Evaluating which plan quality metrics are appropriate for use in lung SBRT

International Patterns of Practice for Stereotactic Ablative Radiotherapy for Early-Stage Non-Small Cell Lung Cancer: Are We All in Sync?: Global patterns of practice for SABR for early-stage NSCLC

PURPOSE

To generate an understanding of the similarities and variations in international practice patterns for stereotactic ablative radiotherapy (SABR) in early-stage non-small cell lung cancer (NSCLC).

METHODS

An online survey was conducted from October to December 2023, addressing general clinical and technical considerations for lung SABR, and for 5 specific anatomical NSCLC locations (peripheral, abutting chest wall, near brachial plexus, central, and ultra-central). Invitations to participate were extended through email and were distributed on social media.

RESULTS

The survey was completed by 255 radiation oncologists, each representing a single institution across 51 countries. Respondents reported treating a median of 20 cases annually. A total of 38% of participants reported using single-fraction SABR, and 54% applied an upper limit on the maximum dose (Dmax). Among those who applied a Dmax limit, 58% reported a Dmax threshold at ≥130% of the prescription, though this limit varied by region and national economy status. Respondents from low- and middle-income countries were less likely to set a Dmax limit at ≥130% (30% vs. 66%, p < 0.01) and less likely to use single-fraction SABR (14% vs. 44%, p < 0.01). Higher annual SABR patient volumes were associated with higher Dmax adoption (г = 0.23, p < 0.01). Across the 5 clinical scenarios presented; 57 distinct dose regimens were recommended. The most common regimen in each scenario was: 54 Gy in 3 fractions for peripheral tumors, 50 Gy in 5 fractions for apical, central, and abutment of chest wall, and 60 Gy in 8 fractions for ultra-central tumors. Approximately two-thirds of practices recommend a biologically effective dose (BED10) <100 Gy for one or more anatomical sites.

CONCLUSION

The findings reveal considerable variation in global SABR practice. These differences highlight the need for further data to guide prescription practices, and an international experts' consensus may be beneficial to standardize practice.

A predictive model for Gamma Knife intermediate dose spill: R50%Analytic-GK

PURPOSE

Minimizing intermediate dose spill in stereotactic radiosurgery (SRS) for brain treatment is crucial. Intermediate dose spill correlates with the exposure of normal brain tissue to high doses, which increases the risk of radionecrosis. R50%, defined as the volume of the 50% of prescription isodose cloud/planning target volume, is one metric for intermediate dose spill. A predictive model for R50% in linear accelerator VMAT-delivered SRS has been developed Desai et al. (2020) and is called R50%Analytic. This study extends the R50%Analytic model to Gamma Knife (GK) delivered SRS, resulting in the R50%Analytic-GK model.

METHODS

Phantom calculations were performed on 11 spherical target volumes ranging from 0.001  to 44 cm3 to develop the R50%Analytic-GK model. R50%Analytic-GK was tested against clinical data from 18 brain metastasis cases with one to 11 targets treated on GK Icon and planned in GammaPlan with lightning dose optimizer. Thirty-five targets with volumes between 0.011  and 27.4 cm3 were analyzed by extracting the R50% achieved clinically (R50%Clinical) for comparison to the predicted intermediate dose spill from R50%Analytic-GK.

RESULTS

The predicted R50%Analytic-GK values generally represent a lower bound for the R50%Clinical values as the model would predict. The Difference, R50%Clinical - R50%Analytic-GK, has a median value of 0.92, which quantifies the lower bound nature of R50%Analytic-GK. The model reflected the character of intermediate dose spill for the clinical cases. A few outliers were likely due to specific planning complexities.

CONCLUSION

The R50%Analytic-GK model for intermediate dose spill successfully extends the theoretical framework of R50%Analytic to GK-delivered SRS. It provides a method to predict the intermediate dose spill for GK Icon treatments. This model can aid in assessing SRS treatment plans by providing a benchmark for the intermediate dose spill for comparison.

Dose-Effect Relationship of Kidney Function After SABR for Primary Renal Cell Carcinoma: TROG 15.03 FASTRACK II

PURPOSE

Stereotactic ablative body radiotherapy (SABR) is a novel option to treat primary renal cell carcinoma. However, a high radiation dose may be received by the treated kidney, which may affect its function posttreatment. This study investigates the dose-effect relationship of kidney SABR with posttreatment renal function.

