SBRT planning for spinal metastasis: indications from a large multicentric study

Refining Treatment Planning in STereotactic Arrhythmia Radioablation: Benchmark Results and Consensus Statement From the STOPSTORM.eu Consortium

PURPOSE

STereotactic Arrhythmia Radioablation (STAR) showed promising results in patients with refractory ventricular tachycardia. However, clinical data are scarce and heterogeneous. The STOPSTORM.eu consortium was established to investigate and harmonize STAR in Europe. The primary goal of this benchmark study was to investigate current treatment planning practice within the STOPSTORM project as a baseline for future harmonization.

METHODS AND MATERIALS

Planning target volumes (PTVs) overlapping extracardiac organs-at-risk and/or cardiac substructures were generated for 3 STAR cases. Participating centers were asked to create single-fraction treatment plans with 25 Gy dose prescriptions based on in-house clinical practice. All treatment plans were reviewed by an expert panel and quantitative crowd knowledge-based analysis was performed with independent software using descriptive statistics for International Commission on Radiation Units and Measurements report 91 relevant parameters and crowd dose-volume histograms. Thereafter, treatment planning consensus statements were established using a dual-stage voting process.

RESULTS

Twenty centers submitted 67 treatment plans for this study. In most plans (75%) intensity modulated arc therapy with 6 MV flattening filter free beams was used. Dose prescription was mainly based on PTV D95% (49%) or D96%-100% (19%). Many participants preferred to spare close extracardiac organs-at-risk (75%) and cardiac substructures (50%) by PTV coverage reduction. PTV D0.035cm3 ranged from 25.5 to 34.6 Gy, demonstrating a large variety of dose inhomogeneity. Estimated treatment times without motion compensation or setup ranged from 2 to 80 minutes. For the consensus statements, a strong agreement was reached for beam technique planning, dose calculation, prescription methods, and trade-offs between target and extracardiac critical structures. No agreement was reached on cardiac substructure dose limitations and on desired dose inhomogeneity in the target.

CONCLUSIONS

This STOPSTORM multicenter treatment planning benchmark study not only showed strong agreement on several aspects of STAR treatment planning, but also revealed disagreement on others. To standardize and harmonize STAR in the future, consensus statements were established; however, clinical data are urgently needed for actionable guidelines for treatment planning.

Patient specific quality assurance in SBRT: a systematic review of measurement-based methods

This topical review focuses on Patient-Specific Quality Assurance (PSQA) approaches to stereotactic body radiation therapy (SBRT). SBRT requires stricter accuracy than standard radiation therapy due to the high dose per fraction and the limited number of fractions. The review considered various PSQA methods reported in 36 articles between 01/2010 and 07/2022 for SBRT treatment. In particular comparison among devices and devices designed for SBRT, sensitivity and resolution, verification methodology, gamma analysis were specifically considered. The review identified a list of essential data needed to reproduce the results in other clinics, highlighted the partial miss of data reported in scientific papers, and formulated recommendations for successful implementation of a PSQA protocol.

Dose prescription and reporting in stereotactic body radiotherapy: A multi-institutional study

BACKGROUND AND PURPOSE

Radiation dose prescriptions are foundational for optimizing treatment efficacy and limiting treatment-related toxicity. We sought to assess the lack of standardization of SBRT dose prescriptions across institutions.

MATERIALS & METHODS

Dosimetric data from 1298 patients from 9 academic institutions treated with IMRT and VMAT were collected. Dose parameters D100, D98, D95, D50, and D2 were used to assess dosimetric variability.

RESULTS

Disease sites included lung (48.3 %) followed by liver (29.7 %), prostate (7.5 %), spine (6.8 %), brain (4.1 %), and pancreas (2.5 %). The PTV volume in lung varied widely with bimodality into two main groups (22.0-28.7 cm3) and (48.0-67.1 cm3). A hot spot ranging from 120-150 % was noted in nearly half of the patients, with significant variation across institutions. A D50 ≥ 110 % was found in nearly half of the institutions. There was significant dosimetric variation across institutions.

CONCLUSIONS

The SBRT prescriptions in the literature or in treatment guidelines currently lack nuance and hence there is significant variation in dose prescriptions across academic institutions. These findings add greater importance to the identification of dose parameters associated with improved clinical outcome comparisons as we move towards more hypofractionated treatments. There is a need for standardized reporting to help institutions in adapting treatment protocols based on the outcome of clinical trials. Dosimetric parameters are subsequently needed for uniformity and thereby standardizing planning guidelines to maximize efficacy, mitigate toxicity, and reduce treatment disparities are urgently needed.

