Van Herk Research Articles

Impact of immobilisation and image guidance protocol on planning target volume margins for supine craniospinal irradiation.

BackgroundThe setup errors during supine-CSI (sCSI) using single or dual immobilisation (SM, DM) subsets from two institutions were reviewed to determine if DM consistently decreased the required planning target volumes (PTV) margins and to identify the optimal image guidance environments.Materials and methodsOurs and a sister institutional cohort, each with a subset of SM or DM sCSI and daily 3-dimensional online image verification sets, were reviewed for the cranial and spinal regions translational shifts. Using descriptive statistics, scatter plots and independent sample Mann-Whitney test we compared shifts in each direction for two subsets in each cohort deriving PTV margins (Van Herk: VH, Strooms: St recipes) for the cranial and spinal regions. Three image guidance (IG) protocols were simulated for two regions on the combined cohort with SM and DM subsets to identify the most optimal option with the smallest PTV margin. The IG protocols: 3F, 5F and 5FB where the systematic error correction was done using the average error from the first three, five and in the cranium alone (applied to both the cranium and spine, otherwise) for the first five set-ups, respectively.Results6968 image sets for 179 patients showed DM could consistently reduce the PTV margin (VH/St) for the cranium from 6/5 to 4/3.5 (31.8/30.8%) and 6/4 to 4/3.5 mm (30.5/16.8%) for primary and validation cohort, respectively. Similarly, for the spine it was 10/8.5 to 6/5.5 (38.6/38.4%) and 9/7.7 to 7/6 (21.6/21.4%), respectively. The “5F-IG” resulted in the smallest margins for both the cranial (3 mm) and spinal region (5 mm) for DM with estimated 95% CTV coverage probability.ConclusionDM with 5F-IG would significantly reduce the required PTV margins for sCSI.

Assessment of planning target volume margins in 1.5 T magnetic resonance‐guided stereotactic body radiation therapy for localized prostate cancer

AbstractObjectiveWe aimed to determine the planning target volume margins applicable to magnetic resonance‐guided stereotactic body radiation therapy (MRgSBRT) of localized prostate cancer on a 1.5 T MR‐Linac.MethodsA total of 10 patients with low‐ to intermediate‐risk prostate cancer were retrospectively enrolled. All patients received five‐fraction MRgSBRT treatments, and underwent daily endorectal ballooning and bladder control. Five patients were implanted with the hydrogel rectal spacer. Three MR scans at each fraction were acquired to evaluate the intrafractional prostate motion. The required planning target volume margin was determined using the van Herk margin recipe based on direct MR‐MR prostate registration. The set‐up margin for localization using bony anatomy was also estimated.ResultsThe mean and standard deviation of translational displacement in left‐right (LR), superior‐inferior (SI), and anterior‐posterior (AP) directions were –0.3 ± 0.7, 2.0 ± 1.5, and 1.4 ± 1.1 mm, respectively. Planning target volume margins over the MRgSBRT fraction duration were 2.8 (LR), 5.3 (SI), and 3.9 mm (AP). Set‐up margins were estimated to 2.6 (LR), 3.3 (SI), and 3.3 mm (AP). Prostate motion increased considerably with treatment duration. Adapt‐to‐position had smaller intrafractional motions than adapt‐to‐shape. The rectal spacer insignificantly (p > 0.1) reduced the prostate motion in SI and AP.ConclusionOnline workflow efficiency enhancement and intrafractional motion monitoring are vital for planning target volume margin reduction and precision enhancement in MRgSBRT.

Calculation of set-up margin in frameless stereotactic radiotherapy accounting for translational and rotational patient positing error.

