprehensive cancer center.

MATERIALS/METHODS

We retrospectively extracted radiation treatment intent from our network-wide clinical pathways for patients diagnosed with prostate cancer between 3/2019 and 10/2022. Patients treated after prostatectomy and those treated with brachytherapy or SBRT were excluded. For the remaining 1529 patients treated with either conventional fractionation or moderate hypofractionation, we identified and merged physician-graded toxicity data using CTCAE version 5.0 recorded in their electronic medical record at each weekly on-treatment visit and follow-up. A total of 1051 patients had toxicity data available. Rates of toxicities were then compared between the cohort of patients who received CF and those who HF using the Chi-square test.

RESULTS

Of the 1051 patients, 450 (43%) received CF and 601 (57%) received HF. These patients were treated by 40 different radiation oncologists (median patients per physician = 18, interquartile range = 7-35). Median age in the CF and HF cohorts was 71 (IQR: 66-76) and 71 (IQR: 66-77; p = 0.51), respectively. The CF cohort had more patients with Gleason 8+ disease (39% vs 19%; p<0.01), PSA >20 (26% vs 11%; p<0.01), or T3a+ (18% vs 8%; p<0.01). Rates of any grade 2+ toxicity were significantly higher in patients who received HF at 45.8% vs 39.6% for those treated with CF (p = 0.04). However, the respective rates of any grade 3+ toxicity were no different at 2.0% vs. 1.8% (p = 0.80). The difference in grade 2 toxicities appeared to be primarily driven by the rates of urinary frequency at 27.1% vs. 17.8% (p<0.01) and prostatic obstruction 14.8% vs. 10.2%, p = 0.03). Rates of grade 2 diarrhea were worse with MF at 5.3% vs. 2.8% for CF (p = 0.04). There were no significant differences between HF and CF in the rates of grade 2 dysuria (6% vs 5.2%), urinary urgency (6.5% vs. 4.2%), proctitis (3.0% vs. 3.6%), urinary incontinence (0.5% vs. 1.3%), rectal bleeding (0.3% vs. 0%), hematuria (0% vs. 0.4%), and fatigue (14.1% vs. 15.1%).

CONCLUSION

In this large network-wide analysis, toxicity was slightly increased among patients with prostate cancer treated with HF compared to CF, consistent with published randomized data. However, the increased toxicity appeared to be primarily GU rather than GI. This study demonstrates the feasibility of analyzing impacts of treatment decisions on a large scale using real-world data through an integrated network of practices.

Evaluation of 68Ga-Fibroblast Activation Protein Inhibitor vs. 18F-FDG as a Novel Radiotracer for Biologically Guided Radiation Therapy

PURPOSE/OBJECTIVE(S)

Real-time biology guided radiation therapy (BgRT) uses real-time positron emissions from a PET tracer during treatment to guide targeted radiation to cancerous lesions. Fibroblast activation protein alpha (FAP) is highly expressed on cancer-associated fibroblasts in tumors with low expression in normal tissues. While 18F-FDG-PET requires fasting and has background in the liver and brain, 68-Gallium labeled FAP inhibitor (FAPI) does not require fasting and has less background uptake. The goal of this study was to investigate the utility of FAPI as a potential universal fiducial for BgRT. We hypothesized that 68Ga-FAPI would be a better radiotracer than 18F-FDG, as assessed by the Normalized Minimal kBq/mL and the Normal Target Signal (NTS), two parameters used to gauge the suitability of BgRT.

MATERIALS/METHODS

PET-CTs were obtained for 50 patients with pancreatic, liver, lung, head & neck, and cervical cancer using 18F-FDG and 68Ga-FAPI (n = 10 for each). Four DICOM images were obtained per patient (FDG PET + CT, FAPI PET + CT). Radiation oncologists delineated the gross tumor volume (GTV) on PET images. A separate set of auto-contours were generated from the PET using an auto-threshold of 40% maximum SUV for all tumors. A 1 cm expansion was added to the GTV to create a ring around the physician-generated contours and auto-contours. The following parameters were measured: GTV volume, SUV max of GTV, SUV mean of GTV, Normalized Minimal kBq/mL within the GTV, and NTS (= SUV max/Ring SUV mean). Values were compared using paired t-test. For the BgRT product with similar calculations, the required Normalized Minimal kBq/mL is > 5 kBq/mL; the required NTS is > 2.7 for treatment planning and > 2.0 for BgRT delivery.

RESULTS

The Normalized Minimal kBq/mL for FAPI was > 5 kBq/mL for all tumors and greater for auto-contoured GTVs compared to physician-contoured GTVs. The mean NTS for the auto-contours for all tumor sites was > 2.0. In addition, there was a statistically significant increase in the NTS for FAPI compared to FDG in pancreatic, liver and head & neck cancers. In pancreatic cancer, there was a statistically significant increase in Normalized Minimal kBq/mL for FAPI compared to FDG (26.0 vs 14.2) (p = 0.01) and the SUVmax of FAPI was almost double that of FDG (15.9 vs 8.2) (p = 0.01). FAPI had no background in the liver, but had high background in the uterus, suggesting it may have a role in liver cancer but not cervical cancer.

CONCLUSION

This is the first study demonstrating the potential superiority of 68Ga-FAPI compared to 18F-FDG as a biologic fiducial for BgRT when treating pancreatic, liver and head & neck cancers, with a similar efficacy for lung cancer. Our results indicate that auto-contoured GTVs generate a higher NTS than physician-contoured GTVs but all are > 2.0. In addition, the Normalized Minimal kBq/mL for auto-contours is > 5 kBq/mL for all tumors. As hypothesized, FAPI-based BgRT is most likely to be successful when treating tumors with significant desmoplastic stroma, such as pancreatic cancer.

