Results of the ACCEL trial: Dosimetry in accelerated partial breast irradiation

Early and Intermediate Treatment Outcome After Postoperative External Beam Accelerated Partial Breast Irradiation in Patients With Early-Stage Breast Cancer

PURPOSE

To prospectively evaluate early and intermediate outcome after accelerated partial breast irradiation (APBI) in patients early-with stage breast cancer.

METHODS AND MATERIALS

Inclusion criteria were defined according to the APBI American Society for Radiation Oncology's ASTRO Evidence-Based Consensus Statement. The prescribed dose was 26 to 28 Gy in 5 fractions on 5 consecutive days. Regular follow-up visits with objective and subjective evaluation of treatment tolerance were performed after 0 and 2 weeks, 6 months, and at annual intervals.

RESULTS

Between February 2017 and January 2020, 175 patients with breast conserving surgery met the inclusion criteria for APBI. Mean age was 65.7 years (range, 46-88). Thirteen percent of patients received a diagnosis with carcinoma in situ, 55%, 35%, and 37% with T1a/b/c, and 10% with T2 stages, respectively. The mean volume of planning target volume (PTV) was 119 cc (range, 45-465), the ratio of mean PTV: whole breast volume ratio was 21% (7%-53%). Mean follow-up was 42 months (median, 45, range, 0-67). Acute toxicity after 2 weeks was low with 69%, 26%, and 5% grade 0, 1, and 2. In addition, 1-, 2-, 3-, 4-, and 5-year follow-up data were available from 146, 134, 107, 73, and 25 patients. Patient-reported cosmetic outcomes were assessed excellent or good in 97.9%, 98.5%, 98.1%, 98.6%, and 100%. Regarding grade 2 toxicities, as by now 3%, 2%, 2%, 0%, and 0% G2 fibrosis, 1%, 1%, 0%, 0%, and 0% G2 atrophy, no G2 skin telangiectasia or breast edema occurred. So far, none of the patients have experienced G3 toxicity or higher. The remaining patients had grade 0 or 1 toxicity only. Five ipsilateral breast recurrences (1 marginally to PTV, 4 out-of-field) and 5 distant recurrences were recorded by March 2023. The 4-year in-breast recurrence rate was 2.5%. Eight patients died, with 2 of them from disease. For all patients, the 4-year overall, cancer specific and disease-free survival rates were 97.1%, 99.4%, and 95.3%, respectively.

CONCLUSIONS

We showed high early- and intermediate-term treatment tolerance and disease control of APBI using 26 to 28 Gy in five fractions in one week in carefully selected patients with early breast cancer. APBI is highly appreciated by patients and efficient, as an additional advantage for busy centers.

Accelerated Partial Breast Irradiation Using 5 Daily Fractions: A Prospective, Phase 2, Multicenter Trial of Cosmetic Outcomes and Toxicity-ACCEL Final Results

PURPOSE

This study investigated the cosmetic degradation and toxicity for an accelerated partial breast irradiation (APBI) prescription delivered in 5 fractions over 1 week and compared the outcomes with those of whole breast irradiation (WBI).

METHODS AND MATERIALS

The trial was a multicenter, single-arm, phase 2 prospective cohort study. Eligible women 50 years of age or older with estrogen receptor-positive and human epidermal growth factor receptor 2-negative invasive ductal carcinoma or ductal carcinoma in situ after breast-conserving surgery received 27 Gy in 5 daily fractions of APBI. The primary endpoint was noninferiority of 2-year cosmesis using the RAPID trial's WBI arm as the control arm. A global consensus cosmetic score using a European Organisation for Research and Treatment of Cancer rating scale score of excellent, good, fair, or poor for each patient at baseline and 2 years was generated by a panel of 5 radiation oncologists using photographs of treated and untreated breasts.

RESULTS

From 2016 to 2019, 298 eligible women were enrolled. By the 2-year follow-up, 76 patients had been lost or withdrawn and 3 had died, resulting in 219 patients available for complete, 2-year photographic cosmetic evaluation. The median follow-up for all participants was 4.7 years (IQR, 3.8-5.5 years). No patient had a fair or poor cosmetic score at the 2-year evaluation. Cosmesis was better or unchanged for 97% of patients and worse for 3% (excellent to good), and no cosmetic failures occurred. The confidence intervals were 0.88 (0.86-0.90) and 1.00 (0.99-1.00) for the RAPID and ACCEL trials, respectively.

CONCLUSIONS

Cosmetic degradation with 5 daily treatments of the ACCEL trial's APBI intervention is noninferior to the WBI arm of the RAPID trial.