Rotation corrected set-up margins in stereotactic radiotherapy (SRT). This study aimed to calculate the rotational positional error corrected set-up margin in frameless SRT. 6D setup errors for the steriotactic radiotherapy patients were converted to 3D translational only error mathematically. Setup margins were calculated with and without considering the rotational error and compared. A total of 79 patients of SRT each received >1 fraction (3-6 fractions) incorporated in this study. Two cone-beam computed tomography (CBCT) scans were acquired for each session of treatment, before and after the robotic couch-aided patient position correction using a CBCT. The postpositional correction set-up margin was calculated using the van Herk formula. Further, a planning target volume_R (PTV_R) (with rotational correction) and PTV_NR (without rotational correction) were calculated by applying the rotation corrected and uncorrected set-up margins on the gross tumor volumes (GTVs). Statistical Analysis Used: General. A total of 380 sessions of pre- (190) and post (190) table positional correction CBCT was analyzed. Posttable position correction mean positional error for lateral, longitudinal, and vertical translational and rotational shifts was (x)-0.01 ± 0.05 cm, (y)-0.02 ± 0.05 cm, (z) 0.00 ± 0.05 cm, and (θ) 0.04° ± 0.3°, (Φ) 0.1° ± 0.4°, (Ψ) 0.0° ± 0.4°, respectively. The GTV volumes show a range of 0.13 cc-39.56 cc, with a mean volume of 6.35 ± 8.65 cc. Rotational correction incorporated postpositional correction set-up margin the in lateral (x), longitudinal (y) and vertical (z) directions were 0.05 cm, 0.12 cm, and 0.1 cm, respectively. PTV_R ranges from 0.27 cc to 44.7 cc, with a mean volume of 7.7 ± 9.8 cc. PTV_NR ranges from 0.32 cc to 46.0 cc, with a mean volume of 8.1 ± 10.1 cc. The postcorrection linear set-up margin matches well with the conventional set-up margin of 1 mm. Beyond a GTV radius of 2 cm, the difference between PTV_NR and PTV_R is ≤2.5%, hence not significant.

Retrospective study of random and systematic errors in Head and Neck cancer Patients with Image guided Helical Intensity Modulated Radiation Therapy

The Precision of radiotherapy is checked based on the matching of pre-treatment 2D portal imaging/CBCT to the reference image. The objective of this study is to find inter-fractional systematic and random setup errors (mm) for head and Neck cancer patients and also find setup margin for Planning Target Volume (PTV) at Mohan Dai Oswal Cancer Hospital Ludhiana Punjab. Inter-fractional motion errors were quantified for 10 Head and neck cancer patients who underwent image guided helical intensity modulated radiation therapy with Radixact X9 machine. One hundred fan beam computed tomography scans of 2mm slice thickness, 3.5 MV average energy and flattening filter free beam has been used to collect the data for the calculation of systematic and random errors. After patient immobilization, patient accuracy is checked with registration of pre-treatment image with reference planning image with the help of Accuray Precision image guidance protocol. For every patient 10 fan beam computed tomography images has been taken. Translational errors have been calculated in X, Y and Z direction to find systematic error (∑) and random error (σ). The final PTV margin is calculated by van Herk’s equation (2.5∑ + 0.7σ). The results of this study shows that mean translational errors varies from -2.3 mm to 3.3 mm in lateral direction (X), -3.6 mm to 1.7 mm in longitudinal direction (Y), -2.7 mm to 1.5 mm in vertical direction (Z). The mean and standard deviation (SD) for systematic errors are 1.467, 0.8, 0.923 and random error 0.0595, 0.02266, 0.03824 in X, Y and Z direction has been calculated. The Total Margin for CTV to PTV which include setup margin (mm) in X, Y and Z direction are 3.7 mm, 2.02 mm and 2.33 mm. In addition to that, a PTV margin of 5.00 mm is the appropriate margin for Mohan Dai Oswal Cancer Hospital’s patients. This work conclude that CTV to PTV margin of 5.00 mm is suitable for image guided helical intensity modulated radiation therapy for Head and neck patients to ensure the minimum 95% coverage of dose to target

Minimum non-isotropic and asymmetric margins for taking into account intrafraction prostate motion during moderately hypofractionated radiotherapy

