Interfractional Variations Research Articles

An MRI framework for respiratory motion modelling validation.

IntroductionRespiratory motion models establish a correspondence between respiratory‐correlated (RC) 4‐dimensional (4D) imaging and respiratory surrogates, to estimate time‐resolved (TR) 3D breathing motion. To evaluate the performance of motion models on real patient data, a validation framework based on magnetic resonance imaging (MRI) is proposed, entailing the use of RC 4DMRI to build the model, and on both (i) TR 2D cine‐MRI and (ii) additional 4DMRI data for testing intra‐/inter‐fraction breathing motion variability.MethodsRepeated MRI data were acquired in 7 patients with abdominal lesions. The considered model relied on deformable image registration (DIR) for building the model and compensating for inter‐fraction baseline variations. Both 2D and 3D validation were performed, by comparing model estimations with the ground truth 2D cine‐MRI and 4DMRI respiratory phases, respectively.ResultsThe median DIR error was comparable to the voxel size (1.33 × 1.33 × 5 mm3), with higher values in the presence of large inter‐fraction motion (median value: 2.97 mm). In the 2D validation, the median estimation error on anatomical landmarks’ position resulted below 4 mm in every scenario, whereas in the 3D validation it was 1.33 mm and 4.21 mm when testing intra‐ and inter‐fraction motion, respectively. The range of motion described in the cine‐MRI was comparable to the motion of the building 4DMRI, being always above the estimation error. Overall, the model performance was dependent on DIR error, presenting reduced accuracy when inter‐fraction baseline variations occurred.ConclusionsResults suggest the potential of the proposed framework in evaluating global motion models for organ motion management in MRI‐guided radiotherapy.

Investigation of interfractional variation in lung tumor position under expiratory-phase breath hold using cone-beam computed tomography in stereotactic body radiation therapy.

We investigated the interfractional variation in the tumor position during lung stereotactic body radiotherapy (SBRT) under expiratory-phase breath hold (BH) using cone-beam computed tomography (CBCT). A total of 79 patients with lung cancer were treated with lung SBRT, wherein the Abches system under expiratory-phase BH was used to study interfractional variation. The tumors were located in the upper lobe in 31 cases, in the middle lobe in 11 cases, and in the lower lobe in 37 cases. Planning CTs were scanned under expiratory-phase BH with the Abches system. The 3-degrees-of-freedom (DOF) tumor-based setup using CBCT images under expiratory-phase BH was performed after a 6-DOF bony vertebrae-based setup using an ExacTrac X-ray system. Interfractional variation in the lung tumor position was defined as the difference in the position of the lung tumor relative to the bone anatomy in the left-right (LR), antero-posterior (AP), and craniocaudal (CC) directions represented as absolute values. The interfractional variation in the lung tumor position was very similar in all the lung regions, and its mean ± standard deviation values in all patients were 1.0 ± 1.1, 1.6 ± 1.9, and 1.6 ± 1.9 mm in the LR, AP, and CC directions, respectively. Further, 99.1%, 92.4%, and 92.7% of all the fractions for the interfractional tumor positional variation in the LR, AP, and CC directions were less than 5 mm, respectively. The interfractional variation in the tumor position was small for lung cancer patients treated with the Abches system under expiratory-phase BH.

Dosimetric changes and interfraction anatomical variations during high dose rate brachytherapy of cervical cancers- Can replanning be avoided?