METHODS AND MATERIALS

This was a prespecified secondary endpoint of the multicenter FASTRACK II (Focal Ablative STereotactic RAdiotherapy for Cancers of the Kidney phase II) clinical trial (National Clinical Trial 02613819). Patients received either 26 Gy in a single fraction (SF) for tumors with a maximal diameter of 4 cm or less or 42 Gy in 3 fractions (multifraction [MF]) for larger tumors. To determine renal function change, 99mTc-dimercaptosuccinic acid (DMSA) single-photon emission computed tomography/computed tomography (SPECT/CT) scans were acquired, and the glomerular filtration rate was estimated at baseline, 12, and 24 months posttreatment. Imaging data sets were rigidly registered to the planning CT where kidneys were segmented to calculate dose-response curves.

RESULTS

From 71 enrolled patients, 36 (51%) and 26 (37%) patients were included in this study based on availability of posttreatment data at 12 and 24 months, respectively. The ipsilateral kidney glomerular filtration rate decreased from baseline by 42% and 39% in the SF cohort and by 45% and 62% in the MF cohort, at 12 and 24 months, respectively (P < .03). The loss in renal function was 3.6%/Gy ± 0.8%/Gy and 4.5%/Gy ± 1.0%/Gy in the SF cohort and 1.7%/Gy ± 0.1%/Gy and 1.7%/Gy ± 0.2%/Gy in the MF cohort at 12 and 24 months, respectively. The major loss in renal function occurred in high-dose regions, where dose-response curves converged to a plateau.

CONCLUSIONS

For the first time in a multicenter study, the dose-effect relationship at 12 and 24 months post-SABR treatment for primary renal cell carcinoma was quantified. Kidney function reduces linearly with dose up to 100 Gy BED3.

Definition of a framework for volumetric modulated arc therapy plan quality assessment with integration of dose-, complexity-, and robustness metrics

Background and purpose

Conventionally, the quality of radiotherapy treatment plans is assessed through visual inspection of dose distributions and dose-volume histograms. This study developed a framework to evaluate plan quality using dose, complexity, and robustness metrics. Additionally, a method for predicting plan robustness metrics using dose and complexity metrics was introduced for cases where plan robustness evaluation is unavailable or impractical.

Materials and methods

The framework and prediction models were developed and validated using 103-bronchial Volumetric Modulated Arc Therapy (VMAT)-plans. The application of the framework was demonstrated using 25-VMAT-plans. To identify significant metrics for plan evaluation, 122-metrics were analysed and narrowed down using multivariate Spearman correlation. Metric limits were set with Statistical process control (SPC). Robustness metrics were predicted using multivariable or single linear regression models based on dose-and complexity-metrics.

Results

Twenty-five-metrics were selected based on the amount and strength of correlations. R95(dose coverage) and HI95/5(homogeneity index) stood out among the dose-metrics, while the complexity-metrics showed similar correlations. Average scenarios dose at 95 % Clinical Target Volume D95mean(CTV) and Errorbar-based Volume-Histograms (EVH) were notable for robustness metrics. Approximately 99 % of evaluated metrics fell within established SPC limits. The prediction model for D95mean(CTV) showed good performance (adjusted R2 = 0.88, mean squared error (MSE) = 3.84 × 10-6), while the model for EVH demonstrated moderate reliability (adjusted R2 = 0.52, MSE = 0.2). No statistically significant differences were found between the predicted (using dose-and complexity-metrics) and calculated robustness metrics (EVH (p-value = 0.9) and D95mean(CTV) (p-value = 1)).

Conclusions

The developed framework enables early detection of sub-optimal, complex and non-robust treatment plans. The predictive model can be used when robustness evaluations are impractical.

Toward an improved assessment of dose conformity in radiotherapy

BACKGROUND

Evaluation of dose conformity is important to ensure minimum dose to normal tissue and sufficient dose coverage of the planning target volume (PTV). The existing conformity indices depend on the PTV volume and do not differentiate between two different scenarios: overdosing normal tissue and underdosing PTV.

PURPOSE

In this study, we introduce a novel index to assess conformity of dose distributions in radiotherapy.