Evaluation of a dedicated software for semi-automated VMAT planning of spine Stereotactic Body Radiotherapy (SBRT)

PURPOSE

To determine whether SBRT of spinal metastasis using a dedicated treatment planning system (TPS) and delivered with a gantry-based LINAC could provide plans of similar quality to the Cyberknife technology. Additional comparison was also done with other commercial TPS used for volumetric modulated arc therapy (VMAT) planning.

MATERIALS AND METHODS

Thirty Spine SBRT patients, previously treated in our institution with CyberKnife (Accuray, Sunnyvale) using Multiplan TPS, were replanned in VMAT with an dedicated TPS (Elements Spine SRS, Brainlab, Munich) and our clinical TPS (Monaco, Elekta LTD, Stockholm), using exactly the same arc geometry. The comparison was done by assessing differences in dose delivered to PTV, CTV and spinal cord, calculating modulation complexity scores (MCS) and performing quality control (QA) of the plans.

RESULTS

Regardless of the vertebra level, in general, no statistical difference was found in PTV coverage between all TPS. Conversely, PTV and CTV D_50% were found significantly higher for the dedicated TPS compared to others. In addition, the dedicated TPS also resulted in better gradient index (GI) than clinical VMAT TPS, whatever the vertebral level, and better GI than Cyberknife TPS for the thoracic level only. The D_2% to the spinal cord was generally significantly lower with the dedicated TPS compared with others. No significant difference was found in the MCS between both VMAT TPS. All QA were clinically acceptable.

CONCLUSION

The Elements Spine SRS TPS offers very effective and user-friendly semi-automated planning tools and is secure and promising for gantry-based LINAC spinal SBRT.

Applications of artificial intelligence in stereotactic body radiation therapy

This topical review focuses on the applications of artificial intelligence (AI) tools to stereotactic body radiation therapy (SBRT). The high dose per fraction and the limited number of fractions in SBRT require stricter accuracy than standard radiation therapy. The intent of this review is to describe the development and evaluate the possible benefit of AI tools integration into the radiation oncology workflow for SBRT automation. The selected papers were subdivided into four sections, representative of the whole radiotherapy process: ‘AI in SBRT target and organs at risk contouring’, ‘AI in SBRT planning’, ‘AI during the SBRT delivery’, and ‘AI for outcome prediction after SBRT’. Each section summarises the challenges, as well as limits and needs for improvement to achieve better integration of AI tools in the clinical workflow.

Treatment Planning for Cardiac Radioablation: Multicenter Multiplatform Benchmarking for the XXX Trial

PURPOSE

Cardiac radioablation is a novel treatment option for patients with refractory ventricular tachycardia (VT) unsuitable for catheter ablation. The quality of treatment planning depends on dose specifications, platform capabilities, and experience of the treating staff. To harmonize the treatment planning, benchmarking of this process is necessary for multicenter clinical studies such as the XXX trial.

METHODS AND MATERIALS

Planning computed tomography data and consensus structures from three patients were sent to five academic centers for independent plan development using a variety of platforms and techniques with the XXX study protocol serving as guideline. Three-dimensional dose distributions and treatment plan details were collected and analyzed. In addition, an objective relative plan quality ranking system for VT treatments was established.

RESULTS

For each case, three coplanar volumetric modulated arc (VMAT) plans for C-arm linear accelerators (LINAC) and three non-coplanar treatment plans for robotic arm LINAC were generated. All plans were suitable for clinical applications with minor deviations from study guidelines in most centers. Eleven of 18 treatment plans showed maximal one minor deviation each for target and cardiac substructures. However, dose-volume histograms showed substantial differences: in one case, the PTV≥30Gy ranged from 0.0% to 79.9% and the RIVA V_14Gy ranged from 4.0% to 45.4%. Overall, the VMAT plans had steeper dose gradients in the high dose region, while the plans for the robotic arm LINAC had smaller low dose regions. Thereby, VMAT plans required only about half as many monitor units, resulting in shorter delivery times, possibly an important factor in treatment outcome.

CONCLUSIONS

Cardiac radioablation is feasible with robotic arm and C-arm LINAC systems with comparable plan quality. Although cross-center training and best practice guidelines have been provided, further recommendations, especially for cardiac substructures, and ranking of dose guidelines will be helpful to optimize cardiac radioablation outcomes.