Interfraction variations occur due to tumour regression during cervical intarcavitary brachytherapy (ICR) requiring replanning with every fraction. This study aimed to determine the correlation of interfraction variations in positioning of applicator and its relationship with relative dosimetry if the dwell time and source positions of the plan of first fraction were applied to other subsequent fractions.This is a retrospective review of the orthogonal radiographs of cervical cancer patients treated from 2013 to 2016 in our institute, receiving ICR with the same dose prescription to point A using a tandem and two ovoids applicators. A hypothetical second and third plan were obtained by applying the same source positions and dwell time as in the first fraction plan. The dose delivered to point A, point B, cervical point, bladder point and rectal point were determined. The actual doses in the treated second and third fractions were compared with these hypothetical plans for any significant change in relative dosimetry. Statistical differences between groups were analysed using 2 tailed paired t test. There was consistant increase in the doses to the point B, bladder point and rectal point in all the hypothetical plans with statistically significant difference observed for point B (p=0.001), rectal point (p=0.000) and bladder point(p=0.041).The conception of avoiding the replanning if similar interfraction conditions are achieved looks promising but the significant findings observed in this study warrants strongly to continue as per current recommendation of replanning with every fraction of brachytherapy.

OC-0444: Impact of deformable image registration on inter-fractional variations in lung cancer proton therapy

OC-0444: Impact of deformable image registration on inter-fractional variations in lung cancer proton therapy

PO-1653: Dosimetric effects of interfractional variations in prostate cancer radiotherapy

PO-1653: Dosimetric effects of interfractional variations in prostate cancer radiotherapy

PO-1172: dose effects due to interfractional variability for hypofractionated radiotherapy of prostate cancer

PO-1172: dose effects due to interfractional variability for hypofractionated radiotherapy of prostate cancer

Inter- and Intra-Fractional Target Changes During Stereotactic Body Radiotherapy to the Prostate Bed With an Endorectal Balloon

Mature toxicity outcomes from the first phase I trial using stereotactic body radiotherapy (SBRT) to the prostate bed for recurrent prostate cancer have been recently reported. From that study, we herein report inter- and intra-fractional positional changes in the clinical target volume (CTV) and rectum with the use of an endorectal balloon. Prostate bed SBRT was delivered over 5 fractions in escalating doses of 35, 40, and 45 Gy. Three fiducial markers were planted in the CTV volume. A subset of 10 patients on the trial received both pre- and post-treatment CBCT for each fraction and were the subject of this analysis. An endorectal balloon was placed during both simulation and daily treatments. Patients were instructed to maintain a full bladder. On pre-treatment CBCT we localized to the anterior surface of the balloon to align the anterior rectal wall and CTV. Inter-fractional variation was measured using registrations based on pelvic anatomy. Intra-fractional changes in balloon position were assessed with soft-tissue registration. The t-test was used to evaluate the significance of correlation coefficient with significance level set at 0.05. The average daily air-filled balloon volume (±standard deviation) was 77.0±5.0 cc (range: 60.6 cc – 88.5 cc), and the average pre-treatment bladder volume was 235.0±109.7 cc. Compared to the planning CT, the pre-treatment bladder volume changed by -35.0±87.9 cc (range: -227.1 – 123.1 cc). Relative to the planning positions, the anterior rectal wall had an average inter-fractional shift of -1.6±5.6 mm (range: -15.2 – 7.5 mm) in the antero-posterior (AP) direction at the mid-section of the balloon (negative value: shifts posteriorly), with ≥ 3mm shift in 66% of total fractions. Correlation between daily bladder volume variations and anterior rectum position variations in the AP direction was statistically significant (Pearson’s correlation coefficient = 0.76, p = 0.02). The average time between the pre- and post-treatment CBCT scans was 9.8±4.0 minutes (range: 6 – 29 minutes). The post-treatment bladder volume increased significantly (p < 0.01) by an average of 58.9±51.6 cc. The balloon volume changed by <5% in each treatment. The center-of-mass of the balloon had average intra-fractional variation of 0.3±0.8 mm in the left-right (LR) direction (range: -1.3 – 2.5 mm), -0.9±2.4 mm in the AP direction (range: -11.0 – 3.0 mm), and -1.7±2.2 mm in the superoinferior (SI) direction (range: -10.9 – 0.7 mm) (negative value: shift anteriorly). No correlation was seen between increased bladder volume and intrafractional AP shifts, since the majority of greater than 3 mm shifts were attributed to one patient. Although not statistically significant, we observed higher than expected intrafractional AP and SI shifts. Our future phase II trial will incorporate larger margins to account for this finding.