Spatiotemporal Optimization of Pelvic Radiation Therapy

PURPOSE/OBJECTIVE(S)

Conformal and adaptive Radiation Therapy (RT) remains critical in the treatment of locally advanced pelvic cancers despite acute and late bowel complications. This work introduces spatiotemporal optimization of pelvic RT including mathematical modelling of intestinal radiobiology to evaluate a novel treatment planning approach for fractionated pelvic RT.

MATERIALS/METHODS

A single-plan fractionated RT delivery has n-fractions at dose/fraction = d and has biologically effective dose BED = nd ( 1 + d / [α/β] ) - ln(2) ([T - Tk] / [αTp]), where Tk and Tp are the kickoff time and doubling time of repopulation, and T is the time to deliver all fractions (including off-days). We present a parameterization of the repopulation BED-term to estimate a range of daily recovered BED including uncertainties in α and Tp. Recovery of BED as currently formulated is independent of the delivered dose; however, the assumption of spatiotemporal optimization is that a regional reduction of dose during RT by mixing treatment plans is critical to allow for normal tissue healing. We evaluate an achievable spatiotemporal optimization treatment planning strategy for an advanced stage prostate and a cervical cancer patient using 25-fraction delivery over 5-weeks and 4-weekends (T = 33 days) and propose 3 treatment plans: (1) a conformal "clinical standard" plan, (2) a right-bowel sparing plan, and (3) a left-bowel sparing plan. The plans are optimized to ensure no increased dose in opposing bowel or normal organs at risk (OARs).

RESULTS

For radiobiological parameters ranging from α = 0.2-0.35/Gy, α/β = 2-3 Gy, Tk = 5-10 days, and Tp = 3-7 days, parameterization shows daily BED recovery of 0.44±0.08 Gy (range = 0.28-0.69 Gy), or an average of 4Gy BED recovery for 5-days treatment surrounded by 2 weekends. Conservatively evaluating the 3.6Gy BED distribution in treatment planning may identify regions for complete recovery (after Tk days). In a prostate 3-plan set, the right- and left- bowel sparing plans deliver equivalent mean dose to targets and OARs to within 5cGy/fx. The clinical plan includes 298cc of bowel under 14Gy, but this same bowel will be spared throughout treatment. Delivery of the clinical plan in week 1 to achieve Tk days, followed by alternating the left and right plan each week has the potential to reduce BED to zero for an additional 116cc of bowel using the left-sparing plan in weeks 2 and 4, and 91cc using the right-bowel sparing plan in weeks 3 and 5. The cervical cancer plan-set was more challenging due to right-sided nodal volumes and higher clinical bowel dose; OAR mean dose variations were >8cGy/fx in a few organs, but the left- and right- sided sparing plans results in 394 cc and 139cc, respectively of unique spared bowel at the 3.6Gy/week threshold to potentially reach zero BED during treatment.

CONCLUSION

A novel spatiotemporal optimization of pelvic RT has the potential to preserve bowel, allow for increased intra-treatment intestinal tissue regeneration, and reduce radiation-induced complications.

Pilot Study of a Novel Ring Gantry-Based PET/CT Linear Accelerator in Patients with Prostate Cancer Receiving [18F]-DCFPyL for PSMA PET Imaging

PURPOSE/OBJECTIVE(S)

The RefleXion X1® system is a hybrid PET imaging-radiotherapy system that uses real-time positron emissions from a PET tracer to deliver biologically guided radiotherapy (BgRT). This study (NCT05470699) evaluated the hypothesis that the X1 PET imaging subsystem would be able to detect [18F]-DCFPyL PSMA PET signal sufficient to generate a deliverable BgRT plan in patients with prostate cancer.

MATERIALS/METHODS

Patients with prostate cancer scheduled for a diagnostic [18F]-DCFPyL PSMA PET scan as part of standard of care were eligible. Upon completion of the diagnostic PSMA PET scan, images were transferred to the radiotherapy planning system for target identification and contouring. If at least one PET avid tumor lesion was identified, the patient was then scanned on the X1 unit. BgRT planning was performed on each X1 scanned patient. The target lesion volume, activity concentration (AC) and normalized target signal (NTS) were acquired. Successful and deliverable BgRT plans required that the target AC was ≥ 5 kBq/ml and NTS ≥ 2.7.

RESULTS

Twenty-six patients underwent [18F]-DCFPyL PET scans (13 with rising PSA after surgery or radiotherapy, 6 with known metastases and 7 with newly diagnosed high-risk prostate cancer). Median (range) PSA was 3.40 (0.04-122). In 16 patients a PET avid tumor was identified and contoured for planning (4 lymph nodes, 5 bone, 6 prostate gland, and 1 prostate bed). In 13 patients the target lesion was visualized on the X1 PET scan, while in 3 patients the target lesion was too close to the bladder to be clearly visualized. BgRT planning was feasible and met standard of care published SBRT organ dose constraints in 8 patients (3 prostate gland, 3 bone, 2 lymph nodes). BgRT planning was not feasible in 8 patients due to insufficient AC, low NTS or proximity of the target lesion to the PET avid bladder. The accompanying table compares median (range) target volume, AC and NTS for feasible versus not feasible plans.

CONCLUSION

This is the first study to investigate the feasibility of using [18F]-DCFPyL PET imaging for BgRT plan generation on the X1 system in patients with prostate cancer. Lesions that are relevant to radiotherapy of prostate cancer can be visualized including lymph node and bone metastases. A dedicated BgRT workflow with PSMA PET imaging on the X1 at 60 minutes post injection will result in higher target AC and will optimize BgRT planning. PET avid lesions < 1 cm or close to the bladder may make BgRT planning challenging. [18F]-DCFPyL-guided BgRT is technically feasible using the RefleXion X1. BgRT using targeted PET radiopharmaceuticals to biologically guide external beam radiotherapy represents a promising new dimension in radiation oncology and warrants further investigation.

Are All Prognostic Stage IB Breast Cancers Equivalent?