Personalized Automation of Treatment Planning for Linac-Based Stereotactic Body Radiotherapy of Spine Cancer

Purpose/Objective(s)

Stereotactic ablative body radiotherapy (SBRT) for vertebral metastases is a challenging treatment process. Planning automation has recently reported the potential to improve plan quality and increase planning efficiency. We performed a dosimetric evaluation of the new Personalized engine implemented in Pinnacle3 for full planning automation of SBRT spine treatments in terms of plan quality, treatment efficiency, and delivery accuracy.

Materials/Methods

The Pinnacle3 treatment planning system was used to reoptimize six patients with spinal metastases, employing two separate automated engines. These two automated engines, the existing Autoplanning and the new Personalized, are both template-based algorithms that employ a wishlist to construct planning goals and an iterative technique to replicate the planning procedure performed by skilled planners. The boost tumor volume (BTV) was defined as the macroscopically visible lesion on RM examination, and the planning target volume (PTV) corresponds with the entire vertebra. Dose was prescribed according to simultaneous integrated boost strategy with BTV and PTV irradiated simultaneously over 3 fractions with a dose of 30 and 21 Gy, respectively. Dose-volume histogram (DVH) metrics and conformance indices were used to compare clinically accepted manual plans (MP) with automated plans developed using both Autoplanning (AP) and Personalized engines (Pers). All plans were evaluated for planning efficiency and dose delivery accuracy.

Results

For similar spinal cord sparing, automated plans reported a significant improvement of target coverage and dose conformity. On average, Pers plans increased near-minimal dose D98% by 10.4% and 8.9% and target coverage D95% by 8.0% and by 4.6% for BTV and PTV, respectively. Automated plans provided significantly superior dose conformity and dose contrast by 37%–47% and by 4.6%–5.7% compared with manual plans. Overall planning times were dramatically reduced to about 15 and 23 min for Pers and AP plans, respectively. The average beam-on times were found to be within 3 min for all plans. Despite the increased complexity, all plans passed the 2%/2 mm γ-analysis for dose verification.

Conclusion

Automated planning for spine SBRT through the new Pinnacle3 Personalized engine provided an overall increase of plan quality in terms of dose conformity and a major increase in efficiency. In this complex anatomical site, Personalized strongly reduce the tradeoff between optimal accurate dosimetry and planning time.

Planning benchmark study for SBRT of liver metastases: Results of the DEGRO/DGMP working group stereotactic radiotherapy and radiosurgery

PURPOSE

To investigate, if liver SBRT treatment planning can be harmonized across different treatment planning systems, delivery techniques and institutions by using a specific prescription method and to minimize the knowledge gap concerning inter-system and inter-user differences. To provide best practice guidelines for all used techniques.

METHODS

A multiparametric specification of target dose (GTV_D50%, GTV_D0.1cc, GTV_V90%, PTV_V70%) with a prescription dose of GTV_D50% = 3 × 20 Gy and OAR limits were distributed with CTs and structure sets from three liver metastases patients. Thirty-five institutions provided 132 treatment plans using different irradiation techniques. These plans were first analyzed for target and OAR doses. Four different renormalization methods were performed (PTV_Dmin, PTV_D98%, PTV_D2%, PTV_Dmax). The resulting 660 treatments plans were evaluated regarding target doses in order to study the effect of dose renormalization to different prescription methods. A relative scoring system was used for comparisons.

RESULTS

GTV_D50% prescription can be performed in all systems. Treatment plan harmonization was overall successful with standard deviations for D_max, PTV_D98%, GTV_D98% and PTV_Dmean of 1.6 Gy, 3.3 Gy, 1.9 Gy and 1.5 Gy, respectively. Primary analysis showed 55 major deviations from clinical goals in 132 plans, while in only <20% of deviations GTV/PTV dose was traded for meeting OAR limits. GTV_D50% prescription produced the smallest deviation from target planning objectives and between techniques, followed by the PTV_Dmax, PTV_D98%, PTV_D2% and PTV_Dmin prescription. Deviations were significant for all combinations but for the PTV_Dmax prescription compared with GTV_D50% and PTV_D98%. Based on the various dose prescription methods, all systems significantly differed from each other, while GTV_D50% and PTV_D98% prescription showed the least differences between the systems.

CONCLUSIONS

This study showed the feasibility of harmonizing liver SBRT treatment plans across different treatment planning systems and delivery techniques when a sufficient set of clinical goals is given.

