The significance of PTV dose coverage on cancer control outcomes in early stage non-small cell lung cancer patients treated with highly ablative stereotactic body radiation therapy

Stereotactic Body Radiotherapy for Management of Pulmonary Oligometastases in Stage IV Colorectal Cancer: A Perspective

In pulmonary oligometastases from colorectal cancer (POM-CRC), metastasectomy is the primarily recommended treatment. Stereotactic body radiotherapy (SBRT) has been suggested as a viable alternative therapy. SBRT efficacy for POM-CRC is poorly delineated compared to selected non-CRC primaries. This perspective article aims to critically summarize the existing evidence regarding efficacy of SBRT in terms of overall survival (OS) and local control (LC), and factors modulating this, in the treatment of POM-CRC. Overall, reasonable LC and OS rates were observed. The wide range of expansions in planning target volume margins introduced variation in pretreatment protocols. Dose-fractionation schedules varied according to patient and tumor characteristics, though leverage of BED10 in select studies enabled standardization. An association between SBRT dose and improved OS and LC was observed across multiple studies. Prognostic factors that were associated with improved LC included: fewer oligometastases, absence of extra-pulmonary metastases, primary tumor histology, and smaller gross tumor volume. Differences in SBRT modality and techniques over time further confounded results. Many studies included patients receiving additional systemic therapies; preprotocol and adjuvant chemotherapies were identified as prognostic factors for LC. SBRT compared with metastasectomy showed no differences in short-term OS and LC outcomes. In conclusion, SBRT is an efficacious treatment for POM-CRC, in terms of OS and LC. Heterogeneity in study design, particularly pertaining to dose protocols, patient selection, and additional therapies should be controlled for future randomized studies to further validate SBRT efficacy.

Dosimetric and workflow impact of synthetic-MRI use in prostate high-dose-rate brachytherapy

PURPOSE

Target and organ delineation during prostate high-dose-rate (HDR) brachytherapy treatment planning can be improved by acquiring both a postimplant CT and MRI. However, this leads to a longer treatment delivery workflow and may introduce uncertainties due to anatomical motion between scans. We investigated the dosimetric and workflow impact of MRI synthesized from CT for prostate HDR brachytherapy.

METHODS AND MATERIALS

Seventy-eight CT and T2-weighted MRI datasets from patients treated with prostate HDR brachytherapy at our institution were retrospectively collected to train and validate our deep-learning-based image-synthesis method. Synthetic MRI was assessed against real MRI using the dice similarity coefficient (DSC) between prostate contours drawn using both image sets. The DSC between the same observer's synthetic and real MRI prostate contours was compared with the DSC between two different observers' real MRI prostate contours. New treatment plans were generated targeting the synthetic MRI-defined prostate and compared with the clinically delivered plans using target coverage and dose to critical organs.

RESULTS

Variability between the same observer's prostate contours from synthetic and real MRI was not significantly different from the variability between different observer's prostate contours on real MRI. Synthetic MRI-planned target coverage was not significantly different from that of the clinically delivered plans. There were no increases above organ institutional dose constraints in the synthetic MRI plans.

CONCLUSIONS

We developed and validated a method for synthesizing MRI from CT for prostate HDR brachytherapy treatment planning. Synthetic MRI use may lead to a workflow advantage and removal of CT-to-MRI registration uncertainty without loss of information needed for target delineation and treatment planning.

Should organs at risk (OARs) be prioritized over target volume coverage in stereotactic ablative radiotherapy (SABR) for oligometastases? a secondary analysis of the population-based phase II SABR-5 trial

BACKGROUND AND PURPOSE

Stereotactic ablative radiotherapy (SABR) for oligometastases may improve survival, however concerns about safety remain. To mitigate risk of toxicity, target coverage was sacrificed to prioritize organs-at-risk (OARs) during SABR planning in the population-based SABR-5 trial. This study evaluated the effect of this practice on dosimetry, local recurrence (LR), and progression-free survival (PFS).

METHODS

This single-arm phase II trial included patients with up to 5 oligometastases between November 2016 and July 2020. Theprotocol-specified planning objective was to cover 95 % of the planning target volume (PTV) with 100 % of the prescribed dose, however PTV coverage was reduced as needed to meet OAR constraints. This trade-off was measured using the coverage compromise index (CCI), computed as minimum dose received by the hottest 99 % of the PTV (D99) divided by the prescription dose. Under-coverage was defined as CCI < 0.90. The potential association between CCI and outcomes was evaluated.

