Consensus on Contouring Primary Breast Tumors on MRI in the Setting of Neoadjuvant Partial Breast Irradiation in Trials

A Contrast-Enhanced Tri-Modal MRI Technique for High-Performance Hypoxia Imaging of Breast Cancer

Personalized radiotherapy strategies enabled by the construction of hypoxia-guided biological target volumes (BTVs) can overcome hypoxia-induced radioresistance by delivering high-dose radiotherapy to targeted hypoxic areas of the tumor. However, the construction of hypoxia-guided BTVs is difficult owing to lack of precise visualization of hypoxic areas. This study synthesizes a hypoxia-responsive T1 , T2 , T2 mapping tri-modal MRI molecular nanoprobe (SPION@ND) and provides precise imaging of hypoxic tumor areas by utilizing the advantageous features of tri-modal magnetic resonance imaging (MRI). SPION@ND exhibits hypoxia-triggered dispersion-aggregation structural transformation. Dispersed SPION@ND can be used for routine clinical BTV construction using T1 -contrast MRI. Conversely, aggregated SPION@ND can be used for tumor hypoxia imaging assessment using T2 -contrast MRI. Moreover, by introducing T2 mapping, this work designs a novel method (adjustable threshold-based hypoxia assessment) for the precise assessment of tumor hypoxia confidence area and hypoxia level. Eventually this work successfully obtains hypoxia tumor target and accurates hypoxia tumor target, and achieves a one-stop hypoxia-guided BTV construction. Compared to the positron emission tomography-based hypoxia assessment, SPION@ND provides a new method that allows safe and convenient imaging of hypoxic tumor areas in clinical settings.

The Use of Magnetic Resonance Imaging in Radiation Therapy Treatment Simulation and Planning

Ever since its introduction as a diagnostic imaging tool the potential of magnetic resonance imaging (MRI) in radiation therapy (RT) treatment simulation and planning has been recognized. Recent technical advances have addressed many of the impediments to use of this technology and as a result have resulted in rapid and growing adoption of MRI in RT. The purpose of this article is to provide a broad review of the multiple uses of MR in the RT treatment simulation and planning process, identify several of the most used clinical scenarios in which MR is integral to the simulation and planning process, highlight existing limitations and provide multiple unmet needs thereby highlighting opportunities for the diagnostic MR imaging community to contribute and collaborate with our oncology colleagues. EVIDENCE LEVEL: 5 TECHNICAL EFFICACY: Stage 5.

Assessing Interobserver Variability in the Delineation of Structures in Radiation Oncology: A Systematic Review

PURPOSE

The delineation of target volumes and organs at risk is the main source of uncertainty in radiation therapy. Numerous interobserver variability (IOV) studies have been conducted, often with unclear methodology and nonstandardized reporting. We aimed to identify the parameters chosen in conducting delineation IOV studies and assess their performances and limits.

METHODS AND MATERIALS

We conducted a systematic literature review to highlight major points of heterogeneity and missing data in IOV studies published between 2018 and 2021. For the main used metrics, we did in silico analyses to assess their limits in specific clinical situations.

RESULTS

All disease sites were represented in the 66 studies examined. Organs at risk were studied independently of tumor site in 29% of reviewed IOV studies. In 65% of studies, statistical analyses were performed. No gold standard (GS; ie, reference) was defined in 36% of studies. A single expert was considered as the GS in 21% of studies, without testing intraobserver variability. All studies reported both absolute and relative indices, including the Dice similarity coefficient (DSC) in 68% and the Hausdorff distance (HD) in 42%. Limitations were shown in silico for small structures when using the DSC and dependence on irregular shapes when using the HD. Variations in DSC values were large between studies, and their thresholds were inconsistent. Most studies (51%) included 1 to 10 cases. The median number of observers or experts was 7 (range, 2-35). The intraclass correlation coefficient was reported in only 9% of cases. Investigating the feasibility of studying IOV in delineation, a minimum of 8 observers with 3 cases, or 11 observers with 2 cases, was required to demonstrate moderate reproducibility.

CONCLUSIONS

Implementation of future IOV studies would benefit from a more standardized methodology: clear definitions of the gold standard and metrics and a justification of the tradeoffs made in the choice of the number of observers and number of delineated cases should be provided.

Adaptive radiotherapy for breast cancer

Research in the field of local and locoregional breast cancer radiotherapy aims to maintain excellent oncological outcomes while reducing treatment-related toxicity. Adaptive radiotherapy (ART) considers variations in target and organs at risk (OARs) anatomy occurring during the treatment course and integrates these in re-optimized treatment plans. Exploiting ART routinely in clinic may result in smaller target volumes and better OAR sparing, which may lead to reduction of acute as well as late toxicities. In this review MR-guided and CT-guided ART for breast cancer patients according to different clinical scenarios (neoadjuvant and adjuvant partial breast irradiation, whole breast, chest wall and regional nodal irradiation) are reviewed and their advantages as well as challenging aspects discussed.

Comparison of dosimetry with magnetic resonance and computed tomography imaging delineation of surgical bed volume in breast cancer irradiation

Background

Postoperative radiotherapy after conservative surgery for patients with breast cancer usually includes focal over-irradiation (boost) to the surgical bed (SB). Irradiation planning using computed tomography (CT) is difficult in many cases because of insufficient intrinsic soft tissue contrast. To ensure appropriate radiation to the tumor, large boost volumes are delineated, resulting in a higher dose to the normal tissue. Magnetic resonance imaging (MRI) provides superior soft tissue contrast than CT and can better differentiate between normal tissue and the SB. However, for SB delineation CT images alone remain the pathway followed in patients undergoing breast irradiation. This study aimed to evaluate the potential advantages in boost dosimetry by using MRI and CT as pre-treatment imaging.