Impact of prescription isodose level and collimator selection on dose homogeneity and plan quality in robotic radiosurgery

Purpose

In stereotactic radiosurgery (SRS), prescription isodoses and resulting dose homogeneities vary widely across different platforms and clinical entities. Our goal was to investigate the physical limitations of generating dose distributions with an intended level of homogeneity in robotic SRS.

Methods

Treatment plans for non-isocentric irradiation of 4 spherical phantom targets (volume 0.27–7.70 ml) and 4 clinical targets (volume 0.50–5.70 ml) were calculated using Sequential (phantom) or VOLO^TM (clinical) optimizers (Accuray, Sunnyvale, CA, USA). Dose conformity, volume of 12 Gy isodose (V12Gy) as a measure for dose gradient, and treatment time were recorded for different prescribed isodose levels (PILs) and collimator settings. In addition, isocentric irradiation of phantom targets was examined, with dose homogeneity modified by using different collimator sizes.

Results

Dose conformity was generally high (nCI ≤ 1.25) and varied little with PIL. For all targets and collimator sets, V12Gy was highest for PIL ≥ 80% and lowest for PIL ≤ 65%. The impact of PIL on V12Gy was highest for isocentric irradiation and lowest for clinical targets (VOLO^TM optimization). The variability of V12Gy as a function of collimator selection was significantly higher than that of PIL. V12Gy and treatment time were negatively correlated. Plans utilizing a single collimator with a diameter in the range of 70–80% of the target diameter were fastest, but showed the strongest dependence on PIL.

Conclusion

Inhomogeneous dose distributions with PIL ≤ 70% can be used to minimize dose to normal tissue. PIL ≥ 90% is associated with a marked and significant increase in off-target dose exposure. Careful selection of collimators during planning is even more important.

Supplementary Information

The online version of this article (10.1007/s00066-021-01872-4) contains supplementary material, which is available to authorized users.

Quality assurance of dysphagia-optimised intensity modulated radiotherapy treatment planning for head and neck cancer

This study aimed to assess the impact of the margin applied to the clinical target volume, to create the planning target volume, on plan quality of a novel dysphagia-optimised intensity modulated radiotherapy technique developed within a head and neck cancer multicentre randomised controlled trial. Protocol compliant plans were used for a single benchmark planning case. Larger margins were associated with higher doses to adjacent organs at risk, particularly the inferior pharyngeal constrictor muscle, but coincided with some improved low dose target coverage. A 3 mm margin is recommended for this technique if local practices allow.

Improving interinstitutional and intertechnology consistency of pulmonary SBRT by dose prescription to the mean internal target volume dose

Purpose

Dose, fractionation, normalization and the dose profile inside the target volume vary substantially in pulmonary stereotactic body radiotherapy (SBRT) between different institutions and SBRT technologies. Published planning studies have shown large variations of the mean dose in planning target volume (PTV) and gross tumor volume (GTV) or internal target volume (ITV) when dose prescription is performed to the PTV covering isodose. This planning study investigated whether dose prescription to the mean dose of the ITV improves consistency in pulmonary SBRT dose distributions.

Materials and methods

This was a multi-institutional planning study by the German Society of Radiation Oncology (DEGRO) working group Radiosurgery and Stereotactic Radiotherapy. CT images and structures of ITV, PTV and all relevant organs at risk (OAR) for two patients with early stage non-small cell lung cancer (NSCLC) were distributed to all participating institutions. Each institute created a treatment plan with the technique commonly used in the institute for lung SBRT. The specified dose fractionation was 3 × 21.5 Gy normalized to the mean ITV dose. Additional dose objectives for target volumes and OAR were provided.

Results

In all, 52 plans from 25 institutions were included in this analysis: 8 robotic radiosurgery (RRS), 34 intensity-modulated (MOD), and 10 3D-conformal (3D) radiation therapy plans. The distribution of the mean dose in the PTV did not differ significantly between the two patients (median 56.9 Gy vs 56.6 Gy). There was only a small difference between the techniques, with RRS having the lowest mean PTV dose with a median of 55.9 Gy followed by MOD plans with 56.7 Gy and 3D plans with 57.4 Gy having the highest. For the different organs at risk no significant difference between the techniques could be found.

Conclusions

This planning study pointed out that multiparameter dose prescription including normalization on the mean ITV dose in combination with detailed objectives for the PTV and ITV achieve consistent dose distributions for peripheral lung tumors in combination with an ITV concept between different delivery techniques and across institutions.