PURPOSE/OBJECTIVE(S)

The 8th edition of the American Joint Committee on Cancer (AJCC) has recognized the prognostic influence of histologic grade and biomarker status for breast cancer (BC). Contemporary BC staging includes both anatomic tumor extent and prognostic stage. However, prognostic stage IB remains heterogeneous and includes patients with locally advanced anatomic pathologic stage IIIA-B (pT3N1 or pT1-3N2, G1-2) hormone-receptor positive/HER2-negative BC (LA-HR+/HER2-) as well as patients with early-stage anatomic clinical/pathologic stage IA (T1cN0, G2-3) triple-negative BC (ES-TNBC). We hypothesized that although both are classified as prognostic stage IB BC, overall survival (OS) is worse for LA-HR+/HER2- compared to ES-TNBC.

MATERIALS/METHODS

We used the National Cancer Database to identify patients with surgically-resected LA-HR+/HER2- BC (pT3N1 or pT1-3N2, grade 1-2) and those with ES-TNBC (T1N0, grade 2-3) from 2004-2017. Patients were excluded if receptor status, tumor grade, and/or TNM staging data were unknown. HR+/HER2- patients treated with neoadjuvant therapy were also excluded. The primary endpoint was OS. Multivariable Cox regression analysis was used to evaluate differences in OS between LA-HR+/HER2- BC and ES-TNBC (adjusting for baseline patient demographic characteristics) in the entire cohort and in the subset of patients that received appropriate treatment based on anatomic stage: radiation (RT), chemotherapy (CT) and hormone therapy for LA-HR+/HER2- BC and CT or CT+RT for ES-TNBC treated with mastectomy or lumpectomy, respectively. We report hazard ratios (HR) with 95% confidence intervals (CI) with p<0.05 considered statistically significant.

RESULTS

A total of 45,818 patients met inclusion criteria (N = 17,359 with LA-HR+/HER2- BC and N = 28,459 with ES-TNBC). Over 75% of the LA-HR+/HER2- BC patients have anatomic pathologic stage IIIB disease (pT1-3N2, G1-2). With a median follow-up of 56 months, the 6-year OS rates were 86.1% (LA-HR+/HER2-) vs. 90.4%patients (ES-TNBC) which corresponded to a 63% relative increased risk of death in LA-HR+/HER2- patients compared to ES-TNBC patients (HR = 1.63, 95% CI 1.53-1.73, p<0.0001) after adjusting for all covariates. Approximately 66% (N = 11,533) LA-HR+/HER2- and 69% (N = 19,512) ES-TNBC received appropriate therapy. The 6-year OS was 91.8% (LA-HR+/HER2-) vs. 93.3% (ES-TNBC) which corresponded to a 35% increased risk of death in the LA-HR+/HER2- patients compared to ES-TNBC (adjusted HR = 1.35, 95% 1.24-1.48, p<0.0001). Other covariates associated with OS were age, income, insurance status, facility type, and ethnicity/race.

CONCLUSION

We found that LA-HR+/HER2- BC has significantly worse OS compared to ES-TNBC despite both being classified as prognostic stage IB, even when accounting for treatments delivered. The categorization of pT3N1 or pT1-3N2, G1-2 HR+/HER2- BC as prognostic stage IB needs to be reconsidered in order to provide patients with more accurate information regarding expected OS.

Omission of Adjuvant Radiotherapy in Low-Risk Elderly Males with Breast Cancer

PURPOSE/OBJECTIVE(S)

Randomized clinical trials demonstrate that lumpectomy + hormone therapy (HT) without radiation therapy (RT) yields equivalent survival and acceptable local-regional outcomes in elderly women with early-stage, node-negative (T1-2N0) hormone-receptor positive (HR+) breast cancer. Whether these data apply to men with the same inclusion criteria remains unknown. We hypothesized that outcomes in males would be comparable to those seen in females, with RT not conferring an overall survival (OS) benefit over HT alone.

MATERIALS/METHODS

We conducted a retrospective matched-cohort study using the National Cancer Database for males ≥65 years with pathologic T1-2N0 (≤3 cm) HR+ breast cancer treated with breast conserving surgery with negative margins from 2004-2019. Patients who received chemotherapy, had nodal or distant metastases, or unknown follow-up were excluded. Adjuvant treatment was classified as HT alone, RT alone, or HT+RT. Due to limitations of survival analysis on retrospective data, male patients were matched with female patients to determine comparable outcomes based on age (± 3 years), Charlson Deyo comorbidity score, T-stage, and adjuvant treatment. Survival analysis was performed using Cox regression and Kaplan-Meier analysis. To adjust for confounding, inverse probability of treatment weighting (IPTW) was used.

RESULTS

A total of 523 patients met inclusion criteria, with 24.4% receiving HT, 16.3% receiving RT, and 59.2% receiving HT+RT. Median follow-up was 6.9 years (IQR: 5.0-9.4 years). Unadjusted 5-yr OS rates in the HT, RT, and HT+RT cohorts were 79.2% (95% CI 70.7-85.5%), 80.9% (95% CI 70.3-88.0%), and 93.3% (95% CI 89.7-95.7%), respectively. Adjusted 10-yr OS rates in the HT, RT, and HT+RT cohorts were 82.3% (95% CI 78.6-85.5%), 83.6% (95% CI 80.0-86.7%), and 92.8% (95% CI 90.1-94.8%), respectively. On unadjusted multivariable Cox regression analysis (MVA), relative to HT, receipt of HT+RT was associated with improvements in OS (HR: 0.603; 95% CI: 0.410-0.888; p = 0.01). RT alone was not associated with improved OS (HR: 1.116; 95% CI: 0.710-1.755; p = 0.633). On adjusted MVA, relative to HT, receipt of HT+RT was associated with improvements in OS (HR: 0.551; 95% CI: 0.370-0.820; p = 0.003). Again, RT alone was not associated with improved OS (HR: 0.991; 95% CI: 0.613-1.604; p = 0.972). Other factors associated with OS included age, Charlson Deyo score, T stage, and grade. Overall, in the matched women, the same trends were found as in the men, the best survival was in HT+RT, but no difference in OS between HT vs. RT.