RESULTS

549 lesions from 381 patients were assessed. Mean CCI was 0.88 (95 % confidence interval [CI], 0.86-0.89), and 196 (36 %) lesions were under-covered. The highest mean CCI (0.95; 95 %CI, 0.93-0.97) was in non-spine bone lesions (n = 116), while the lowest mean CCI (0.71; 95 % CI, 0.69-0.73) was in spine lesions (n = 104). On multivariable analysis, under-coverage did not predict for worse LR (HR 0.48, p = 0.37) or PFS (HR 1.24, p = 0.38). Largest lesion diameter, colorectal and 'other' (non-prostate, breast, or lung) primary predicted for worse LR. Largest lesion diameter, synchronous tumor treatment, short disease free interval, state of oligoprogression, initiation or change in systemic treatment, and a high PTV Dmax were significantly associated with PFS.

CONCLUSION

PTV under-coverage was not associated with worse LR or PFS in this large, population-based phase II trial. Combined with low toxicity rates, this study supports the practice of prioritizing OAR constraints during oligometastatic SABR planning.

Dose-effect relationship of stereotactic body radiotherapy in non-small cell lung cancer patients

OBJECTIVE

To establish the dose effect relationship between the dose parameters of stereotactic body radiation therapy (SBRT) for early non-small cell lung cancer (NSCLC) and the local tumor control rate.

MATERIALS AND METHODS

A comprehensive literature search was conducted using PubMed, the Web of Science and the Cochrane databases to determine the articles treated with SBRT in early-stage NSCLC. Original studies with complete prescription dose information, tumor local control rate and other important parameters were screened and reported. Probit model in XLSTAT 2016 was used for regression analysis, and P < 0.05 was set as a statistically significant level.

RESULTS

After literature screening, 22 eligible studies were included in probit model regression analysis, involving 1861 patients. There is no significant dose effect relationship between nominal BED₁₀ and peripheral BED₁₀ versus 3 years local control probability. There were significant dose effect relationships between the center BED₁₀ and the average BED₁₀ versus the 3 years local control probability, with P values are 0.001 and < 0.0001, respectively. According to the results of this model, the 3 years local control rate of 90.5% (87.5-92.1%) and 89.5% (86.7-91.0%) can be expected at the center BED₁₀ of 180 Gy or the average BED₁₀ of 140 Gy, prospectively.

CONCLUSIONS

For NSCLC treated with SBRT, more attention should be paid to the central dose and average dose of PTV. A set of clear definition in the dose prescription should be established to ensure the effectiveness and comparability of treatment.

How 3 Academic Centers Prescribe Stereotactic Body Radiation Therapy for Primary Lung Cancer

Stereotactic body radiation therapy (SBRT) is commonly used to treat early-stage, node-negative primary lung cancer, but society guidelines provide limited information regarding several technical aspects of SBRT, leading to potential variation in practice. In this report, we present the technical details used by 3 academic institutions when treating a solitary primary lung tumor up to 5 cm in dimension with curative-intent SBRT. We provide specifications outlined in major active or recently completed clinical trials. Among the participating institutions, we discovered multiple divergences in treatment parameters, including, but not limited to, prescription dose and desired degree of heterogeneity within the target volume. It is unclear to what extent these differences in parameters might affect tumor control or toxicity, but updated consensus guidelines addressing the relevant SBRT prescription details may help standardize practice patterns.

Relationship between Dose Prescription Methods and Local Control Rate in Stereotactic Body Radiotherapy for Early Stage Non-Small-Cell Lung Cancer: Systematic Review and Meta-Analysis

Variations in dose prescription methods in stereotactic body radiotherapy (SBRT) for early stage non-small-cell lung cancer (ES-NSCLC) make it difficult to properly compare the outcomes of published studies. We conducted a comprehensive search of the published literature to summarize the outcomes by discerning the relationship between local control (LC) and dose prescription sites. We systematically searched PubMed to identify observational studies reporting LC after SBRT for peripheral ES-NSCLC. The correlations between LC and four types of biologically effective doses (BED) were evaluated, which were calculated from nominal, central, and peripheral prescription points and, from those, the average BED. To evaluate information on SBRT for peripheral ES-NSCLC, 188 studies were analyzed. The number of relevant articles increased over time. The use of an inhomogeneity correction was mentioned in less than half of the articles, even among the most recent. To evaluate the relationship between the four BEDs and LC, 33 studies were analyzed. Univariate meta-regression revealed that only the central BED significantly correlated with the 3-year LC of SBRT for ES-NSCLC (p = 0.03). As a limitation, tumor volume, which might affect the results of this study, could not be considered due to a lack of data. In conclusion, the central dose prescription is appropriate for evaluating the correlation between the dose and LC of SBRT for ES-NSCLC. The standardization of SBRT dose prescriptions is desirable.