Microwave imaging for monitoring breast cancer treatment: A pilot study

BACKGROUND

Microwave imaging has been proposed for medical applications, creating maps related to water content of tissues. Breast imaging has emerged as a key application because the signals can be coupled directly into the breast and experience limited attenuation in fatty tissues. While the literature contains reports of tumor detection with microwave approaches, there is limited exploration of treatment monitoring.

PURPOSE

This study aims to detect treatment-related changes in breast tissue with a low-resolution microwave scanner.

METHODS

Microwave scans of 15 patients undergoing treatment for early-stage breast cancer are collected at up to 4 time points: after surgery (baseline), 6 weeks after accelerated partial breast radiation, as well as 1 and 2 years post-treatment. Both the treated and untreated breast are scanned at each time point. The microwave scanner consists of planar transmit and receive arrays and uses signals from 0.1 to 10 GHz. The average microwave frequency properties (permittivity) are calculated for each scan to enable quantitative comparison. Baseline and 6-week results are analyzed with a two-way ANOVA with blocking.

RESULTS

Consistent properties are observed for the untreated breast over time, similar to a previous study. Comparison of the scans of the treated and untreated breast suggests increased properties related to treatment, particularly at baseline and 6-weeks following radiotherapy. Analysis of the average properties of the scans with ANOVA indicates statistically significant differences (p < 0.05 $p < 0.05$ ) in the treated and untreated breast at these time points.

CONCLUSIONS

Microwave imaging has the potential to track treatment-related changes in breast tissues.

Assessment of Tissue Toxicity Risk in Breast Radiotherapy using Bayesian Networks

PURPOSE

The purpose of this analysis is to predict worsening post-treatment normal tissue toxicity in patients undergoing accelerated partial breast irradiation (APBI) therapy and to quantitatively identify which diagnostic, anatomical, and dosimetric features are contributing to these outcomes.

METHODS

A retrospective study of APBI treatments was performed using 32 features pertaining to various stages of the patient's treatment journey. These features were used to inform and construct a Bayesian network (BN) based on both statistical analysis of feature distributions and relative clinical importance. The target feature for prediction was defined as a measurable worsening of telangiectasia, subcutaneous tissue induration, or fibrosis when compared against the observed baseline. Parameter learning for the network was performed using data from the 299 patients included in the ACCEL trial and predictive performance was measured. Feature importance for the BN was quantified using a novel information-theoretic approach.

RESULTS

Cross validated performance of the BN for predicting toxicity was consistently higher when compared against conventional machine learning (ML) techniques. The measured BN receiver operating characteristic area under the curve was 0.960±0.013 against the best ML result of 0.942±0.021 using 5-fold cross validation with separate test data across 100 trials. The volume of the clinical target volume, gross target volume, and baseline toxicity measurements were found to have the highest feature importance and mutual dependence with normal tissue toxicity in the network, representing the strongest contribution to patient outcomes.

CONCLUSIONS

The BN outperformed conventional ML techniques in predicting tissue toxicity outcomes and provided deeper insight into which features are contributing to these outcomes. This article is protected by copyright. All rights reserved.

Performance of a knowledge-based planning model for optimizing intensity-modulated radiotherapy plans for partial breast irradiation

Purpose

To evaluate a knowledge‐based (KB) planning model for RapidPlan, generated using a five‐field intensity‐modulated radiotherapy (IMRT) class solution beam strategy and rigorous dosimetric constraints for accelerated partial breast irradiation (APBI).

Materials and methods

The RapidPlan model was configured using 64 APBI treatment plans and validated for 120 APBI patients who were not included in the training dataset. KB plan dosimetry was compared to clinical plan dosimetry, the clinical planning constraints, and the constraints used in phase III APBI trials. Dosimetric differences between clinical and KB plans were evaluated using paired two‐tailed Wilcoxon signed‐rank tests.

Results

KB planning was able to produce IMRT‐based APBI plans in a single optimization without manual intervention that are comparable or better than the conventionally optimized, clinical plans. Comparing KB plans to clinical plans, differences in PTV, heart, contralateral breast, and ipsilateral lung dose–volume metrics were not clinically significant. The ipsilateral breast volume receiving at least 50% of the prescription dose was statistically and clinically significantly lower in the KB plans.

Conclusion

KB planning for IMRT‐based APBI provides equivalent or better dosimetry compared to conventional inverse planning. This model may be reliably applied in clinical practice and could be used to transfer planning expertise to ensure consistency in APBI plan quality.