PurposeTo investigate the impact on dose distribution of intrafraction motion during moderate hypofractionated prostate cancer treatments and to estimate minimum non-isotropic and asymmetric (NI-AS) treatment margins taking motion into account. MethodsProstate intrafraction 3D displacements were recorded with a transperineal ultrasound probe and were evaluated in 46 prostate cancer patients (876 fractions) treated by moderate hypofractionated radiation therapy (60 Gy in 20 fractions). For 18 patients (346 fractions), treatment plans were recomputed increasing CTV-to-PTV margins from 0 to 6 mm with an auto-planning optimization algorithm. Dose distribution was estimated using the voxel shifting method by displacing CTV structure according to the retrieved movements. Time-dependent margins were finally calculated using both van Herk’s formula and the voxel shifting method. ResultsMean intrafraction prostate displacements observed were −0.02 ± 0.52 mm, 0.27 ± 0.78 mm and −0.43 ± 1.06 mm in left–right, supero-inferior and antero-posterior directions, respectively. The CTV dosimetric coverage increased with increased CTV-to-PTV margins but it decreased with time. Hence using van Herk’s formula, after 7 min of treatment, a margin of 0.4 and 0.5 mm was needed in left and right, 1.5 and 0.7 mm in inferior and superior and 1.1 and 3.2 mm in anterior and posterior directions, respectively. Conversely, using the voxel shifting method, a margin of 0 mm was needed in left–right, 2 mm in superior, 3 mm in inferior and anterior and 5 mm in posterior directions, respectively. With this latter NI-AS margin strategy, the dosimetric target coverage was equivalent to the one obtained with a 5 mm homogeneous margin. ConclusionsNI-AS margins would be required to optimally take into account intrafraction motion.

Ramifications of Setup Margin Use During Frameless Stereotactic Radiosurgery/Therapy With Gamma Knife Icon Cone-Beam Computed Tomography (CBCT): A Dosimetric Study.

ObjectiveThe objective is to explore the possibility of optimal/rational application of setup margin during treatment planning for frameless stereotactic Gamma Knife radiosurgery/therapy.MethodsUncertainty measurements for frameless Gamma Knife Icon treatment were used to calculate the necessary setup margin via four different published recipes and these margins were subsequently applied to treatment plans of 30 previously treated patients and replans were generated meeting comparable plan quality metrics. All plans were then analyzed based on the ability to maintain normal tissue dose tolerances and the relative increase in target dose coverage probability using a pass/fail scoring system based on published normal tissue dose constraints and an in-house developed optimal scoring method.ResultsGross tumor volume/planning target volume (GTV/PTV) size strongly correlated with both meeting normal tissue tolerances and optimal scores for single fraction plans corroborating published clinical outcomes. The Van Herk Margin Formula (VHMF) and Parker margin formulae were indicated as good candidates for high probabilities of both meeting normal tissue goals and high optimal scores which generally translated to just over 1 mm in GTV to PTV margin.ConclusionFor single fraction treatment, GTV size is highly significant in predicting failure to meet normal tissue goals whereas whether setup margin was used was not a significant predictor. Setup margin can rationally be applied when fraction number is dictated by clinically indicated metrics regarding GTV size of greater or less than 4 cc. 1 mm is a reasonable practical application of margin added to GTV to ensure physical prescription dose target coverage for most cases when clinically desired based on disease type and intended outcome.

Target margin design through analyzing a large cohort of clinical log data in the cyberknife system.

PurposeCalculating the adequate target margin for real‐time tumor tracking using the Cyberknife system is a challenging issue since different sources of error exist. In this study, the clinical log data of the Cyberknife system were analyzed to adequately quantify the planned target volume (PTV) margins of tumors located in the lung and abdomen regions.MethodsIn this study, 45 patients treated with the Cyberknife module were examined. In this context, adequate PTV margins were estimated based on the Van Herk formulation and the uncertainty estimation method by considering the impact of errors and uncertainties. To investigate the impact of errors and uncertainties on the estimated PTV margins, a statistical analysis was also performed.ResultsOur study demonstrates five different sources of errors, including segmentation, deformation, correlation, prediction, and targeting errors, which were identified as the main sources of error in the Cyberknife system. Furthermore, the clinical evaluation of the current study reveals that the two different formalisms provided almost identical PTV margin estimates. Additionally, 4–5 mm and 5 mm margins on average could provide adequate PTV margins at lung and abdomen tumors in all three directions, respectively. Overall, it was found that concerning the PTV margins, the impact of correlation and prediction errors is very high, while the impact of robotics error is low.ConclusionsThe current study can address two limitations in previous researches, namely insufficient sample sites and a smaller number of patients. A comparison of the present results concerning the lung and abdomen areas with other studies reveals that the proposed strategy could provide a better reference in selection the PTV margins. To our knowledge, this study is one of the first attempts to estimate the PTV margins in the lung and abdomen regions for a large cohort of patients treated using the Cyberknife system.