CONCLUSION

Among men ≥65 years old with T1-2N0 HR+ breast cancer, RT alone did not confer an OS benefit over HT alone. Combined RT+HT did yield improvements in OS, though there are likely significant unmeasured confounders contributing to these outcomes in patients treated with the most aggressive approach. Our findings support that RT omission may be a reasonable option in elderly men with T1-2N0 HR+ breast cancer treated with lumpectomy + HT.

Dosimetric Plan Evaluation of Biology Guided Radiotherapy Using [18F]-DCFPyL PSMA Radiotracer in Patients with Prostate Cancer

PURPOSE/OBJECTIVE(S)

The X1 system represents a cutting-edge solution in radiotherapy delivery, with its capability to perform Biology Guided Radiotherapy (BgRT). The system utilizes real-time positron emission tomography (PET) signal as biological fiducials to provide tracked dose delivery and is initially available for use with [18F]-Fluorodeoxyglucose (FDG). The aim of this research study is to assess the quality of BgRT treatment plans for prostate cancer using patients' PSMA PET images obtained on the X1 system.

MATERIALS/METHODS

Sixteen patients with at least one PET-avid tumor identified on their whole-body diagnostic PSMA PET scan were selected. These patients were scanned on X1 following their diagnostic scan without additional radiotracer administration. Based on the X1 PET images, a BgRT plan was created for each patient, with the prescription dose determined by the location of treatment sites. The planning objectives of organs-at-risk (OARs) were established in accordance with the 2018 Timmerman guidelines. Target coverage objective was the dose covering 95% (D95%) of the planning target volume (PTV) to be higher than 100%. The following parameters were analyzed: PTV D95%, the minimal dose (Dmin) of gross tumor volume (GTV), plan maximum dose (Dmax), conformity index (CI), gradient index (GI), and maximum point dose (D0.03cc) to the nearest OARs. The X1 BgRT planning system also generated dose volume histogram (DVH) bounds, which model variations in BgRT delivery. The low boundary of GTV Dmin, representing the minimum GTV dose in the worst-case scenario, was recorded.

RESULTS

BgRT plans were created for all patients, except for one where the target signal was indistinguishable from the bladder. The prescription dose was 2700 cGy or 3000 cGy in 3 fractions for lymph node lesions, 2400 cGy to 3000 cGy in 3 fractions for bone metastasis, and 4500 cGy in 5 fractions for lesions in prostate. All plans met the dose constraints for OARs as per the Timmerman guidelines. The Dmax of all plans was 129.9% ± 6.9% (mean ± standard deviation). The PTV D95% and GTV Dmin were 101.7% ± 1.0% and 111.0% ± 7.6%, respectively. The low boundary of GTV Dmin was 95.9% ± 5.8%. The CI and GI were 1.22 ± 0.11 and 9.40 ± 2.12, respectively. The D0.03cc to nearest OARs was 84.6% ± 25.4%. The estimated treatment time was 699 ± 228 seconds.

CONCLUSION

This study is a pioneering effort to evaluate the quality of BgRT plans for prostate cancer patients using the [18F]-DCFPyL PSMA radiotracer. Our results showed that all BgRT plans met the planning objectives defined in the Timmerman protocol. BgRT with [18F]-DCFPyL represents a promising treatment modality for patients with prostate cancer. Further research is needed to validate this approach, including a comprehensive assessment of the dosimetric and tracking accuracy through physical measurements.

Machine Learning and Explainable Artificial Intelligence to Predict Occult Pelvic Nodal Metastases in Prostate Cancer

PURPOSE/OBJECTIVE(S)

Determination of risk of occult pelvic lymph node involvement (LNI) in patients with cN0 prostate cancer is critical for determination of optimal treatment options. Though several nomograms exist, machine learning (ML) approaches might enable physicians to better assess individual risk by incorporating multiple clinical risk factors. Herein, we developed a ML model to predict occult LNI, and explained its composition using an explainable artificial intelligence (XAI) framework.

MATERIALS/METHODS

Patients with cN0 prostate adenocarcinoma diagnosed from 2018-2020 were identified in the National Cancer Database. The query was limited to patients with known clinical staging and biopsy results who did not receive neoadjuvant therapy prior to pelvic nodal examination. Occult LNI was defined as pN1 disease based on surgical evaluation, with a minimum of 10 nodes examined. Five ML models were trained to predict LNI. Variables incorporated into the model were age, core biopsy results, Gleason scores, preoperative prostate specific antigen (PSA), and clinical T-stage. Model performance, measured using area under the receiver operator characteristic curve (AUC) on a holdout testing dataset, was compared to multivariable logistic regression. The best-performing model was explained using SHapley Additive exPlanation (SHAP) values. To permit more clinically-meaningful statistical interpretation, using a novel approach SHAP values were converted into odds ratios (OR), confidence intervals (CI), and p-values.

RESULTS

A total of 23,131 patients met inclusion criteria; 2,676 (11.6%) had occult LNI. The Extreme Gradient Boosting model outperformed all other models with an AUC of 0.82 (95% CI: 0.78-0.86) compared to 0.80 (95% CI: 0.76-0.84) for logistic regression. Increasing PSA (OR: 1.031; p<0.001), number of positive biopsy cores (OR: 1.055; p<0.001), and percent positive biopsy cores (OR: 1.01; p<0.001) were all associated with increased risk of LNI. Based on observation of SHAP dependence plots, risk of LNI plateaued at PSA>20 ng/dL and >11 positive cores, while no plateau was observed for percent positive biopsy cores. Relative to T1c disease, patients with T3b were at highest risk of LNI (OR: 1.461; p = 0.003). Gleason score of 9 was associated with significant risk of LNI (Ref: Gleason 6; OR: 1.891; p<0.001). This was primarily driven by the primary Gleason score; primary Gleason 5 disease was associated with significant risk of LNI (Ref: Gleason 3; OR: 1.915; p<0.001) while a secondary Gleason score of 5 was the only grade with significant increased risk of LNI (Ref: Gleason 3; OR: 1.185; p = 0.004). Age and number of cores examined were not significant predictors of LNI.