METHODS

The suggested conformity indexC I d e x p $C{I_{{d_{exp}}}}$ is defined by the ratio of the volume representing actual "non-conformity" of the planned dose and the volume representing acceptable "non-conformity." The latter volume is produced by expanding the PTV. If both the average distance (d ¯ $\overline d $ ) between the reference isodose surface and planning target volume and arbitrarily selected PTV expansion margin (d e x p ${d_{exp}}$ ) are much smaller than the size of the PTV,C I d e x p $C{I_{{d_{exp}}}}$ approximately equals the ratiod ¯ d e x p $\dfrac{{\bar d}}{{{d_{exp}}}}$ . In this work,C I d e x p $C{I_{{d_{exp}}}}$ was utilized to analyze 90 cases of brain metastases treated with stereotactic radiation therapy (SRS) and 102 cases of lung cancer treated with stereotactic body radiation therapy (SBRT).

RESULTS

Ford e x p ${d_{exp}}$  = 0.1 cm, all considered SRS treatment plans were characterized byC I d e x p < 1 $C{I_{{d_{exp}}}} < 1$ while 2 out of 102 SBRT plans hadC I d e x p > 1 $C{I_{{d_{exp}}}} > 1$ . The average values ofC I d e x p $C{I_{{d_{exp}}}}$ for SRS and SBRT plans were 0.31 and 0.43, respectively. Ford e x p ${d_{exp}}$  = 0.2 cm, all studied treatment plans hadC I d e x p < 1 $C{I_{{d_{exp}}}} < 1$ , and the average values ofC I d e x p $C{I_{{d_{exp}}}}$ for SRS and SBRT plans were 0.15 and 0.25, respectively.

CONCLUSIONS

The suggested conformity indexC I d e x p $C{I_{{d_{exp}}}}$ varies less with PTV volume than the RTOG and Riet-Paddick indices frequently used for evaluation of dose conformity. In addition,C I d e x p $C{I_{{d_{exp}}}}$ can be expressed as a sum of two terms which describe "over-coverage" and "under-coverage" of the treatment target. The results confirm thatC I d e x p $C{I_{{d_{exp}}}}$ can be used for evaluation of dose conformity in SRS and SBRT.

Verification of an optimizer algorithm by the beam delivery evaluation of intensity-modulated arc therapy plans

Background

In the case of dynamic radiotherapy plans, the fractionation schemes can have dosimetric effects. Our goal was to define the effect of the fraction dose on the plan quality and the beam delivery.

Materials and methods

Treatment plans were created for 5 early-stage lung cancer patients with different dose schedules. The planned total dose was 60 Gy, fraction dose was 2 Gy, 3 Gy, 5 Gy, 12 Gy and 20 Gy. Additionally renormalized plans were created by changing the prescribed fraction dose after optimization. The dosimetric parameters and the beam delivery parameters were collected to define the plan quality and the complexity of the treatment plans. The accuracy of dose delivery was verified with dose measurements using electronic portal imaging device (EPID).

Results

The plan quality was independent from the used fractionation scheme. The fraction dose could be changed safely after the optimization, the delivery accuracy of the treatment plans with changed prescribed dose was not lower. According to EPID based measurements, the high fraction dose and dose rate caused the saturation of the detector, which lowered the gamma passing rate. The aperture complexity score, the gantry speed and the dose rate changes were not predicting factors for the gamma passing rate values.

Conclusions

The plan quality and the delivery accuracy are independent from the fraction dose, moreover the fraction dose can be changed safely after the dose optimization. The saturation effect of the EPID has to be considered when the action limits of the quality assurance system are defined.

A systematically compiled set of quantitative metrics to describe spatial characteristics of radiotherapy dose distributions and aid in treatment planning

PURPOSE

Many quantitative metrics have been proposed in literature for characterization of spatial dose properties. The aim of this study is to work towards much-needed consensus in the radiotherapy community on which of these metrics to use. We do this by comparing characteristics of the metrics and providing a systematically selected set of metrics to comprehensively quantify properties of the spatial dose distribution.

METHODS

We searched the literature for metrics to quantitatively evaluate dose conformity, homogeneity, gradient (overall and directional), and distribution and location of over- and under-dosed sub-volumes. For each spatial dose property, we compared the responses of its corresponding metrics to simulated dose variations in a virtual water phantom. Selection criteria were a metric's ability to describe simulated scenarios robustly and to be visualized in an intuitive way.