The transition from conformal to advanced radiotherapy techniques in the treatment planning of gynecological cancer patients

Background/Aim. The transition from standard to highly conformal radiation therapy techniques requires the implementation of complex advanced dosimetry. The aim of the study was to compare dosimetric parameters of the three-dimensional conformal radiotherapy (3DCRT) and volumetric modulated arc therapy (VMAT) plan, as well as complications after treatment in relation to dosimetric parameters in gynecological cancer patients. Methods. A total of 49 gynecological cancer patients were included in the study. All patients were planned for 3DCRT, but due to un-acceptable doses to organs at risk (OARs), treatment plans for intensity modulated radiation therapy (IMRT), or VMAT, were generated for 21 patients. The patients were prescribed 50.4 Gy/28 fractions (4 patients) and 45 Gy/25 fractions (45 patients). The coverage of planning target volume (PTV) and doses to OARs were recorded. PTV margins were evaluated for both techniques according to the Van Herk formula. Results. ICRU 83 criteria were fulfilled in all 3DCRT/VMAT/IMRT plans providing optimal coverage of PTV. Doses to OARs, on average, the V45Gy in the small bowel in IMRT/VMAT plans was four times smaller than the same in 3DCRT plans. The V45 Gy of small bowels was, on average, 49.4 cm3 in IMRT/VMAT plans, while in 3DCRT plans, it was 211.6 cm3. In the case of the femoral head, a significant reduction in V30Gy (10.8% vs. 33.1%) and mean dose in the case of IMRT/VMAT plans was recorded (30.4 Gy in 3DCRT vs. 23.6 Gy). Rectum was planned with a significantly lower dose in terms of V30Gy (79.5% vs. 95.2%) in IMRT/VMAT plans. The bladder was better spared in VMAT plans in terms of V40Gy (51% vs. 91%), but the maximum dose was higher in VMAT plans than in 3DCRT (50.1 Gy to 48.1 Gy on average). For all OARs, there was a statistically significant difference registered at p ? 0.05. Toxicities recorded in VMAT and 3DCRT patients included mainly radiation-induced cystitis and enteritis. Patients treated with 3DCRT generally had longer recovery time. The homogeneity index was 0.11 for VMAT plans and 0.09 for 3DCRT plans. Conclusions. Analysis of dosimetric parameters revealed significant differences in normal tissue doses for the same 3DCRT and VMAT patients, which confirmed the necessity for the implementation of advanced techniques for as many patients as possible.

About Intelligence (Artificial or Not) – A Discussion with Marcel van Herk

Professor Marcel van Herk is the chair of the radiotherapy physics at the University of Manchester. As a medical physics student he started on the development of a liquid ionization imaging device that later became the Varian PortalVision system for electronic portal imaging. Starting during a Postdoc at Harvard Medical School, USA in 1992, his work incorporated 3D image processing, first CT-MR image registration for treatment planning, later measuring organ motion and other uncertainties in radiotherapy such as delineation variability. Much of this work has been integrated in the Elekta Synergy cone-beam CT image guidance system developed. He also held a part-time teaching role as a Professor at the University of Amsterdam working amongst others at the integration of microscopic optical imaging into radiotherapy treatment planning. His goal is to do research to improve the clinical practice of radiotherapy.