CONCLUSION

Our ML achieved improved performance relative to logistic regression at predicting occult LNI. XAI provided insight into the inner-working of the ML model. ML can be used to identify patients at risk for occult LNI and therefore inform clinical decision-making.

Patterns of Care for Patients with Locally Advanced Rectal Cancer Treated with Total Neoadjuvant Therapy between 2016-2020: An NCDB Analysis

PURPOSE/OBJECTIVE(S)

Treatment for locally advanced rectal cancer (LARC), defined as T3/4 or any T with N+ disease, typically requires multi-modality management consisting of radiation (RT), chemotherapy (CHT), and surgery. Despite emerging evidence that total neoadjuvant therapy (TNT) is the preferred treatment of LARC, it remains unknown what proportion of patients are receiving TNT in the United States. Our objective was to (1) determine the proportion of patients with LARC receiving TNT over time, (2) determine the most common method in which TNT is being delivered, and (3) determine what factors are associated with a lower likelihood of receiving TNT in the United States.

MATERIALS/METHODS

Retrospective data was obtained from the National Cancer Database (NCDB) for patients diagnosed with rectal cancer between 2016-2020. Patients were excluded if they had M1 disease, T1-2 N0 disease, incomplete staging information, non-adenocarcinoma histology, received RT to a non-rectum site, or received a non-definitive RT dose. Patients were determined to have received TNT if they (1) received RT and multi-agent (MA)-CHT prior to surgery, (2) had an interval of >180 days from the onset of neoadjuvant therapy to surgery if they received long course (LC)-chemoradiation (CRT) (based on 35 days for LC-CRT + 112 days for 8 cycles of MA-CHT + 30 days to surgery), or (3) had an interval of >150 days from the onset of neoadjuvant therapy to surgery if they received short course (SC)-RT (based on 5 days for SC-RT + 112 days for 8 cycles of MA-CHT + 30 days to surgery). Data were analyzed using linear regression, Chi-square test, and binary logistic regression.

RESULTS

Of the 26,375 patients included, the median age was 60 (range 21-90) years, with the majority of patients being <65 years old (65.6%), male (62.1%), and non-Hispanic white (77.0%). A total of 5,003 (19.0%) patients received TNT, and 21,372 (81.0%) patients received classical combined modality therapy (CMT). The proportion of patients receiving TNT increased significantly over time, from 6.1% in 2016, 9.0% in 2017, 15.3% in 2018, 25.8% in 2019, to 34.6% in 2020 (slope = 7.36, 95% CI 4.58-10.15, R2 = 0.96, p = 0.040). The most common TNT regimen was MA-CHT followed by LC-CRT (73.2% of cases from 2016-2020). The proportion of patients receiving SC-RT as part of TNT significantly increased from 2.8% in 2016, 1.7% in 2017, 4.6% in 2018, 7.3% in 2019, to 13.7% in 2020 (slope = 2.74, 95% CI 0.37-5.11, R2 = 0.82, p = 0.035). On multivariate analysis, factors associated with a lower likelihood of TNT use included age >65 (OR 0.66, 95% CI 0.61-0.71, p<0.001), female gender (OR 0.92, 95% CI 0.86-0.98, p = 0.014), Black race (OR 0.87, 95% CI 0.77-0.98, p = 0.024), and T3 N0 disease (OR 0.60, 95% CI 0.52-0.70, p<0.001).

CONCLUSION

TNT utilization rates have significantly increased in recent years, from 6.1% in 2016 to 34.6% in 2020. The observed trend appears to be in line with the recent National Comprehensive Cancer Network (NCCN) guidelines recommending TNT as the preferred approach.

Brachytherapy boost for prostate cancer: Trends in care and survival outcomes

PURPOSE

Androgen suppression combined with elective nodal and dose-escalated radiation therapy recently demonstrated an improved biochemical failure-free survival in men who received external beam radiation therapy (EBRT) plus a brachytherapy boost (BB) compared with dose-escalated external beam radiotherapy (DE-EBRT). We sought to analyze the factors predictive for use of EBRT + BB as compared with DE-EBRT and report resulting survival outcomes on a national level using a hospital-based registry.

METHODS AND MATERIALS

We identified 113,719 men from the National Cancer Database from 2004 to 2013 with intermediate- or high-risk prostate cancer who were treated with EBRT + BB or DE-EBRT. We performed univariate and multivariate analyses of all available factors potentially predictive of receipt of treatment selection. Survival was evaluated in a multivariable model with propensity adjustment.

RESULTS

For intermediate-risk patients, utilization of BB decreased from 33.1% (n = 1742) in 2004 to 12.5% (n = 766) in 2013 and for high-risk patients, utilization dropped from 27.6% (n = 879) to 10.8% (n = 479). Numerous factors predictive for use of BB were identified. Cox proportional hazards analysis was performed-adjusting for age, Charlson-Deyo comorbidity score, T stage, prostate-specific antigen, Gleason score, and sociodemographic factors-and demonstrated BB use was associated with a hazard ratio of 0.71 (95% confidence interval, 0.67-0.75; p < 0.0005) and 0.73 (95% confidence interval, 0.68-0.78; p < 0.0005) for intermediate- and high-risk patients, respectively.

CONCLUSIONS

There has been a concerning decline in the utilization of BB for intermediate- and high-risk prostate cancer patients despite an association with improved on overall survival. Numerous factors predictive for use of BB have been identified.