RESULTS

We saw substantial differences in the responses of metrics to the simulated dose variations. Some conformity and homogeneity metrics were unable to quantify certain types of changes (e.g. target under-coverage). Others showed a large dependency on the shape and volume of targets and isodoses. Metric values differed between calculations in a static plan and in simulated full treatment courses including setup errors, especially for metrics quantifying distribution and location of hot and cold spots. We provide an Eclipse plugin script to calculate and visualize selected metrics.

CONCLUSION

The selected set of metrics provides complementary and comprehensive quantitative information about the spatial dose distribution. This work serves as a step towards broader consensus on the use of spatial dose metrics.

Effect of abdominal compression on target movement and extension of the external boundary of peripheral lung tumours treated with stereotactic radiotherapy based on four-dimensional computed tomography

BACKGROUND

This study aimed to investigate the effect of abdominal compression on tumour motion and target volume and to determine suitable planning target volume (PTV) margins for patients treated with lung stereotactic body radiotherapy (SBRT) based on four-dimensional computed tomography (4DCT).

METHODS

Twenty-three patients diagnosed to have a peripheral pulmonary tumour were selected and divided into an all lesions group (group A), an upper middle lobe lesions group (group B), and a lower lobe lesions group (group C). Two 4DCT scans were performed in each patient, one with and one without abdominal compression. Cone beam computed tomography (CBCT) was performed before starting treatment. The gross target volumes (GTVs) were delineated and internal gross target volumes (IGTVs) were defined. IGTVs were generated using two methods: (1) the maximum intensity projections (MIPs) based on the 4DCT were reconstructed to form a single volume and defined as the IGTVMIP and (2) GTVs from all 10 phases were combined to form a single volume and defined as the IGTV10. A 5-mm, 4-mm, and 3-mm margin was added in all directions on the IGTVMIP and the volume was constructed as PTVMIP_5mm, PTVMIP_4mm, and PTVMIP_3mm.

RESULTS

There was no significant difference in the amplitude of tumour motion in the left-right, anterior-posterior, or superior-inferior direction according to whether or not abdominal compression was applied (group A, p = 0.43, 0.27, and 0.29, respectively; group B, p = 0.46, 0.15, and 0.45; group C, p = 0.79, 0.86, and 0.37; Wilcoxon test). However, the median IGTVMIP without abdominal compression was 33.67% higher than that with compression (p = 0.00), and the median IGTV10 without compression was 16.08% higher than that with compression (p = 0.00). The median proportion of the degree of inclusion of the IGTVCBCT in PTVMIP_5mm, PTVMIP_4mm, and PTVMIP_3mm ≥ 95% was 100%, 100%, and 83.33%, respectively.

CONCLUSIONS

Abdominal compression was useful for reducing the size of the IGTVMIP and IGTV10 and for decreasing the PTV margins based on 4DCT. In IGTVMIP with abdominal compression, adding a 4-mm margin to account for respiration is feasible in SBRT based on 4DCT.

The surface area effect: How the intermediate dose spill depends on the PTV surface area in SRS

PURPOSE

Stereotactic radiosurgery (SRS) is rapidly becoming the standard of care for many intracranial targets. The characteristics of the planning target volume (PTV) can affect the intermediate dose spill and thus normal brain volume dose which is correlated with brain toxicity. R50% (volume receiving 50% of prescription dose divided by PTV volume) is a useful metric to quantify the intermediate dose spill. We propose a novel understanding of how the PTV surface area (SAPTV ) affects the intermediate dose spill of SRS treatments.

METHODS

Using a phantom model provided by a computed tomography (CT) of the IROC Head Phantom® and Eclipse® Treatment Planning System, we investigate the relationship of R50% and SAPTV in single-target SRS treatments. The planning studies are conducted for SRS treatments on a Varian TrueBeam® linear accelerator with high-definition MLC and a 6 MVFFF beam mode. These data are analyzed to ascertain trends in R50% related to SAPTV . Since SAPTV is not available as a structure property in the Eclipse RTPS, we introduce an Eclipse script to extract PTV surface area of arbitrary-shaped PTVs. We compare a physically reasonable theoretical prediction of R50%, R50%Analytic , to the R50% achieved in treatment planning studies.