A Prospective Study of a Resorbable Intravesical Fiducial Marker for Bladder Cancer Radiation Therapy

PurposeWe conducted a prospective pilot study to evaluate safety and feasibility of TraceIT, a resorbable radiopaque hydrogel, to improve image guidance for bladder cancer radiation therapy (RT). Methods and MaterialsPatients with muscle invasive bladder cancer receiving definitive RT were eligible. TraceIT was injected intravesically around the tumor bed during maximal transurethral resection of bladder tumor. The primary endpoint was the difference between radiation treatment planning margin on daily cone beam computed tomography based on alignment to TraceIT versus standard-of-care pelvic bone anatomy. The Van Herk margin formula was used to determine the optimal planning target volume margin. TraceIT visibility, recurrence rates, and survival were estimated by Kaplan-Meier method. Toxicity was measured by Common Terminology Criteria for Adverse Events version 4.03. ResultsThe trial was fully accrued and 15 patients were analyzed. TraceIT was injected in 4 sites/patient (range, 4-6). Overall, 94% (95% confidence interval [CI], 90%-98%) of injection sites were radiographically visible at RT initiation versus 71% (95% CI, 62%-81%) at RT completion. The median duration of radiographic visibility for injection sites was 106 days (95% CI, 104-113). Most patients were treated with a standard split-course approach with initial pelvic radiation fields, then midcourse repeat transurethral resection of bladder tumor followed by bladder tumor bed boost fields, and 14/15 received concurrent chemotherapy. Alignment to fiducials could allow for reduced planning target volume margins (0.67 vs 1.56 cm) for the initial phase of RT, but not for the boost (1.01 vs 0.96 cm). This allowed for improved target coverage (D95% 80%-83% to 91%-94%) for 2 patients retrospectively planned with both volumetric-modulated arc therapy and 3-dimensional conformal RT. At median follow-up of 22 months, no acute or late complications attributable to TraceIT placement occurred. No patients required salvage cystectomy. ConclusionsTraceIT intravesical fiducial placement is safe and feasible and may facilitate tumor bed delineation and targeting in patients undergoing RT for localized muscle invasive bladder cancer. Improved image guided treatment may facilitate strategies to improve local control and minimize toxicity.

Theoretical Calculation of Population-Based Margins in Fiducial Marker-Based Real-time Tumor Tracking

We introduced a theoretical margin calculation method based on a combination of the guide to the expression of uncertainty in measurement (GUM) and the probability distribution of the tracking error, and determined the population-based margins in fiducial marker-based real-time tumor tracking (RTTT) radiotherapy.A total of 74,705, 73,235, and 164,968 sets of intrafractional X-ray images were analyzed for 47 lung, 48 liver, and 25 pancreatic cancer patients, respectively, in an RTTT clinical trial using a gimbaled Linac. The 2.5th and 97.5th percentiles of the tracking error, defined as the difference between the predicted and detected target positions, were regarded as representative values in each fraction of the disease site. The population-based statistics of the probability distribution of these representative tracking errors (PD-RTEs) were calculated in six directions. The goodness of fit of the PD-RTEs with the normal distribution were tested. The novel margin theory is expressed as follows: Margin = Υ+k⋅√(υ^2+τ^2), where Υ, υ, and τ are the population-based mean difference, interpatient uncertainty, and intrapatient uncertainty of the population of a representative tracking error in each direction, respectively. k is the factor to expect the margin satisfying 95% of the population. Considering that k depended on the target shape, k = 2.0 for the lung and liver cancer, 2.33 for the pancreatic cancer were applied. To verify the margin formula, the content rate in which the CTV was included in the PTV was calculated through Monte Carlo simulation (MCS) using the PD-RTE. The same calculation was also performed using the van Herk's theory intending to cover 90% of the population.The calculated results are shown in Table. The margins on the left, right, anterior, and posterior were within 4.1 mm for all sites. The superior and inferior margins were up to 6.2, 4.7, and 3.9 mm for lung, liver, and pancreatic cancer, respectively. Although some of the PD-RTEs could not be considered normal distributions, MCS revealed that the median content rates using the proposed margins satisfied 95% for lung and liver cancer, and 93% for pancreatic cancer. By contrast, although the median content rate according to van Herk's equation was 89% for pancreatic cancer, the content rates were more than 96% for lung and liver cancer, which was higher than their expected coverage of the population.In this study, we proposed a theoretically determined margin calculation formula for RTTT based on a combination of the GUM and the probability distribution of the tracking errors. The proposed formula appropriately calculated the practical margin size in fiducial marker-based RTTT for lung, liver, and pancreatic cancer.