Supplementary Information

The online version of this article (10.1007/s00066-021-01799-w) contains supplementary material, which is available to authorized users.

Revisiting the formalism of equivalent uniform dose based on the linear-quadratic and universal survival curve models in high-dose stereotactic body radiotherapy

PURPOSE

To examine the equivalent uniform dose (EUD) formalism using the universal survival curve (USC) applicable to high-dose stereotactic body radiotherapy (SBRT).

MATERIALS AND METHODS

For nine non-small-cell carcinoma cell (NSCLC) lines, the linear-quadratic (LQ) and USC models were used to calculate the EUD of a set of hypothetical two-compartment tumor dose-volume histogram (DVH) models. The dose was varied by ±5%, ±10%, and ±20% about the prescription dose (60 Gy/3 fractions) to the first compartment, with fraction volume varying from 1% and 5% to 30%. Clinical DVHs of 21 SBRT treatments of NSCLC prescribed to the 70-83% isodose lines were also considered. The EUD of non-standard SBRT dose fractionation (EUDSBRT) was further converted to standard fractionation of 2 Gy (EUDCFRT) using the LQ and USC models to facilitate comparisons between different SBRT dose fractionations. Tumor control probability (TCP) was then estimated from the LQ- and USC-EUDCFRT.

RESULTS

For non-standard SBRT fractionation, the deviation of the USC- from the LQ-EUDSBRT is largely limited to 5% in the presence of dose variation up to ±20% to fractional tumor volume up to 30% in all NSCLC cell lines. Linear regression with zero constant yielded USC-EUDSBRT = 0.96 × LQ-EUDSBRT (r2 = 0.99) for the clinical DVHs. Converting EUDSBRT into standard 2‑Gy fractions by the LQ formalism produced significantly larger EUDCFRT than the USC formalism, particularly for low [Formula: see text] ratios and large fraction dose. Simplified two-compartment DVH models illustrated that both the LQ- and USC-EUDCFRT values were sensitive to cold spot below the prescription dose with little volume dependence. Their deviations were almost constant for up to 30% dose increase above the prescription. Linear regression with zero constant yielded USC-EUDCFRT = 1.56 × LQ-EUDCFRT (r2 = 0.99) for the clinical DVHs. The clinical LQ-EUDCFRT resulted in median TCP of almost 100% vs. 93.8% with USC-EUDCFRT.

CONCLUSION

A uniform formalism of EUD should be defined among the SBRT community in order to apply it as a single metric for dose reporting and dose-response modeling in high-dose-gradient SBRT because its value depends on the underlying cell survival model and the model parameters. Further investigations of the optimal formalism to derive the EUD through clinical correlations are warranted.

Head and neck radiotherapy on the MR linac: a multicenter planning challenge amongst MRIdian platform users

Purpose

Purpose of this study is to evaluate plan quality on the MRIdian (Viewray Inc., Oakwood Village, OH, USA) system for head and neck cancer (HNC) through comparison of planning approaches of several centers.

Methods

A total of 14 planners using the MRIdian planning system participated in this treatment challenge, centrally organized by ViewRay, for one contoured case of oropharyngeal carcinoma with standard constraints for organs at risk (OAR). Homogeneity, conformity, sparing of OARs, and other parameters were evaluated according to The International Commission on Radiation Units and Measurements (ICRU) recommendations anonymously, and then compared between centers. Differences amongst centers were assessed by means of Wilcoxon test. Each plan had to fulfil hard constraints based on dose–volume histogram (DVH) parameters and delivery time. A plan quality metric (PQM) was evaluated. The PQM was defined as the sum of 16 submetrics characterizing different DVH goals.

Results

For most dose parameters the median score of all centers was higher than the threshold that results in an ideal score. Six participants achieved the maximum number of points for the OAR dose parameters, and none had an unacceptable performance on any of the metrics. Each planner was able to achieve all the requirements except for one which exceeded delivery time. The number of segments correlated to improved PQM and inversely correlated to brainstem D_0.1cc and to Planning Target Volume1 (PTV) D_0.1cc. Total planning experience inversely correlated to spinal canal dose.

Conclusion

Magnetic Resonance Image (MRI) linac-based planning for HNC is already feasible with good quality. Generally, an increased number of segments and increasing planning experience are able to provide better results regarding planning quality without significantly prolonging overall treatment time.