Prediction of Need for Proton Adaptive Plan in Head and Neck Cancers Using Clinical and Radiographic Features

Head and neck cancers can be very aggressive and life-threatening with an estimated annual incidence of 65,000 and annual death toll of 14,000 in the USA. The mainstays of treatment are surgical resection and radiotherapy. One of the challenges of delivering radiation is inter-fraction variability in targets due to growth or shrink of tumor, decreased size of grafted tissue, and weight loss. The typical solution is to create new plans based on interval CT simulation scans during treatment. However, with proton radiotherapy this problem becomes magnified due to greater changes in plan uncertainties with varying volume and density of tissue and longer time required for re-planning as compared to photon-based radiotherapy. We aim to see if there are factors that can help predict which patients are at higher risk for requiring re-plan with proton radiotherapy. Patients with primary cancer of the head and neck treated at our institution’s proton therapy center who had a Quality Assurance CT Scan (QACT) done during the course of their treatment were reviewed. Clinical factors from consultation prior to radiation start and radiographic factors from planning CT scan and QACT were collected for comparison using Pearson Correlation Coefficient to assess for potential significance. Sixty-three patients were treated in 2019 at our institution’s proton therapy center. Of the 35 patients who had radiation in the post-operative setting, 9 required re-planning. Factors significantly associated with re-plan included time from surgery to radiation start (median 34 vs 50 days, p = 0.017) and size of maximal warp vector of the body contour from sim to QACT (median 1.48 vs 2.5, p = 0.031). Additionally, the radial length of graft trended towards association (median 43 vs 58 mm, p = 0.061). Replanning was not associated with weight at consultation (median 79.3 kg vs 75.2 kg), concurrent chemotherapy, or maximal warp vector of the air cavity from sim to QACT (median 4.61 vs 4.23). Disease had varying proportions of re-plans according to site, with 5 of 13 oral cavity cancers requiring re-plan (38%) while only 1 of 10 salivary cancers required re-plan. The median dose at re-plan was 40 Gy (range of 10-64 Gy). Clinical and radiographic factors can help predict patients at higher risk for requiring proton radiation adaptive plans. This work could potentially offer insight into using clinical and radiomic features from cone beam CT scans to help predict when patients may need adaptive plans and where initial plans are likely to fail, which could help reduce time treated with inadequate proton plans, which is compounded by inherent challenges in the proton planning process.

Delivery Uncertainty Estimation Using Daily Breath-Hold Cone-Beam CTs For Liver Proton Stereotactic Body Radiotherapy

To estimate for proton stereotactic body radiotherapy (SBRT) plan delivery uncertainty with daily cone-beam CT (CBCT) imaging for liver patients under breath-hold with active breath coordinator (ABC). Images from 11 liver patients previously treated with photon SBRT with ABC at our institution are used for this study. The prescribed dose for the proton SBRT plan is 75 Gy (RBE) in 5 fractions. There is a total of 11 patients with 113 setup CBCT images collected during the patient’s previous photon SBRT treatment. The planning CT and all CBCT images are imported into Raystation 9A. A CT relative linear stopping power (RLSP) curve is calibrated for CBCT and used for proton dose calculation. A four beams proton SBRT plan is optimized with the single field optimization (SFO) technique. All 113 CBCT images are registered to planning CT with bony anatomy (rib cage and vertebrae) or liver to evaluate the delivery uncertainty. The delivery dose is estimated from the proton dose calculation on each CBCT image with the same proton plan. The dose coverages for targets are evaluated for: gross tumor volume (GTV), gtv_multABC_2mmRad_5mmSI, and ptv_5mmRad_7mmSI. Liver positions under DIBH with ABC are not consistent under bony alignment as measured. The inter-fractional and intra-fractional variation of DIBH for liver are analyzed. Proton plan delivery uncertainty can dramatically reduce under well liver alignment (up to 5% prescription dose). Daily CBCT images could be potentially used to estimate proton plan delivery uncertainty for liver patients.