Conformal Avoidance of Normal Organs at Risk by Perfusion-Modulated Dose Sculpting in Tumor Single-Dose Radiation Therapy

PURPOSE

Although 24 Gy single-dose radiation therapy (SDRT) renders >90% 5-year local relapse-free survival in human solid tumor lesions, SDRT delivery is not feasible in ∼50% of oligometastatic lesions owing to interference by dose/volume constraints of a serial organ at risk (OAR). Conformal OAR avoidance is based on a hypothetical model positing that the recently described SDRT biology specifically permits volumetric subdivision of the SDRT dose, such that high-intensity vascular drivers of SDRT lethality, generated within a major tumor subvolume exposed to a high 24 Gy dose (high-dose planning target volume [PTV_HD]), would equilibrate SDRT signaling intensity throughout the tumor interstitial space, rendering bystander radiosensitization of a minor subvolume (perfusion-modulated dose sculpting PTV [PTV_PMDS]), dose-sculpted to meet a serial OAR dose/volume constraint. An engineered PTV_PMDS may thus yield tumor ablation despite PMDS dose reduction and conformally avoiding OAR exposure to a toxic dose.

METHODS AND MATERIALS

Dose fall-off within the PTV_PMDS penumbra of oligometastatic lesions was planned and delivered by intensity modulated inverse dose painting. SDRT- and SDRT-PMDS-treated lesions were followed with periodic positron emission tomography/computed tomography imaging to assess local tumor control.

RESULTS

Cumulative baseline 5-year local relapse rates of oligometastases treated with 24 Gy SDRT alone (8% relapses, n = 292) were similar in moderate PTV_PMDS dose-sculpted (23-18 Gy, n = 76, 11% relapses, P = .36) and extreme dose-sculpted (<18 Gy, n = 61, 14% relapses, P = .29) lesions, provided the major 24 Gy PTV_HD constituted ≥60% of the total PTV. In contrast, 28% of local relapses occurred in 26 extreme dose-sculpted PTV_PMDS lesions when PTV_HD constituted <60% of the total PTV (P = .004), suggesting a threshold for the PTV_PMDS bystander effect.

CONCLUSION

The study provides compelling clinical support for the bystander radiosensitization hypothesis, rendering local cure of tumor lesions despite a ≥25% PTV_PMDS dose reduction of the 24 Gy PTV_HD dose, adapted to conformally meet OAR dose/volume constraints. The SDRT-PMDS approach thus provides a therapeutic resolution to otherwise radioablation-intractable oligometastatic disease.

Stereotactic body radiotherapy for primary non-small cell lung cancer patients with clinical T3-4N0M0 (UICC 8th edition): outcomes and patterns of failure

The evidence for stereotactic body radiotherapy (SBRT) is meagre for patients with clinical T3-4N0M0 non-small cell lung cancer (8th Edition of the Union for International Cancer Control (UICC)). This study retrospectively investigated clinical outcomes following SBRT for such patients. Among consecutive patients treated with SBRT, patients staged as cT3-4N0M0 by all criteria were examined, most of whom were unsuitable to chemoradiotherapy due to their fragile characters. Clinical outcomes were evaluated and factors associated with outcomes were investigated. Between 2005 and 2017, 70 eligible patients (T3: 58, T4: 12; median age 81 (63-93) years) were identified. Median follow-up duration was 28.6 (1.0-142.5) months. No adjuvant chemotherapy was administered. The 3-year local recurrence rates were 15.8% and 16.7% in T3 and T4 patients, respectively, and they were significantly lower in the high-dose group (3.1% vs 28.6%, P < 0.01). Multivariate analyses showed that the dose-volumetric factor was the significant factor for local recurrence. The 3-year regional and distant metastasis rates, cancer-specific mortality, and overall survival in T3 and T4 patients were 22.7% and 25.0%, 26.5% and 33.3%, 32.2% and 41.7%, and 39.5% and 41.7%, respectively. Only age was correlated with overall survival. Radiation pneumonitis ≥grade 3 and fatal hemoptysis occurred in 3 and 1 patients, respectively. SBRT for cT3-4N0M0 lung cancer patients achieved good local control. Survival was rather good considering that patients were usually frail, staged with clinical staging, and were not given adjuvant chemotherapy, and it may be comparable to surgery. To validate these outcomes following SBRT, a prospective study is warranted.