Supplementary Information

The online version of this article (10.1007/s00066-021-01771-8) contains supplementary material, which is available to authorized users.

Lean Thinking to manage a national working group on physics aspects of Stereotactic Body Radiation Therapy

PURPOSE

To report how the adoption of a Lean Thinking mindset in the management of a national working group (WG) on the physics of stereotactic body radiation therapy (SBRT) contributed to achieve SBRT standardization objectives.

METHODS

Vision for the WG has been established as fragmentation reduction and process harmonization enhancement in SBRT for Italian centers. Two main research themes of the technical aspects of SBRT emerged as areas with major standardization improvement needs, small field dosimetry and SBRT planning comparisons, to be investigated through multi-institutional studies. The management of the WG leveraged on the Lean concept of fostering self-organization in a non-hierarchical environment. Four progressive involvement levels were defined for each study. No specific "scientific" pre-experience was required to propose and coordinate a project, just requiring a voluntary commitment. People engagement was measured in terms of number of published articles. The standardization goals have been conducted through a simplified "5S" (Sort, Set in Order, Shine, Standardize, and Sustain) methodology, first considering a phase of awareness (the first three "S"), then identifying and implementing standardization actions (the last two "S").

RESULTS

Since the beginning, 157 medical physicists joined the AIFM/SBRT-WG. Twenty-four papers/reviews/letters have been published in the period 2014-2019 on major radiation oncology journals, authored by >100 physicists (>50% working in small hospitals). Six over 12 first authors worked in peripheral/small hospitals, with no prior publication as first author. These studies contributed to the awareness and standardization phases for both small-field dosimetry and planning. In particular, errors in small-field measurements in 8% of centers were detected thanks to a generalized output factor curve in function of the effective field size created by averaging data available from different Linacs. Furthermore, planner's experience in SBRT was correlated with dosimetric parameters in the awareness phase; while sharing median dose volume histograms (DVHs) reduced variability among centers while keeping the same level of plan complexity. Finally, all the dosimetric parameters statistically significant to the planner experience during the awareness phase, were no longer significantly different in the standardization phase.

CONCLUSIONS

The experience of our SBRT-WG has shown how a Lean Thinking mindset could foster the SBRT procedure standardization and spread the physics of SBRT knowledge, enhancing personal growth. Our expectation is to inspire other scientific societies that have to deal with fragmented contexts or pursue processes harmonization through Lean principles.

Precision Stereotactic Radiotherapy for Spinal Tumors: Mechanism, Efficacy, and Issues

Stereotactic ablative radiotherapy (SABR/SBRT) is a revolutionary technique for tumor therapy. Its advantages are especially beneficial for the treatment spinal tumors. It has a wide range of indications in radiotherapy alone and in preoperative and postoperative treatments for spinal tumor. The mechanism of stereotactic radiotherapy for spinal tumors is special, and completely different from traditional radiotherapy. Compared with traditional radiotherapy, SBRT creates more DNA double-strand breaks, leads to less DNA damage repair, and also has anti-vascular effects, in situ vaccine effects and abscopal effect. In the present study, the literature regarding SABR for the treatment of spinal tumors is summarized, and we reviewed characteristics of SABR and spinal tumors, as well as the clinical efficacy and toxicity of SABR in treating spinal tumors. In addition, we proposed several issues around the SABR treatment of spinal tumor, the standard of treatment dose, and the post-treatment follow-up. We also made predictions with respect to future management of spinal tumors, SABR development, multi-modality integration between SABR and other treatments, and other future development trends, thereby providing future research directions as a contribution to the field.

Estimating dose delivery accuracy in stereotactic body radiation therapy: A review of in-vivo measurement methods

Stereotactic body radiation therapy (SBRT) has been recognized as a standard treatment option for many anatomical sites. Sophisticated radiation therapy techniques have been developed for carrying out these treatments and new quality assurance (QA) programs are therefore required to guarantee high geometrical and dosimetric accuracy. This paper focuses on recent advances on in-vivo measurements methods (IVM) for SBRT treatment. More specifically, all of the online QA methods for estimating the effective dose delivered to patients were compared. Determining the optimal IVM for performing SBRT treatments would reduce the risk of errors that could jeopardize treatment outcome. A total of 89 papers were included. The papers were subdivided into the following topics: point dosimeters (PD), transmission detectors (TD), log file analysis (LFA), electronic portal imaging device dosimetry (EPID), dose accumulation methods (DAM). The detectability capability of the main IVM detectors/devices were evaluated. All of the systems have some limitations: PD has no spatial data, EPID has limited sensitivity towards set-up errors and intra-fraction motion in some anatomical sites, TD is insensitive towards patient related errors, LFA is not an independent measure, DAMs are not always based on measures. In order to minimize errors in SBRT dose delivery, we recommend using synergic combinations of two or more of the systems described in our review: on-line tumor position and patient information should be combined with MLC position and linac output detection accuracy. In this way the effects of SBRT dose delivery errors will be reduced.