Image-Guided Radiotherapy for Pelvic Cancers: A Review of Current Evidence and Clinical Utilisation

The meticulous selection and utilisation of image-guided radiotherapy (IGRT) are essential for optimal radiotherapy treatment delivery when using highly conformal treatment techniques in pelvic radiotherapy. Pelvic IGRT has several general IGRT issues to consider (such as choice of match strategy, prioritisation between multiple treatment targets and margin estimates) as well as issues specific to pelvic radiotherapy, in particular large inter-fraction organ variation. A range of interventions, including adaptive treatment strategies, have been developed to address these challenges. This review covers general considerations for the clinical implementation of pelvic IGRT in routine practice and provides an overview of current knowledge regarding pelvic inter-fraction organ motion. Published IGRT evidence for each of the major tumour sites (gynaecological, prostate, bladder, rectal and anal cancer) is summarised, as are state-of-the-art adaptive approaches. General recommendations for the implementation of an institutional pelvic IGRT strategy include.•Ensuring consistency between treatment intent and the IGRT approach utilised.•Ensuring minimum national and international IGRT guidance is followed while considering the benefit of daily volumetric IGRT.•Ensuring the appropriate allied health professionals (namely therapy radiographers/radiation therapists) lead on undertaking IGRT.•Ensuring the IGRT workflow procedure is clear and includes an escalation process for difficult set-ups.•Ensuring a robust IGRT service is in place before implementing advanced adaptive approaches.

Strict bladder filling and rectal emptying during prostate SBRT: Does it make a dosimetric or clinical difference?

BackgroundTo evaluate inter-fractional variations in bladder and rectum during prostate stereotactic body radiation therapy (SBRT) and determine dosimetric and clinical consequences.MethodsEighty-five patients with 510 computed tomography (CT) images were analyzed. Median prescription dose was 40 Gy in 5 fractions. Patients were instructed to maintain a full bladder and empty rectum prior to simulation and each treatment. A single reviewer delineated organs at risk (OARs) on the simulation (Sim-CT) and Cone Beam CTs (CBCT) for analyses.ResultsBladder and rectum volume reductions were observed throughout the course of SBRT, with largest mean reductions of 86.9 mL (19.0%) for bladder and 6.4 mL (8.7%) for rectum noted at fraction #5 compared to Sim-CT (P < 0.01). Higher initial Sim-CT bladder volumes were predictive for greater reduction in absolute bladder volume during treatment (ρ = − 0.69; P < 0.01). Over the course of SBRT, there was a small but significant increase in bladder mean dose (+ 4.5 ± 12.8%; P < 0.01) but no significant change in the D2cc (+ 0.8 ± 4.0%; P = 0.28). The mean bladder trigone displacement was in the anterior direction (+ 4.02 ± 6.59 mm) with a corresponding decrease in mean trigone dose (− 3.6 ± 9.6%; P < 0.01) and D2cc (− 6.2 ± 15.6%; P < 0.01). There was a small but significant increase in mean rectal dose (+ 7.0 ± 12.9%, P < 0.01) but a decrease in rectal D2cc (− 2.2 ± 10.1%; P = 0.04). No significant correlations were found between relative bladder volume changes, bladder trigone displacements, or rectum volume changes with rates of genitourinary or rectal toxicities.ConclusionsDespite smaller than expected bladder and rectal volumes at the time of treatment compared to the planning scans, dosimetric impact was minimal and not predictive of detrimental clinical outcomes. These results cast doubt on the need for excessively strict bladder filling and rectal emptying protocols in the context of image guided prostate SBRT and prospective studies are needed to determine its necessity.