Fiducial marker motion relative to the tumor bed has a significant impact on PTV margins in partial breast irradiation

INTRODUCTION

With the introduction of accelerated partial breast irradiation (APBI) and the trend of reducing the number of fractions, the geometric accuracy of treatment delivery becomes critical. APBI patient setup is often based on fiducials, as the seroma is frequently not visible on pretreatment imaging. We assessed the motion of fiducials relative to the tumor bed between planning CT and treatment, and calculated margins to compensate for this motion.

METHODS

A cohort of seventy patients treated with APBI on a Cyberknife was included. Planning and in-room pretreatment CT scans were registered on the tumor bed. Residual motion of the centers of mass of surgical clips and interstitial gold markers was calculated. We calculated the margins required per desired percentage of patients with 100% CTV coverage, and the systematic and random errors for fiducial motion.

RESULTS

For a single fraction treatment, a margin of 1.8 mm would ensure 100% CTV coverage in 90% of patients when using surgical clips for patient set-up. When using interstitial markers, the margin should be 2.2 mm. The systematic and random errors were 0.46 mm for surgical clip motion and 0.60 mm for interstitial marker motion. No clinical factors were found predictive for fiducial motion.

CONCLUSIONS

Fiducial motion relative to the tumor bed between planning CT and APBI treatment is non-negligible and should be included in the PTV margin calculation to prevent geographical miss. Systematic and random errors of fiducial motion were combined with other geometric uncertainties to calculate comprehensive PTV margins for different treatment techniques.

The influence of respiratory motion on dose distribution in accelerated partial breast irradiation using volumetric modulated arc therapy

PURPOSE

Accelerated partial breast irradiation (APBI) is alternative treatment option for patients with early stage breast cancer. The interplay effect on volumetric modulated arc therapy APBI (VMAT-APBI) has not been clarified. This study aimed to evaluate the feasibility of VMAT-APBI for patients with small breasts and investigate the amplitude of respiratory motion during VMAT-APBI delivery that significantly affects dose distribution.

METHODS

The VMAT-APBI plans were generated with 28.5 Gy in five fractions. We performed patient-specific quality assurance using Delta⁴ phantom under static conditions. We also measured point dose and dose distribution using the ionization chamber and radiochromic film under static and moving conditions of 2, 3 and 5 mm. We compared the measured and calculated point doses and dose distributions by dose difference and gamma passing rates.

RESULTS

A total of 20 plans were generated; the dose distributions were consistent with those of previous reports. For all measurements under static conditions, the measured and calculated point doses and dose distributions showed good agreement. The dose differences for chamber measurement were within 3%, regardless of moving conditions. The mean gamma passing rates with 3%/2 mm criteria in the film measurement under static conditions and with 2 mm, 3 mm, and 5 mm of amplitude were 95.0 ± 2.0%, 93.3 ± 3.3%, 92.1 ± 6.2% and 84.8 ± 7.8%, respectively. The difference between 5 mm amplitude and other conditions was statistically significant.

CONCLUSIONS

Respiratory management should be considered for the risk of unintended dose distribution if the respiratory amplitude is >5 mm.

Characterization and registration of 3D ultrasound for use in permanent breast seed implant brachytherapy treatment planning

PURPOSE

Permanent breast seed implant (PBSI) brachytherapy is a novel technique for early-stage breast cancer. Computed tomography (CT) images are used for treatment planning and freehand 2D ultrasound for implant guidance. The multimodality imaging approach leads to discrepancies in target identification. To address this, a prototype 3D ultrasound (3DUS) system was recently developed for PBSI. In this study, we characterize the 3DUS system performance, establish QA baselines, and develop and test a method to register 3DUS images to CT images for PBSI planning.

METHODS AND MATERIALS

3DUS system performance was characterized by testing distance and volume measurement accuracy, and needle template alignment accuracy. 3DUS-CT registration was achieved through point-based registration using a 3D-printed model designed and constructed to provide visible landmarks on both images and tested on an in-house made gel breast phantom.

RESULTS

The 3DUS system mean distance measurement accuracy was within 1% in axial, lateral, and elevational directions. A volumetric error of 3% was observed. The mean needle template alignment error was 1.0° ± 0.3 ° and 1.3 ± 0.5 mm. The mean 3DUS-CT registration error was within 3 mm when imaging at the breast centre or across all breast quadrants.

CONCLUSIONS

This study provided baseline data to characterize the performance of a prototype 3DUS system for PBSI planning and developed and tested a method to obtain accurate 3DUS-CT image registration for PBSI planning. Future work will focus on system validation and characterization in a clinical context as well as the assessment of impact on treatment plans.