Technological quality requirements for stereotactic radiotherapy : Expert review group consensus from the DGMP Working Group for Physics and Technology in Stereotactic Radiotherapy

This review details and discusses the technological quality requirements to ensure the desired quality for stereotactic radiotherapy using photon external beam radiotherapy as defined by the DEGRO Working Group Radiosurgery and Stereotactic Radiotherapy and the DGMP Working Group for Physics and Technology in Stereotactic Radiotherapy. The covered aspects of this review are 1) imaging for target volume definition, 2) patient positioning and target volume localization, 3) motion management, 4) collimation of the irradiation and beam directions, 5) dose calculation, 6) treatment unit accuracy, and 7) dedicated quality assurance measures. For each part, an expert review for current state-of-the-art techniques and their particular technological quality requirement to reach the necessary accuracy for stereotactic radiotherapy divided into intracranial stereotactic radiosurgery in one single fraction (SRS), intracranial fractionated stereotactic radiotherapy (FSRT), and extracranial stereotactic body radiotherapy (SBRT) is presented. All recommendations and suggestions for all mentioned aspects of stereotactic radiotherapy are formulated and related uncertainties and potential sources of error discussed. Additionally, further research and development needs in terms of insufficient data and unsolved problems for stereotactic radiotherapy are identified, which will serve as a basis for the future assignments of the DGMP Working Group for Physics and Technology in Stereotactic Radiotherapy. The review was group peer-reviewed, and consensus was obtained through multiple working group meetings.

Credentialing of vertebral stereotactic ablative body radiotherapy in a multi-centre trial

PURPOSE/OBJECTIVE

Stereotactic ablative body radiotherapy (SABR) in multi-centre trials requires rigorous quality assurance to ensure safe and consistent treatment for all trial participants. We report results of vertebral SABR dosimetry credentialing for the ALTG/TROG NIVORAD trial.

MATERIAL/METHODS

Centres with a previous SABR site visit performed axial film measurement of the benchmarking vertebral plan in a local phantom and submitted radiochromic film images for analysis. Remaining centres had on-site review of SABR processes and axial film measurement of the vertebral benchmarking plan. Films were analysed for dosimetric and positional accuracy: gamma analysis (>90% passing 2%/2mm/10% threshold) and ≤ 1 mm positional accuracy at target-cord interface was required.

RESULTS

19 centres were credentialed; 11 had on-site measurement. Delivery devices included linear accelerator, TomoTherapy and CyberKnife systems. Five centres did not achieve 90% gamma passing rate. Of these, three were out of tolerance (OOT) in low (<5Gy) dose regions and > 80% passing rate and deemed acceptable. Two were OOT over the full dose range: one elected not to remeasure; the other also had positional discrepancy greater than 1 mm and repeat measurement with a new plan was in tolerance. The original OOT was attributed to inappropriate MLC constraints. All centres delivered planned target-cord dose gradient within 1 mm.

CONCLUSION

Credentialing measurements for vertebral SABR in a multi-centre trial showed although the majority of centres delivered accurate vertebral SABR, there is high value in independent audit measurements. One centre with inappropriate MLC settings was detected, which may have resulted in delivery of clinically unacceptable vertebral SABR plans.

Dosimetric Multicenter Planning Comparison Studies for Stereotactic Body Radiation Therapy: Methodology and Future Perspectives

In this review a summary of the published literature pertaining to the stereotactic body radiation therapy multiplanning comparison, data sharing strategies, and implementation of benchmark planning cases to improve the skills and knowledge of the participating centers was investigated. A total of 30 full-text articles were included. The studies were subdivided in 3 categories: multiplanning studies on dosimetric variability, planning harmonization before clinical trials, and technical and methodologic studies. The methodology used in the studies were critically analyzed to find common and original elements with the pros and cons. Multicenter planning studies have played a key role in improving treatment plan harmonization, treatment plan compliance, and even clinical practices. This review has highlighted that some fundamental steps should be taken to transform a simple treatment planning comparison study into a potential credentialing method for stereotactic body radiation therapy accreditation. In particular, prescription and general requirements should always be well defined; data analysis should be performed with independent dose volume histogram or dose calculations; quality score indices should be constructed; feedback and correction strategies should be provided; and a simple web-based collaboration platform should be used. The results reported clearly showed that a crowd-based replanning approach is a viable method for achieving harmonization and standardization of treatment planning among centers using different technologies.