Investigating the use of virtual 4DCT from 4DMRI in gated carbon ion radiation therapy of abdominal tumors

PurposeTo generate virtual 4DCT from 4DMRI with field of view (FOV) extended to the entire involved patient anatomy, in order to evaluate its use in carbon ion radiation therapy (CIRT) of the abdominal site in a clinical scenario. Materials and methodsThe virtual 4DCT was generated by deforming a reference CT in order to (1) match the anatomy depicted in the 4DMRI within its FOV, by calculating deformation fields with deformable image registration to describe inter-fractional and breathing motion, and (2) obtain physically plausible deformation outside of the 4DMRI FOV, by propagating and modulating the previously obtained deformation fields. The implemented method was validated on a digital anthropomorphic phantom, for which a ground truth (GT) 4DCT was available. A CIRT treatment plan was optimized at the end-exhale reference CT and the RBE-weighted dose distribution was recalculated on both the virtual and GT 4DCTs. The method estimation error was quantified by comparing the virtual and GT 4DCTs and the corresponding recomputed doses. The method was then evaluated on 8 patients with pancreas or liver tumors treated with CIRT using respiratory gating at end-exhale. The clinical treatment plans adopted at the National Center for Oncological Hadrontherapy (CNAO, Pavia, Italy) were considered and the dose distribution was recomputed on all respiratory phases of the planning and virtual 4DCTs. By comparing the two datasets and the corresponding dose distributions, the geometrical and dosimetric impact of organ motion was assessed. ResultsFor the phantom, the error outside of the 4DMRI FOV was up to 4.5mm, but it remained sub-millimetric in correspondence to the target within the 4DMRI FOV. Although the impact of motion on the target D95% resulted in variations ranging from 22% to 90% between the planned dose and the doses recomputed on the GT 4DCT phases, the corresponding estimation error was ≤2.2%. In the patient cases, the variation of the baseline tumor position between the planning and the virtual end-exhale CTs presented a median (interquartile range) value of 6.0 (4.9) mm. For baseline variations larger than 5mm, the tumor D95% variation between the plan and the dose recomputed on the end-exhale virtual CT resulted larger than 10%. Median variations higher than 10% in the target D95% and gastro-intestinal OARs D2% were quantified at the end-inhale, whereas close to the end-exhale phase, limited variations of relevant dose metrics were found for both tumor and OARs. ConclusionsThe negligible impact of the geometrical inaccuracy in the estimated anatomy outside of the 4DMRI FOV on the overall dosimetric accuracy suggests the feasibility of virtual 4DCT with extended FOV in CIRT of the abdominal site. In the analyzed patient group, inter-fractional variations such as baseline variation and breathing variability were quantified, demonstrating the method capability to support treatment planning in gated CIRT of the abdominal site.

Magnitude and dosimetric impact of inter-fractional positional variations of the metal port of tissue expanders in postmastectomy patients treated with radiation.

Postmastectomy breast reconstruction involves the insertion of a temporary tissue expander, which contains a metal injection port. The purpose of this study was to determine the magnitude and dosimetric impact of the inter-fractional positional variations of the port for patients treated with radiation. For nine breast cases treated on Tomotherapy, the deviation of the port in the daily MVCT from its reference position was measured in the three cardinal directions. The dosimetric effects of the measured errors were evaluated for two classes of error: Internal Port Error (IPE) and Patient Registration Error (PRE). For each class, dose accumulation was done for daily measured errors and a systematic error. Inter-fractional positional errors of the port were small, with 87% of the deviations below 5mm, but errors larger than 1.5cm were observed. The cumulative effect of the daily measured and systematic IPE decreased target coverage by as much as 2.8% and 3.5%, respectively. The cumulative effect of the daily measured PRE decreased target coverage by an average of 3.5%. The cumulative effect of a systematic PRE significantly decreased target coverage by an average of 16%. The presence of IPE over the course of treatment had minimal clinical impact while PRE had a greater impact on clinically-relevant regions. The robustness of treatment delivery can be improved by assigning the port its appropriate density during planning despite contouring uncertainties due to metal artefacts, and by prioritizing anatomical alignment over port alignment during daily registration.