Methods

Eighteen boost volumes were drawn on CT and MRI and elastically co-registered using commercial image registration software. The radiotherapy treatment plan was optimized using the CT volumes as the baseline. The dose distributions of the target volumes on CT and MRI were compared using dose-volume histogram cutoff points.

Results

The radiation volumes to the SB varied considerably between CT and MRI (conformity index between 0.24 and 0.67). The differences between the MRI and CT boost doses in terms of the volume receiving 98% of the prescribed dose (V98%) varied between 10% and 30%. Smaller differences in the V98% were observed when the boost volumes were delineated using MRI.

Conclusion

Using MRI to delineate the volume of the SB may increase the accuracy of boost dosimetry.

Prone vs. supine accelerated partial breast irradiation on an MR-Linac: A planning study

BACKGROUND AND PURPOSE

Accelerated partial breast irradiation (APBI) may benefit from the MR-Linac for target definition, patient setup, and motion monitoring. In this planning study, we investigated whether prone or supine position is dosimetrically beneficial for APBI on an MR-Linac and we evaluated patient comfort.

MATERIALS AND METHODS

Twenty-patients (9 postoperative, 11 preoperative) with a DCIS or breast tumor <3 cm underwent 1.5 T MRI in prone and supine position. The tumor or tumor bed was delineated as GTV and a 2 cm CTV-margin and 0.5 cm PTV-margin were added. 1.5 T MR-Linac treatment plans (5 × 5.2 Gy) with 11 beams were created for both positions in each patient. We evaluated the number of plans that achieved the planning constraints and performed a dosimetric comparison between prone and supine position using the Wilcoxon signed-rank test (p-value <0.01 for significance). Patient experience during scanning was evaluated with a questionnaire.

RESULTS

All 40 plans met the target coverage and OAR constraints, regardless of position. Heart D_mean was not significantly different (1.07 vs. 0.79 Gy, p-value: 0.027). V5Gy to the ipsilateral lung (4.4% vs. 9.8% median, p-value 0.009) and estimated delivery time (362 vs. 392 s, p-value: 0.003) were significantly lower for prone position. PTV coverage and dose to other OAR were comparable between positions. The majority of patients (13/20) preferred supine position.

CONCLUSION

APBI on the MR-Linac is dosimetrically feasible in prone and supine position. Mean heart dose was similar in both positions. Ipsilateral lung V5Gy was lower in prone position.

Use of a healthy volunteer imaging program to optimize clinical implementation of stereotactic MR-guided adaptive radiotherapy

PURPOSE

MR-linacs (MRLs) have enabled the use of stereotactic magnetic resonance (MR) guided online adaptive radiotherapy (SMART) across many cancers. As data emerges to support SMART, uncertainty remains regarding optimal technical parameters, such as optimal patient positioning, immobilization, image quality, and contouring protocols. Prior to clinical implementation of SMART, we conducted a prospective study in healthy volunteers (HVs) to determine optimal technical parameters and to develop and practice a multidisciplinary SMART workflow.

METHODS

HVs 18 years or older were eligible to participate in this IRB-approved study. Using a 0.35 T MRL, simulated adaptive treatments were performed by a multi-disciplinary treatment team in HVs. For each scan, image quality parameters were assessed on a 5-point scale (5 = extremely high, 1 = extremely poor). Adaptive recontouring times were compared between HVs and subsequent clinical cases with a t-test.

RESULTS

18 simulated treatments were performed in HVs on MRL. Mean parameters for visibility of target, visibility of nearby organs, and overall image quality were 4.58, 4.62, and 4.62, respectively (range of 4-5 for all measures). In HVs, mean ART was 15.7 min (range 4-35), comparable to mean of 16.1 (range 7-33) in the clinical cases (p = 0.8963). Using HV cases, optimal simulation and contouring guidelines were developed across a range of disease sites and have since been implemented clinically.

CONCLUSIONS

Prior to clinical implementation of SMART, scans of HVs on an MRL resulted in acceptable image quality and target visibility across a range of organs with similar ARTs to clinical SMART. We continue to utilize HV scans prior to clinical implementation of SMART in new disease sites and to further optimize target tracking and immobilization. Further study is needed to determine the optimal duration of HV scanning prior to clinical implementation.

Optimizing MR-Guided Radiotherapy for Breast Cancer Patients

Current research in radiotherapy (RT) for breast cancer is evaluating neoadjuvant as opposed to adjuvant partial breast irradiation (PBI) with the aim of reducing the volume of breast tissue irradiated and therefore the risk of late treatment-related toxicity. The development of magnetic resonance (MR)-guided RT, including dedicated MR-guided RT systems [hybrid machines combining an MR scanner with a linear accelerator (MR-linac) or 60Co sources], could potentially reduce the irradiated volume even further by improving tumour visibility before and during each RT treatment. In this position paper, we discuss MR guidance in relation to each step of the breast RT planning and treatment pathway, focusing on the application of MR-guided RT to neoadjuvant PBI.