PTV margin calculation for head and neck patients treated with VMAT: a systematic literature review

AbstractAim:The intent of the review was to identify different methodological approaches used to calculate the planning target volume (PTV) margin for head and neck patients treated with volumetric arc therapy (VMAT), and whether the necessary factors to calculate the margin size with the selected formula were used.Materials and Methods:A comprehensive, systematic search of related studies was done using the Hydi search engine and different databases: MEDLINE, PubMed, CINAHL, ProQuest (Nursing and Allied Health), Scopus, ScienceDirect and tipsRO. The literature search included studies published between January 2007 and December 2020. Eligibility screening was performed by two reviewers.Results:A total of seven studies were found. All the reviewed studies used the Van Herk formula to measure the PTV margin. None of the studies incorporated the systematic errors of target volume delineation in the PTV equation. Inter-fraction translational errors were assessed in all the studies, whilst intra-fraction errors were only included in the margin equation for two studies. The studies showed great heterogeneity in the key characteristics, aims and methods.Findings:Since systemic errors from target volume delineation were not considered and not all studies assess intra-fraction errors, PTV margins may be underestimated. The recommendations are that studies need to determine the effect of target volume variance on PTV margins. It is also recommended to compare PTV margin results using various formulas.

Using 4DCBCT simulation and guidance to evaluate inter-fractional tumor variance during SABR for lung tumor within the lower lobe

Inter-fractional tumor variance would lead to insufficient dosage or overdose in tumor region during lung cancer radiotherapy. However, previous works have not considered influence of inter-fractional tumor amplitude variance at treatment position due to lack of effective evaluation method during radiotherapy, especially for lung tumor within the lower lobe. Our objective was to investigate inter-fractional tumor baseline shift and amplitude variance due to respiratory motion with 4DCBCT simulation and guidance during stereotactic ablative body radiotherapy (SABR) for lung tumor. Subject included 19 patients with lung tumor within the lower lobe. 4DCBCT-simulated images at treatment position were acquired sequentially to determine internal tumor volume (ITV) and reference tumor motion at simulation process. Compared with reference tumor motion, 95 4DCBCT-guided images were acquired during each treatment to evaluate inter-fractional tumor baseline shift and amplitude variance, which were − 0.0 ± 1.3 mm and − 0.2 ± 1.4 mm in left–right(LR) direction, 0.9 ± 2.3 mm and 0.4 ± 2.9 mm in superior-inferior (SI) direction, 0.1 ± 1.5 mm and − 0.4 ± 2.0 mm in anterior–posterior (AP) direction. ITV margin were 3.5 mm, 7.5 mm and 5.3 mm in LR, SI and AP directions with van Herk’s (Int J Radiat Oncol Biol Phys 52(5):1407–1422, 2002) formula. 4DCBCT simulation and guidance is a reliable method to evaluate inter-fractional tumor variance during SABR for lung tumor within the lower lobe. ITV margin of 3.5 mm, 7.5 mm and 5.3 mm in LR, SI and AP directions would ensure greater tumor coverage during SABR for lung tumor within the lower lobe.

PD-0904 Limitation of van Herk’s recipe in robust optimization of clinical IMPT for head and neck cancer

PD-0904 Limitation of van Herk’s recipe in robust optimization of clinical IMPT for head and neck cancer

Margin calculation for multiple lung metastases treated with single-isocenter SBRT