Feasibility of Optical Surface-Guidance for Position Verification and Monitoring of Stereotactic Body Radiotherapy in Deep-Inspiration Breath-Hold

BackgroundReductions in tumor movement allow for more precise and accurate radiotherapy with decreased dose delivery to adjacent normal tissue that is crucial in stereotactic body radiotherapy (SBRT). Deep inspiration breath-hold (DIBH) is an established approach to mitigate respiratory motion during radiotherapy. We assessed the feasibility of combining modern optical surface-guided radiotherapy (SGRT) and image-guided radiotherapy (IGRT) to ensure and monitor reproducibility of DIBH and to ensure accurate tumor localization for SBRT as an imaging-guided precision medicine.MethodsWe defined a new workflow for delivering SBRT in DIBH for lung and liver tumors incorporating SGRT and IGRT with cone beam computed tomography (CBCT) twice per treatment fraction. Daily position corrections were analyzed and for every patient two points retrospectively characterized: an anatomically stable landmark (predominately Schmorl’s nodes or spinal enostosis) and a respiratory-dependent landmark (predominately surgical clips or branching vessel). The spatial distance of these points was compared for each CBCT and used as surrogate for intra- and interfractional variability. Differences between the lung and liver targets were assessed using the Welch t-test. Finally, the planning target volumes were compared to those of free-breathing plans, prepared as a precautionary measure in case of technical or patient-related problems with DIBH.ResultsTen patients were treated with SBRT according this workflow (7 liver, 3 lung). Planning target volumes could be reduced significantly from an average of 148 ml in free breathing to 110 ml utilizing DIBH (p < 0.001, paired t-test). After SGRT-based patient set-up, subsequent IGRT in DIBH yielded significantly higher mean corrections for liver targets compared to lung targets (9 mm vs. 5 mm, p = 0.017). Analysis of spatial distance between the fixed and moveable landmarks confirmed higher interfractional variability (interquartile range (IQR) 6.8 mm) than intrafractional variability (IQR 2.8 mm). In contrast, lung target variability was low, indicating a better correlation of patients’ surface to lung targets (intrafractional IQR 2.5 mm and interfractional IQR 1.7 mm).ConclusionSBRT in DIBH utilizing SGRT and IGRT is feasible and results in significantly lower irradiated volumes. Nevertheless, IGRT is of paramount importance given that interfractional variability was high, particularly for liver tumors.

Intra-fraction and Inter-fraction analysis of a dedicated immobilization device for intracranial radiation treatment