Determination of the relative affinities of antibody fragments expressed in Escherichia coli by enzyme-linked immunosorbent assay

We have previously utilized an M13 phage expression system and codon-based mutagenesis to increase the avidity of the tumor-specific antibody, BR96, 65-fold (Yelton et al., 1995, J. Immunol. 155, 1994-2004). Mutants with improved affinity were identified by screening phage-expressed antibodies on a carcinoma cell line. In this study we describe a more broadly applicable assay which permits rapid and quantitative comparison of affinities of related antibodies produced in an M13 phage expression system. BR96 variants displaying a range of affinities were expressed as soluble antibody fragments (Fabs) in the periplasmic space of bacteria and isolated from small-scale cultures grown in a 96-well format, yielding between 142 ng and 1.06 microg of Fab. Although the small-scale cultures expressed variable levels of Fab, the lower quantities were sufficient to saturate microtiter plates coated with a limiting amount of anti-human Fab antibody, resulting in the capture of uniform quantities of the Fab variants. The relative affinities of the variants were then compared by assessing binding to biotinylated antigen followed by detection with streptavidin-alkaline phosphatase conjugates. This approach permitted the direct comparison of the relative affinities of large numbers of antibody variants in a single step without multiple antibody dilutions. The assay is readily adaptable for screening phage-expressed antibody libraries against any biotinylated target and does not require purified antigen. For instance, the biotinylated BR96 antigen utilized in these studies was from a total cell extract prepared by labeling the surface of live tumor cells followed by detergent extraction. Thus, this approach may be applicable to the screening of antibody libraries against other cell surface antigens, such as transmembrane receptors, which are difficult to purify.

A combinatorial library strategy for the rapid humanization of anticarcinoma BR96 Fab

We have used a combinatorial mutagenesis strategy to humanize BR96, a monoclonal antibody that binds to the Lewis Y class of tumor antigens. This approach allows simultaneous assessment of hundreds of humanized variable regions to identify the molecules that best preserve affinity, thus overcoming the major drawback of current humanization procedures, the requirement to construct and analyze each humanized antibody separately. Murine residues of BR96 were mutated to human if they were solvent-exposed residues that did not participate in the formation of the antigen binding site and were not at the interface of the light and heavy chain. At positions that might be involved in binding to antigen, the choice between the murine and human residue was more difficult. Murine and human alternatives were incorporated into a combinatorial library at positions representing buried residues that might affect the structural integrity of the antigen binding site. By encoding this library of humanized BR96 Fabs in an M13 phage vector, we rapidly identified several candidates with nearly identical antigen binding, within 2-fold, of the chimeric Fab. Additional mutagenesis directed at sites suggested in the literature as potentially important for antigen binding in a similar anti-Lewis Y antibody yielded no further improvements.

Affinity maturation of the BR96 anti-carcinoma antibody by codon-based mutagenesis

We have increased up to 65-fold the avidity of BR96, a mAb recognizing Lewis Y (Le(y))-related Ags expressed on the surface of many human carcinomas. Libraries of mutations in the complementarity-determining regions (CDRs) of BR96 were constructed in an M13 phage Fab expression vector by codon-based mutagenesis, a method that efficiently introduces large numbers and potentially all combinations of amino acid substitutions. Two mutants that improved the affinity of BR96 to tumor Ag were identified by screening the libraries on carcinoma cell lines. One mutant, M1, at position 97 (Asp to Ala) in CDR3 of the heavy chain, resulted in an 8- to 10-fold improvement in Ag binding, as assessed by ELISA. A second mutant, M2, at position 53 (Gly to Asp) in CDR2 of VH increased binding three- to fivefold. When these mutations were combined, the resulting Fab M3 was improved approximately 30-fold. An additional library was constructed in CDR1 of M1. M4, a mutation with three amino acid substitutions in CDR1, was isolated by screening the library with an enzyme conjugate of synthetic Le(y) tetrasaccharide (sLe(y)). This mutant improved BR96 Fab affinity to sLe(y) an estimated 15- to 20-fold by ELISA, and 14-fold as measured by surface plasmon resonance. The M4 IgG had 65-fold improved avidity to sLe(y) relative to the BR96 IgG. The mutants will be useful for comparison of the efficacy of Abs with different affinities for delivery of cytotoxic agents to tumor cells.

Affinity maturation of the BR96 anti-carcinoma antibody by codon-based mutagenesis

We have increased up to 65-fold the avidity of BR96, a mAb recognizing Lewis Y (Le(y))-related Ags expressed on the surface of many human carcinomas. Libraries of mutations in the complementarity-determining regions (CDRs) of BR96 were constructed in an M13 phage Fab expression vector by codon-based mutagenesis, a method that efficiently introduces large numbers and potentially all combinations of amino acid substitutions. Two mutants that improved the affinity of BR96 to tumor Ag were identified by screening the libraries on carcinoma cell lines. One mutant, M1, at position 97 (Asp to Ala) in CDR3 of the heavy chain, resulted in an 8- to 10-fold improvement in Ag binding, as assessed by ELISA. A second mutant, M2, at position 53 (Gly to Asp) in CDR2 of VH increased binding three- to fivefold. When these mutations were combined, the resulting Fab M3 was improved approximately 30-fold. An additional library was constructed in CDR1 of M1. M4, a mutation with three amino acid substitutions in CDR1, was isolated by screening the library with an enzyme conjugate of synthetic Le(y) tetrasaccharide (sLe(y)). This mutant improved BR96 Fab affinity to sLe(y) an estimated 15- to 20-fold by ELISA, and 14-fold as measured by surface plasmon resonance. The M4 IgG had 65-fold improved avidity to sLe(y) relative to the BR96 IgG. The mutants will be useful for comparison of the efficacy of Abs with different affinities for delivery of cytotoxic agents to tumor cells.

The anti-idiotypic response by cynomolgus monkeys to humanized anti-Tac is primarily directed to complementarity-determining regions H1, H2, and L3

The anti-ld response developed by cynomolgus monkeys to the humanized anti-Tac antibody was analyzed by using 12 humanized anti-Tac variants differing in V region structure. The majority of the monkey response was directed against idiotopes composed wholly or in part of complementarity-determining regions H1, H2, and L3. There was no detectable response directed solely to five single complementarity-determining regions examined or solely to the modified human V region framework.