Initial clinical observations of intra- and interfractional motion variation in MR-guided lung SBRT

OBJECTIVE

To evaluate variations in intra- and interfractional tumour motion, and the effect on internal target volume (ITV) contour accuracy, using deformable image registration of real-time two-dimensional-sagittal cine-mode MRI acquired during lung stereotactic body radiation therapy (SBRT) treatments.

METHODS

Five lung tumour patients underwent free-breathing SBRT treatments on the ViewRay system, with dose prescribed to a planning target volume (defined as a 3-6 mm expansion of the 4DCT-ITV). Sagittal slice cine-MR images (3.5 × 3.5 mm² pixels) were acquired through the centre of the tumour at 4 frames per second throughout the treatments (3-4 fractions of 21-32 min). Tumour gross tumour volumes (GTVs) were contoured on the first frame of the MR cine and tracked for the first 20 min of each treatment using offline optical-flow based deformable registration implemented on a GPU cluster. A ground truth ITV (MR-ITV_20 min) was formed by taking the union of tracked GTV contours. Pseudo-ITVs were generated from unions of the GTV contours tracked over 10 s segments of image data (MR-ITV_10 s).

RESULTS

Differences were observed in the magnitude of median tumour displacement between days of treatments. MR-ITV_10 s areas were as small as 46% of the MR-ITV_20 min.

CONCLUSION

An ITV offers a "snapshot" of breathing motion for the brief period of time the tumour is imaged on a specific day. Real-time MRI over prolonged periods of time and over multiple treatment fractions shows that ITV size varies. Further work is required to investigate the dosimetric effect of these results. Advances in knowledge: Five lung tumour patients underwent free-breathing MRI-guided SBRT treatments, and their tumours tracked using deformable registration of cine-mode MRI. The results indicate that variability of both intra- and interfractional breathing amplitude should be taken into account during planning of lung radiotherapy.

Stereotactic body radiotherapy for operable early-stage non-small cell lung cancer

PURPOSE

To analyze outcomes of stereotactic body radiotherapy (SBRT) for operable patients with early-stage non-small cell lung cancer (NSCLC) and to evaluate factors associated with outcomes.

METHODS

We retrospectively analyzed operable patients with NSCLC, staged as cT1-2N0M0, treated with SBRT between 2006 and 2015. Both biopsy-proven and clinically diagnosed NSCLC were included. Local control and survival rates were calculated and compared between subsets of patients. We investigated factors associated with outcomes.

RESULTS

We identified 88 operable patients among 661 patients with cT1-2N0M0 NSCLC. The median age was 79 years (range: 55-88). The median follow-up time after SBRT was 40 months (range: 4-121). Fifty-nine patients had been pathologically diagnosed and the other 29 had been clinically diagnosed as having NSCLC. Local control, cause-specific survival (CSS) and overall survival (OS) at 3 years were 91%, 97% and 90% for T1, and 100%, 82% and 74% for T2, respectively. The CSS and OS at 3 years were 100% and 100% for GGO and 83% and 59% for solid tumors, respectively (p=0.005). On univariate analysis, age and T stage were significantly associated with CSS, and age, the Charlson Comorbidity Index (CCI), and opacity were significantly associated with OS. On multivariate analysis, age and CCI were significantly associated with OS. As for toxicities, Grades 0, 1, 2 and 3 radiation pneumonitis occurred in 37.5%, 47.7%, 13.6% and 1.1% of patients, respectively. No Grade 4 or 5 radiation pneumonitis occurred, and no other toxicities of Grade 2 or above were observed.

CONCLUSION

Outcomes of SBRT for operable early stage NSCLC were as good as previous SBRT and surgery studies. Further investigation for selecting good SBRT candidates is warranted in high-risk operable patients.