Background and purposeA single-isocenter stereotactic body radiotherapy (SBRT) approach for multiple lung metastases has the potential to lower cumulative patient dose and reduce overall treatment time. However, the magnitude of inter-lesion position variation is currently unknown and not incorporated in margin calculations. The aim of this study was to quantify inter-lesion position variation and calculate safety margins for single-isocenter lung SBRT. Materials and methodsA total of 83 pairs of pulmonary metastases from 42 NSCLC patients were used to calculate relative inter-lesion position variation by lesion-based registration of planning CT and verification CBCT. Furthermore, β-value assessment of van Herk’s margin formula was performed by evaluating the distance between planned and blurred dose profiles of simulated spherical lesions, to evaluate its validity for heterogeneously planned dose distributions. Population-based ITV to PTV margins were calculated using the entire dataset and using subgroups with significant differences in relative inter-lesion position variation. ResultsThe mean ± SD inter-lesion position variation was 1.2 ± 1.1 mm as 3D-vector. Inter-lesion position variation was significantly increased if ≥1 lesion was not attached to the pleura or lesions were distant. The simulation showed that the combined SD of the random errors contributed to the margin only in the SI direction with 0.25∙σtot for a 65% dose prescription. When incorporating inter-lesion position variation, the safety margins increased from 5.6, 5.8, 5.2 mm (AP, SI, LR) to 6.0, 6.6, 5.5 mm for the entire cohort. ConclusionRelative inter-lesion position variation is influenced by inter-target distance and location and can be compensated with additional safety margins of <1 mm using single-isocenter SBRT.

Automated algorithm for calculation of setup corrections and planning target volume margins for offline image-guided radiotherapy protocols.

PurposeEach radiotherapy center should have a site‐specific planning target volume (PTV) margins and image‐guided (IG) radiotherapy (IGRT) correction protocols to compensate for the geometric errors that can occur during treatment. This study developed an automated algorithm for the calculation and evaluation of these parameters from cone beam computed tomography (CBCT)‐based IG‐intensity modulated radiotherapy (IG‐IMRT) treatment.Methods and materialsA MATLAB algorithm was developed to extract the setup errors in three translational directions (x, y, and z) from the data logged by the CBCT system during treatment delivery. The algorithm also calculates the resulted population setup error and PTV margin based on the van Herk margin recipe and subsequently estimates their respective values for no action level (NAL) and extended no action level (eNAL) offline correction protocols. The algorithm was tested on 25 head and neck cancer (HNC) patients treated using IG‐IMRT.ResultsThe algorithms calculated that the HNC patients require a PTV margin of 3.1, 2.7, and 3.2 mm in the x‐, y‐, and z‐direction, respectively, without IGRT. The margin can be reduced to 2.0, 2.2, and 3.0 mm in the x‐, y‐, and z‐direction, respectively, with NAL and 1.6, 1.7, and 2.2 mm in the x‐, y‐, and z‐direction, respectively, with eNAL protocol. The results obtained were verified to be the same with the margins calculated using an Excel spreadsheet. The algorithm calculates the weekly offline setup error correction values automatically and reduces the risk of input data error observed in the spreadsheet.ConclusionsIn conclusion, the algorithm provides an automated method for optimization and reduction of PTV margin using logged setup errors from CBCT‐based IGRT.

Evaluation of set-up errors and estimation of set-up margin during external beam radiation therapy of prostate cancer using electronic portal imaging device (EPID)

AbstractAim:In radiation therapy, accurate dose distribution in target volume requires accurate treatment setup. The set-up errors are unwanted and inherent in the treatment process. By achieving these errors, a set-up margin (SM) of clinical target volume (CTV) to planning target volume (PTV) can be determined. In the current study, systematic and random set-up errors that occurred during prostate cancer radiotherapy were measured by an electronic portal imaging device (EPID). The obtained values were used to propose the optimum CTV-to-PTV margin in prostate cancer radiotherapy.Materials and methods:A total of 21 patients with prostate cancer treated with external beam radiation therapy (EBRT) participated in this study. A total of 280 portal images were acquired during 12 months. Gross, population systematic (Σ) and random (σ) errors were obtained based on the portal images in Anterior–Posterior (AP), Medio-Lateral (ML) and Superior–Inferior (SI) directions. The SM of CTV to PTV were then calculated and compared by using the formulas presented by the International Commission on Radiation Units and Measurements (ICRU) 62, Stroom and Heijmen and Van Herk et al.Results:The findings showed that the population systematic errors during prostate cancer radiotherapy in AP, ML and SI directions were 1·40, 1·95 and 1·94 mm, respectively. The population random errors in AP, ML and SI directions were 2·09, 1·85 and 2·29 mm, respectively. The SM of CTV to PTV calculated in accordance with the formula of ICRU 62 in AP, ML and SI directions were 2·51, 2·68 and 3·00 mm, respectively. And according to Stroom and Heijmen, formula were 4·23, 5·19 and 5·48 mm, respectively. And Van Herk et al. formula were 4·96, 6·17 and 6·45 mm, respectively.Findings:The SM of CTV to PTV in all directions, based on the formulas of ICRU 62, Stroom and Heijmen and van Herk et al., were equal to 2·73, 4·98 and 5·86 mm, respectively; these values were obtained by averaging the margins in all directions.