The anti-idiotypic response by cynomolgus monkeys to humanized anti-Tac is primarily directed to complementarity-determining regions H1, H2, and L3

The anti-ld response developed by cynomolgus monkeys to the humanized anti-Tac antibody was analyzed by using 12 humanized anti-Tac variants differing in V region structure. The majority of the monkey response was directed against idiotopes composed wholly or in part of complementarity-determining regions H1, H2, and L3. There was no detectable response directed solely to five single complementarity-determining regions examined or solely to the modified human V region framework.

Application of a filamentous phage pVIII fusion protein system suitable for efficient production, screening, and mutagenesis of F(ab) antibody fragments

We describe the application of a novel filamentous phage vector system suitable for efficient screening and production of F(ab) antibody fragments. The vector system can concurrently produce free F(ab) fragments and F(ab) displayed on the surface of M13 bacteriophage via a VHCH1-pVIII fusion protein. When expressed in a supO (nonsuppressor) strain of Escherichia coli free F(ab) can be produced. Antibody F(ab) fragments are secreted into culture medium at concentrations up to 0.3 mg/liter and conveniently subjected to detailed analysis with little or no purification. Higher concentrations of F(ab) (approximately 10 mg/liter) were found to accumulate in the periplasmic space. In this report the vector system is shown to produce correctly folded and assembled F(ab) fragments of chimeric L6, a mAb against a tumor-associated Ag expressed by many human carcinomas.

Application of a filamentous phage pVIII fusion protein system suitable for efficient production, screening, and mutagenesis of F(ab) antibody fragments

We describe the application of a novel filamentous phage vector system suitable for efficient screening and production of F(ab) antibody fragments. The vector system can concurrently produce free F(ab) fragments and F(ab) displayed on the surface of M13 bacteriophage via a VHCH1-pVIII fusion protein. When expressed in a supO (nonsuppressor) strain of Escherichia coli free F(ab) can be produced. Antibody F(ab) fragments are secreted into culture medium at concentrations up to 0.3 mg/liter and conveniently subjected to detailed analysis with little or no purification. Higher concentrations of F(ab) (approximately 10 mg/liter) were found to accumulate in the periplasmic space. In this report the vector system is shown to produce correctly folded and assembled F(ab) fragments of chimeric L6, a mAb against a tumor-associated Ag expressed by many human carcinomas.

Antibody engineering by codon-based mutagenesis in a filamentous phage vector system

A novel codon-based mutagenesis procedure is described that allows rapid and efficient modification of antibody amino acid sequences expressed as F(ab) fragments in M13. The procedure succeeded in generating a library of mutations in the complementarity-determining regions of chimeric L6, an antibody against a tumor-associated Ag. A set of anti-Id antibodies (anti-Id 1, 3, and 7) that bind near the L6 Ag-binding site served as model Ag. The goal was to select mutant antibody sequences that altered the L6 reactivity with the anti-Id in subtle ways, i.e., to eliminate the binding to one anti-Id while preserving other reactivities or to identify mutants with increased binding. A high frequency of variant M13 phage clones exhibiting altered specificity for the anti-Id were identified. Codon-based mutagenesis in conjunction with the M13 antibody expression and screening system should provide an efficient and general approach for redirecting the specificity and potentially improving the affinity of antibodies in vitro.

Antibody engineering by codon-based mutagenesis in a filamentous phage vector system

A novel codon-based mutagenesis procedure is described that allows rapid and efficient modification of antibody amino acid sequences expressed as F(ab) fragments in M13. The procedure succeeded in generating a library of mutations in the complementarity-determining regions of chimeric L6, an antibody against a tumor-associated Ag. A set of anti-Id antibodies (anti-Id 1, 3, and 7) that bind near the L6 Ag-binding site served as model Ag. The goal was to select mutant antibody sequences that altered the L6 reactivity with the anti-Id in subtle ways, i.e., to eliminate the binding to one anti-Id while preserving other reactivities or to identify mutants with increased binding. A high frequency of variant M13 phage clones exhibiting altered specificity for the anti-Id were identified. Codon-based mutagenesis in conjunction with the M13 antibody expression and screening system should provide an efficient and general approach for redirecting the specificity and potentially improving the affinity of antibodies in vitro.

Dissection of the combining site in a humanized anti-Tac antibody

The genetically engineered "humanized" anti-Tac antibody (HAT) has been shown to bind the p55 chain of the human IL-2R with an affinity close to that of the murine anti-Tac. Although the HAT molecule contains all six mouse CDR, it was not known which, and to what extent, each of the CDR contributes to Ag binding. These questions were addressed by constructing a series of variant HAT antibodies, each substituting a single HAT CDR with a heterologous CDR. The association constants of the variant HAT antibodies to p55 were determined by competitive binding analysis. We find that CDR 1 and 3 of the H chain and CDR 3 of the L chain are essential for maintaining binding. The remaining three CDR appear to be involved to a lesser degree.

Dissection of the combining site in a humanized anti-Tac antibody

The genetically engineered "humanized" anti-Tac antibody (HAT) has been shown to bind the p55 chain of the human IL-2R with an affinity close to that of the murine anti-Tac. Although the HAT molecule contains all six mouse CDR, it was not known which, and to what extent, each of the CDR contributes to Ag binding. These questions were addressed by constructing a series of variant HAT antibodies, each substituting a single HAT CDR with a heterologous CDR. The association constants of the variant HAT antibodies to p55 were determined by competitive binding analysis. We find that CDR 1 and 3 of the H chain and CDR 3 of the L chain are essential for maintaining binding. The remaining three CDR appear to be involved to a lesser degree.