Results of a multicenter intensity modulated radiation therapy treatment planning comparison study for a sample prostate cancer case

PURPOSE

To determine the influence of different medical physicists, photon energies, treatment planning systems and treatment machines on the resulting external beam radiotherapy dose distribution for a sample prostate cancer case.

METHODS

A pre-contoured computed tomography (CT) dataset containing planning target volume 1 (PTV1) prostate and seminal vesicles (single dose [SD] 1.8 Gy, total dose [TD] 59.4 Gy), PTV2 prostate (simultaneously integrated boost [SIB], SD 2.0 Gy, TD 66 Gy), PTV3 prostate and seminal vesicles approach (SD 1.8 Gy, TD 73.8 Gy/80.4 Gy SIB) as well as organs at risk (OAR: rectum, bladder, femoral heads, bowel, anus) was offered to the members of the task group IMRT (intensity-modulated radiation therapy) of the German Society for Medical Physics. The purpose was to calculate one combined treatment plan (TP) for PTV1 and PTV2, as well as a separate one for PTV3. Dose volume histograms (DVH), different dose values, conformity index (CI), homogeneity index (HI), gradient index (GI) and a new "better than average score" were used to analyse the dose distributions.

RESULTS

Altogether 44 institutions took part in this study and submitted acceptable dose distributions for the PTVs. However, there were statistically significant differences, especially for the doses administered to the OAR, such as rectum, bladder and femoral heads. Differences between the treatment plans were not easily detectable by visual inspection of the isodose distribution. Dose maxima may occur outside the PTV. Even though scoring indices are already published, the new "better than average score" was needed to identify a plan that minimises dose to all OAR simultaneously.

CONCLUSION

Different medical physicists or dosimetrists, photon energies, treatment planning systems, and treatment machines have an impact on the resulting dose distribution. However, the differences only become apparent when comparing DVH, analysing dose values, comparing CI, HI, GI, as well as reviewing the dose distribution in every single plane. A new score was introduced to identify treatment plans that simultaneously deliver a low dose to all OAR. Such inter- and intra-institutional comparison studies are needed to explore different treatment planning strategies; however, there is still no automatic solution for an "optimal" treatment plan.

Plan quality improvement by DVH sharing and planner's experience: Results of a SBRT multicentric planning study on prostate

PURPOSE

To evaluate, in a multi-institutional context, the role of Dose Volume Histogram (DVH) sharing in order to achieve higher plan quality, to harmonize prostate Stereotactic Body Radiation Therapy (SBRT) plans and to assess if the planner's experience in SBRT could lead to lower dose at organs at risk (OARs).

METHODS

During the first phase five patients enrolled for prostate SBRT were planned by multiple physicists according to common protocol. The prescription dose was 35 Gy in 5 fractions. Dosimetric parameters, modulation index (MIt), plan parameters, and planner experience level (EL) were statistically analyzed. During the second phase median DVHs from all centers were shared and physicists replanned one patient of the five, aiming at inter-planner harmonization and further OARs sparing. Data were summarized by Spearman-correlogram (p < 0.05) and boxplots. The Kruskal-Wallis test was used to compare the re-plans to the original plans.

RESULTS

Seventy-eight SBRT plans from 13 centers were evaluated. EL correlated with modulation of plan parameters and reduction of OARs doses, such as volume receiving 28 Gy of rectum (rectum-V28Gy), rectum-V32Gy, and bladder-V30Gy. The re-plans showed significant reduced variability in rectum-V28Gy and increased PTV dose homogeneity. No significant difference in plan complexity metrics and plan parameters between plans and re-plans were obtained.

CONCLUSIONS

Planner's experience in prostate SBRT was correlated with dosimetric parameters. Sharing median DVHs reduced variability among centers whilst keeping the same level of plan complexity. SBRT planning skills can benefit from a replanning phase after sharing DVHs from multiple centers, improving plan quality and concordance among centers.