Flogging a Dead Salmon? Reduced Dose Posterior to Prostate Correlates With Increased PSA Progression in Voxel-Based Analysis of 3 Randomized Phase 3 Trials

Flogging a Dead Salmon? Reduced Dose Posterior to Prostate Correlates With Increased PSA Progression in Voxel-Based Analysis of 3 Randomized Phase 3 Trials

Intrafraction motion during partial breast irradiation depends on treatment time

IntroductionAs the prognosis of early-stage breast cancer patients is excellent, prevention of radiation-induced toxicity has become crucial. Reduction of margins compensating for intrafraction motion reduces non-target dose. We assessed motion of the tumor bed throughout APBI treatment fractions and calculated CTV-PTV margins for breathing and drift. MethodsThis prospective clinical trial included patients treated with APBI on a Cyberknife with fiducial tracking. Paired orthogonal kV images made throughout the entire fraction were used to extract the tumor bed position. The images used for breathing modelling were used to calculate breathing amplitudes. The margins needed to compensate for breathing and drift were calculated according to Engelsman and Van Herk respectively. ResultsTwenty-two patients, 110 fractions and 5087 image pairs were analyzed. The margins needed for breathing were 0.3–0.6 mm. The margin for drift increased with time after the first imaging for positioning. For a total fraction duration up to 8 min, a margin of 1.0 mm is sufficient. For a fraction of 32 min, 2.5 mm is needed. Techniques that account for breathing motion can reduce the margin by 0.1 mm. There was a systematic trend in the drift in the caudal, medial and posterior direction. To compensate for this, 0.7 mm could be added to the margins. ConclusionsThe margin needed to compensate for intrafraction motion increased with longer fraction duration due to drifting of the target. It doubled for a fraction of 24 min compared to 8 min. Breathing motion has a limited effect.

New target volume delineation and PTV strategies to further personalise radiotherapy

Target volume delineation uncertainty (DU) is arguably one of the largest geometric uncertainties in radiotherapy that are accounted for using planning target volume (PTV) margins. Geometrical uncertainties are typically derived from a limited sample of patients. Consequently, the resultant margins are not tailored to individual patients. Furthermore, standard PTVs cannot account for arbitrary anisotropic extensions of the target volume originating from DU. We address these limitations by developing a method to measure DU for each patient by a single clinician. This information is then used to produce PTVs that account for each patient’s unique DU, including any required anisotropic component. We do so using a two-step uncertainty evaluation strategy that does not rely on multiple samples of data to capture the DU of a patient’s gross tumour volume (GTV) or clinical target volume. For simplicity, we will just refer to the GTV in the following. First, the clinician delineates two contour sets; one which bounds all voxels believed to have a probability of belonging to the GTV of 1, while the second includes all voxels with a probability greater than 0. Next, one specifies a probability density function for the true GTV boundary position within the boundaries of the two contours. Finally, a patient-specific PTV, designed to account for all systematic errors, is created using this information along with measurements of the other systematic errors. Clinical examples indicate that our margin strategy can produce significantly smaller PTVs than the van Herk margin recipe. Our new radiotherapy target delineation concept allows DUs to be quantified by the clinician for each patient, leading to PTV margins that are tailored to each unique patient, thus paving the way to a greater personalisation of radiotherapy.