A synthetic presequence reversibly inhibits protein import into yeast mitochondria

We show that a synthetic peptide corresponding to the N-terminal 22 residues of the cytochrome c oxidase subunit IV presequence blocked import of pre-subunit IV into yeast mitochondria. The 22-residue peptide pL4-(1-22) did not alter the electrical potential across the mitochondrial inner membrane (the delta psi). Inhibition of import was reversible and could be overcome by the addition of increased amounts of precursor. Two other peptides, pL4-(1-16) and pL4-(1-23), which correspond to, respectively, the N-terminal 16 and 23 residues of the same presequence, also blocked import of pre-subunit IV. However, pL4-(1-16) was a much weaker inhibitor of import, while the inhibitory effect of pL4-(1-23) was due to its ability to completely collapse the delta psi. pL4-(1-22) seems to be a general inhibitor of mitochondrial import, in that it also blocked uptake of several other proteins. These included the precursors of the yeast proteins cytochrome c oxidase subunit Va, the F1-ATPase beta subunit, mitochondrial malate dehydrogenase, and the ATP/ADP carrier. In addition, uptake of two non-yeast precursor proteins (human ornithine transcarbamylase and a cytochrome oxidase subunit IV-dihydrofolate reductase fusion), was also blocked by the peptide. Subsequent studies revealed that pL4-(1-22) did not block the initial recognition or binding of proteins to mitochondria. Rather, our results suggest that the peptide acts at a subsequent translocation step which is common to the import pathways of many different precursor proteins.

Localization of a synthetic presequence that blocks protein import into mitochondria

In the accompanying paper (Glaser, S. M., and Cumsky, M. G. (1990) J. Biol. Chem. 265, 8808-8816) we demonstrated that pL4-(1-22), a synthetic peptide corresponding to the N-terminal 22 residues of the cytochrome c oxidase subunit IV presequence, blocked protein import into mitochondria. Import inhibition was reversible and occurred at a step subsequent to the initial recognition and binding of precursor proteins to the mitochondrial surface. In the present work we have studied the nature of the association between the peptide and mitochondria, as well as determined its intramitochondrial location. We found that pL4-(1-22) was imported into mitochondria in a manner that was dependent upon the delta psi and that the majority of the mitochondrially associated peptide was in the membrane fraction. Density gradient analysis of total membranes indicated that pL4-(1-22) cofractionated with the inner membrane, although the possibility that it was present in both membranes could not be ruled out. It appeared to be inserted within the bilayer since it could not be extracted with salts, chaotropic agents, or high pH. We observed a steady decrease in the amount of pL4-(1-22) found within peptide-treated mitochondria over time. Coincident with this decrease was an increase in the ability of those mitochondria to import and process precursor proteins, suggesting that the peptide was ultimately turned over. The results presented here correlate well with those of the accompanying paper. Together they suggest that pL4-(1-22) blocks import at the level of the mitochondrial membranes, although the exact nature of the import block is not yet clear.

Removal of a hydrophobic domain within the mature portion of a mitochondrial inner membrane protein causes its mislocalization to the matrix

We have examined the import and intramitochondrial localization of the precursor to yeast cytochrome c oxidase subunit Va, a protein of the mitochondrial inner membrane. The results of studies on the import of subunit Va derivatives carrying altered presequences suggest that the uptake of this protein is highly efficient. We found that a presequence of only 5 amino acids (Met-Leu-Ser-Leu-Arg) could direct the import and localization of subunit Va with wild-type efficiency, as judged by several different assays. We also found that subunit Va could be effectively targeted to the mitochondrial inner membrane with a heterologous presequence that failed to direct import of its cognate protein. The results presented here confirmed those of an earlier study and showed clearly that the information required to "sort" subunit Va to the inner membrane resides in the mature protein sequence, not within the presequence per se. We present additional evidence that the aforementioned sorting information is contained, at least in part, in a hydrophobic stretch of 22 amino acids residing within the C-terminal third of the protein. Removal of this domain caused subunit Va to be mislocalized to the mitochondrial matrix.

Functional analysis of mitochondrial protein import in yeast

In order to facilitate studies on protein localization to and sorting within yeast mitochondria, we have designed an experimental system that utilizes a new vector and a functional assay. The vector, which we call an LPS plasmid (for leader peptide substitution), employs a yeast COX5a gene (the structural gene for subunit Va of the inner membrane protein complex cytochrome c oxidase) as a convenient reporter for correct mitochondrial localization. Using in vitro mutagenesis, we have modified COX5a so that the DNA sequences encoding the wild-type subunit Va leader peptide can be precisely deleted and replaced with a given test sequence. The substituted leader peptide can then be analyzed for its ability to direct subunit Va to the inner mitochondrial membrane (to target and sort) by complementation or other in vivo assays. In this study we have tested the ability of several heterologous sequences to function in this system. The results of these experiments indicate that a functional leader peptide is required to target subunit Va to mitochondria. In addition, leader peptides, or portions thereof, derived from proteins located in other mitochondrial compartments can also be used to properly localize this polypeptide. The results presented here also indicate that the information necessary to sort subunit Va to the inner mitochondrial membrane does not reside in the leader peptide but rather in the mature subunit Va sequence.

SL12 murine T-lymphoma: a new model for tumor cell heterogeneity

It has been observed that subclones from the spontaneous murine AKR/J T-lymphoma cell line SL12 with similar in vitro growth characteristics exhibit stable differences in tumorigenicity. The cell line is composed of at least three distinct cloned cell types that are highly, moderately, or poorly tumorigenic in syngeneic host animals. When healthy, young, syngeneic host animals were given iv injections with the same number of viable growth phase cells, each cloned cell type had a different tumor incidence, latent period, and pattern of tumor spread. The unusual stability of the cloned cell lines is shown by a similar incidence, latency, and spread of the tumors when studied after more than 1 year of continuous in vitro culture. The SL12 clones also differ in several phenotypic characteristics commonly used to classify thymocyte maturation, e.g., a) the expression of three of seven surface antigens examined, b) the cellular response to glucocorticoid hormone, and c) the expression of terminal deoxynucleotidyl transferase.