RESULTS

The SRS phantom study indicates good correlation between the plan R50% and SAPTV . A near-linear relationship of plan R50% vs SAPTV is observed as predicted by the R50%Analytic model. Agreement between plan R50% values and R50%Analytic predictions is good for all but the very smallest PTV volumes.

CONCLUSIONS

We demonstrate dependence of the intermediate dose spill measured by R50% on the SAPTV . We call that dependence the surface area effect. This dependence is explicit in the R50%Analytic prediction model. The predicted value of R50%Analytic for a given PTV could be used for guidance during SRS treatment plan optimization, and plan evaluation for that PTV.

Radiation treatment planning study to investigate feasibility of delivering Immunotherapy in Combination with Ablative Radiosurgery to Ultra-High DoSes (ICARUS)

PURPOSE

Immune checkpoint inhibitors improve survival in metastatic diseases for some cancers. Multisite SBRT with pembrolizumab (SBRT + Pembro) was shown to be safe with promising local control using biologically effective doses (BEDs) = 95-120 Gy. Increased BED may improve response rate; however, SBRT doses are limited by surrounding organs at risk (OARs). The purpose of this work was to develop and validate methods for safe delivery of ultra-high doses of radiation (BED₁₀  > 300) to be used in future clinical trials.

METHODS AND MATERIALS

The radiation plans from 15 patients enrolled on a phase I trial of SBRT + pembro were reanalyzed. Metastatic disease sites included liver (8/15), inguinal region (1/15), pelvis (2/15), lung (1/15), abdomen (1/15), spleen (1/15), and groin (1/15). Gross tumor volumes (GTVs) ranged from 80 to 708 cc. Following the same methodology used in the Phase I trial on which these patients were treated, GTVs > 65 cc were contracted to a 65 cc subvolume (SubGTV) resulting in only a portion of the GTV receiving prescription dose. Volumetric modulated arc therapy (VMAT) was used to plan treatments BED₁₀  = 360 Gy. Plans utilizing both 6FFF and 10FFF beams were compared to clinical plans delivering BED₁₀  = 112.50 Gy. The target primary goal was V100% > 95% with a secondary goal of V70% > 99% and OAR objectives per the trial. To demonstrate feasibility, plans were delivered to a diode array phantom and evaluated for fidelity using gamma analysis.

RESULTS

All 30 plans met the secondary coverage goal and satisfied all OAR constraints. The primary goal was achieved in 12/15 of the 6FFF plans and 13/15 of the 10FFF plans. Average gamma analysis passing rate using criteria of 3% dose difference and 3, 2, and 1 mm were 99.1  ±  1.0%, 98.5  ±  1.6%, and 95.1  ±  3.8%, respectively.

CONCLUSION

Novel VMAT planning approaches with clinical treatment planning software and linear accelerators prove capable of delivering radiation doses in excess of 360 Gy BED₁₀ to tumor subvolumes, while maintaining safe OAR doses.

What is plan quality in radiotherapy? The importance of evaluating dose metrics, complexity, and robustness of treatment plans

Plan evaluation is a key step in the radiotherapy treatment workflow. Central to this step is the assessment of treatment plan quality. Hence, it is important to agree on what we mean by plan quality and to be fully aware of which parameters it depends on. We understand plan quality in radiotherapy as the clinical suitability of the delivered dose distribution that can be realistically expected from a treatment plan. Plan quality is commonly assessed by evaluating the dose distribution calculated by the treatment planning system (TPS). Evaluating the 3D dose distribution is not easy, however; it is hard to fully evaluate its spatial characteristics and we still lack the knowledge for personalising the prediction of the clinical outcome based on individual patient characteristics. This advocates for standardisation and systematic collection of clinical data and outcomes after radiotherapy. Additionally, the calculated dose distribution is not exactly the dose delivered to the patient due to uncertainties in the dose calculation and the treatment delivery, including variations in the patient set-up and anatomy. Consequently, plan quality also depends on the robustness and complexity of the treatment plan. We believe that future work and consensus on the best metrics for quality indices are required. Better tools are needed in TPSs for the evaluation of dose distributions, for the robust evaluation and optimisation of treatment plans, and for controlling and reporting plan complexity. Implementation of such tools and a better understanding of these concepts will facilitate the handling of these characteristics in clinical practice and be helpful to increase the overall quality of treatment plans in radiotherapy.

A physically meaningful relationship between R50% and PTV surface area in lung SBRT

Purpose

We propose a novel understanding of two characteristics of the planning target volume (PTV) that affect the intermediate‐dose spill in lung stereotactic body radiation therapy (SBRT) as measured by R50%. This phantom model research investigates two characteristics of the PTV that have a marked effect on the value of R50%: the mean dose deposited within the PTV (D_av) and the surface area of the PTV (SA_PTV).

Methods

Using a phantom model provided by a CT of the IROC Thorax‐Lung Phantom® (IROC Houston QA Center, Houston, TX) and Eclipse® Treatment Planning System (Varian Medical Systems, Palo Alto, CA), we investigate the two characteristics for spherical and cylindrical PTVs. A total of 135 plans with tightly controlled PTV characteristics are employed. A lower bound for R50% (R50%min_∆r) is derived and clearly establishes a relationship between R50% and SA_PTV that has not been fully appreciated previously.

Results

The study of PTV D_av revealed a local minimum for R50% as a function of the PTV D_av at D_av ≈ 110% of Rx dose. As PTV D_av increases above this local minimum, R50% increases; while for PTV D_av less than this local minimum, the R50% value also increases. The study of PTV surface area (SA_PTV) demonstrated that as the SA_PTV increases, the R50% increases if the PTV volume stays the same. The SA_PTV result is predicted by the theoretical investigation that yields the R50% lower bound, R50%min_∆r.

Conclusions

This research has identified two characteristics of the PTV that have a marked influence on R50%: PTV D_av and SA_PTV. These characteristics have not been clearly articulated in the vast body of previous research in SBRT. These results could help explain plans that cannot meet the RTOG criteria for R50%. With further development, these concepts could be extended to provide additional guidance for creating acceptable SBRT plans.

Plan evaluation indices: A journey of evolution

AIM

A systemic review and analysis of evolution journey of indices, such as conformity index (CI), homogeneity index (HI) and gradient index (GI), described in the literature.

BACKGROUND

Modern radiotherapy techniques like VMAT, SRS and SBRT produce highly conformal plans and provide better critical structure and normal tissue sparing. These treatment techniques can generate a number of competitive plans for the same patients with different dose distributions. Therefore, indices like CI, HI and GI serve as complementary tools in addition to visual slice by slice isodose verification while plan evaluation. Reliability and accuracy of these indices have been tested in the past and found shortcomings and benefits when compared to one another.

MATERIAL AND METHODS

Potentially relevant studies published after 1993 were identified through a pubmed and web of science search using words "conformity index", "Homogeneity index", "Gradient index"," Stereotactic radiosurgery"," stereotactic Body radiotherapy" "complexity metrics" and "plan evaluation index". Combinations of words "plan evaluation index conformity index" were also searched as were bibliographies of downloaded papers.

RESULTS AND CONCLUSIONS

Mathematical definitions of plan evaluation indices modified with time. CI definitions presented by various authors tested at their own and could not be generalized. Those mathematical definitions of CI which take into account OAR sparing grant more confidence in plan evaluation. Gradient index emerged as a significant plan evaluation index in addition to CI whereas homogeneity index losing its credibility. Biological index base plan evaluation is becoming popular and may replace or alter the role of dosimetrical indices.

Variation in current prescription practice of stereotactic body radiotherapy for peripherally located early stage non-small cell lung cancer: Recommendations for prescribing and recording according to the ACROP guideline and ICRU report 91

BACKGROUND AND PURPOSE

In 2017 the ACROP guideline on SBRT for peripherally located early stage NSCLC was published. Later that year ICRU-91 about prescribing, recording and reporting was published. The purpose of this study is to quantify the current variation in prescription practice in the institutions that contributed to the ACROP guideline and to establish the link between the ACROP and ICRU-91 recommendations.

MATERIAL AND METHODS

From each of the eight participating centres, 15 SBRT plans for stage I NSCLC were analyzed. Plans were generated following the institutional protocol, centres prescribed 3 × 13.5 Gy, 3 × 15 Gy, 3 × 17 Gy or 3 × 18 Gy. Dose parameters of the target volumes were reported as recommended by ICRU-91 and also converted to BED10Gy.

RESULTS

The intra-institutional variance in D98%, Dmean and D2% of the PTV and GTV/ITV is substantially smaller than the inter-institutional spread, indicating well protocollised planning procedures are followed. The median values per centre ranged from 56.1 Gy to 73.1 Gy (D2%), 50.4 Gy to 63.3 Gy (Dmean) and 40.5 Gy to 53.6 Gy (D98%) for the PTV and from 57.1 Gy to 73.6 Gy (D2%), 53.7 Gy to 68.7 Gy (Dmean) and 48.5 Gy to 62.3 Gy (D98%) for the GTV/ITV. Comparing the variance in PTV D98% with the variance in GTV Dmean per centre, using an F-test, shows that four centres have a larger variance in GTV Dmean, while one centre has a larger variance in PTV D98% (p values <0.01). This shows some centres focus on achieving a constant PTV coverage while others aim at a constant GTV coverage.

CONCLUSION

More detailed recommendations for dose planning and reporting of lung SBRT in line with ICRU-91 were formulated, including a minimum PTV D98% of 100 Gy BED10Gy and minimum GTV/ITV mean dose of 150 Gy BED10Gy and a D2% in the range of 60-70 Gy.

Multi-center evaluation of dose conformity in stereotactic body radiotherapy

BACKGROUND AND PURPOSE

Stereotactic body radiotherapy (SBRT) is an emerging technique for treating oligometastases, but limited data is available on what plan quality is achievable for a range of modalities and clinical sites.

METHODS

SBRT plans for lung, spine, bone, adrenal, liver and node sites from 17 participating centers were reviewed. Centers used various delivery techniques including static and rotational intensity-modulation and multiple non-coplanar beams. Plans were split into lung and other body sites and evaluated with different plan quality metrics, including two which are independent of target coverage; "prescription dose spillage" (PDS) and "modified gradient index" (MGI). These were compared to constraints from the ROSEL and RTOG 0813 clinical trials.

RESULTS

Planning target volume (PTV) coverage was compromised (PTV V100% < 90%) in 29% of patient plans in order to meet organ-at-risk (OAR) tolerances, supporting the use of plan quality metrics which are independent of target coverage. Both lung (n = 48) and other body (n = 99) site PDS values agreed well with ROSEL constraints on dose spillage, but RTOG 0813 values were too high to detect sub-optimal plans. MGI values for lung plans were mis-matched to both sets of previous constraints, with ROSEL values too high and RTOG 0813 values too low. MGI values were lower for other body plans as expected, though this was only statistically significant for PTV volumes <20 cm3.

CONCLUSIONS

Updated guidance for lung and other body site SBRT plan quality using the PDS and MGI metrics is presented.

Dose escalation for locally advanced pancreatic cancer: How high can we go?

Purpose

There are limited treatment options for locally advanced, unresectable pancreatic cancer (LAPC) and no likelihood of cure without surgery. Radiation offers an option for local control, but radiation dose has previously been limited by nearby bowel toxicity. Advances in on-board imaging and treatment planning may allow for dose escalation not previously feasible and improve local control. In preparation for development of clinical trials of dose escalation in LAPC, we undertook a dosimetric study to determine the maximum possible dose escalation while maintaining known normal tissue constraints.

Methods and Materials

Twenty patients treated at our institution with either SBRT or dose-escalated hypofractionated IMRT (DE-IMRT) were re-planned using dose escalated SBRT to 70 Gy in 5 fractions to the GTV and 40 Gy in 5 fractions to the PTV. Standard accepted organ at risk (OAR) constraints were used for planning. Descriptive statistics were generated for homogeneity, conformality, OAR's and GTV/PTV.

Results

Mean iGTV coverage by 50 Gy was 91% (±0.07%), by 60 Gy was 61.3% (±0.08%) and by 70 Gy was 24.4% (±0.05%). Maximum PTV coverage by 70 Gy was 33%. Maximum PTV coverage by 60 Gy was 77.5%. The following organ at risk (OAR) constraints were achieved for 90% of generated plans: Duodenum V20 < 30 cc, V30 < 3 cc, V35 < 1 cc; Small Bowel V20 < 15 cc, V30 < 1 cc, V35 < 0.1 cc; Stomach V20 < 20 cc, V30 < 2 cc, V35 < 1 cc. V40 < 0.5 cc was achieved for all OAR.

Conclusions

Dose escalation to 60 Gy is dosimetrically feasible with adequate GTV coverage. The identified constraints for OAR's will be used in ongoing clinical trials.