BackgroundImmobilization devices are crucial to minimize patient positioning uncertainties in radiotherapy (RT) treatments. Accurate inter and intra-fraction motions is particularly important for intracranial and stereotactic radiation treatment which require high precision in dose delivery. Recently, a new immobilization device has been developed specifically for the radiation treatment of intracranial malignancies. To date, no data are available on the use of this device in daily clinical practice. The aim of this study is to investigate the intra and inter-fraction variations, patient comfort and radiographer confidence of the immobilization system from two distinct institutions: HagaZiekenhuis, Den Haag, Netherlands and IRCCS Ospedale Sacro Cuore Don Calabria, Negrar, Italy.Material and methodSixteen patients (10 diagnosed with brain metastases and 6 with primary central nervous systemic tumor) from IRCCS Ospedale Sacro Cuore Don Calabria and 17 patients (all diagnosed with brain metastases tumor) from HagaZiekenhuis were included in this study. The median target volume was 436 cc (range 3.2–1628 cc) and 4.58 cc (range 0.4–27.19 cc) for IRCCS and Haga, respectively. For patients treated in IRCCS Sacro Cuore Don Calabria, the median dose prescription was 30 Gy (range 27–60 Gy) and median number of fractions 10 (range 3–30). In Haga the median dose prescription was 21 Gy (range 8–21 Gy) and the median number of fraction was 1 (range 1–3). The immobilization device was assembled during CT simulation. A short interview to the patient regarding the device’s comfort level was conducted at the end of the simulation procedure. Additionally, simulation setup time and radiographer (RTT) procedures (i.e. mask preparation) were evaluated. Prior to radiation treatment delivery, an automatic rigid match on the cranial bones between cone beam computed tomography (CBCT) and planning-CT was performed. A couch shift was performed subsequently. An extra post-treatment CBCT was acquire after the treatment delivery. This post-treatment CBCT was matched with pre-treatment CBCT to identify any possible intra-fraction motion. All online matches were validated by experienced radiation oncologist or RTT. A total of 126 CBCT’s were analyzed offline by radiation oncologist/medical physicist. The data of the pre-treatment CBCT match was used to quantify inter-fraction motion. The post-treatment CBCT was matched with pre-treatment CBCT to identify any possible intra-fraction motion.ResultsDuring the molding of the mask, all patients responded positive to the comfort. Median time required by the RTTs to assemble the immobilization system was 9 min (range 6–12 min). In terms of comfort, all patients reported a good-to high level of satisfaction. The RTTs also respond positively towards the use of the locking mechanism and clips. Results of positioning uncertainties were comparable between the two institutes. The mean inter-fraction motion for all translational and rotational directions were < 2 mm (SD < 4 mm) and < 0.5°(SD < 1.5°), respectively, while the mean intra-fraction motions were < 0.4 mm (SD < 0.6 mm) and 0.3° (SD < 0.5°).ConclusionsThis study demonstrates the efficacy and feasibility of the immobilization device in the intracranial radiation treatment. Both patient comfort and preparation time by RTTs are considered adequate. In combination with online daily imaging procedure, this device can achieve submillimeter accuracy required for intracranial and stereotactic treatments.

Adaptive radiotherapy based on statistical process control for oropharyngeal cancer.

PurposeThe purpose of this study is to quantify dosimetric changes throughout the delivery of oropharyngeal cancer treatment and to investigate the application of statistical process control (SPC) for the management of significant deviations during the course of radiotherapy.MethodsThirteen oropharyngeal cancer patients with daily cone beam computed tomography (CBCT) were retrospectively reviewed. Cone beam computed tomography images of every other fraction were imported to the Velocity software and registered to planning CT using the 6 DOF (degrees of freedom) couch shifts generated during patient setup. Using Velocity “Adaptive Monitoring” module, the setup‐corrected CBCT was matched to planning CT using a deformable registration. Volumes and dose metrics at each fraction were calculated and rated with plan values to evaluate interfractional dosimetric variations using a SPC framework. T‐tests between plan and fraction volumes were performed to find statistically insignificant fractions. Average upper and lower process capacity limits (UCL, LCL) of each dose metric were derived from these fractions using conventional SPC guidelines.ResultsGross tumor volume (GTV) and organ at risk (OAR) volumes in the first 13 fractions had no significant changes from the pretreatment planning CT. The GTV and the parotid glands subsequently decreased by 10% at the completion of treatment. There were 3–4% increases in parotid mean doses, but no significant differences in dose metrics of GTV and other OARs. The changes were organ and patient dependent. Control charts for various dose metrics were generated to assess the metrics at each fraction for individual patient.ConclusionsDaily CBCT could be used to monitor dosimetric variations of targets and OARs resulting from volume changes and tissue deformation in oropharyngeal cancer radiotherapy. Treatment review with the guidance of a SPC tool allows for an objective and consistent clinical decision to apply adaptive radiotherapy.

TO ASSESS THE DOSIMETRIC IMPACT ON PROSTATE, BLADDER AND RECTUM IN CARCINOMA PROSTATE DUE TO INTERFRACTIONALANATOMICAL VARIATION IN IMAGE GUIDED RADIATION THERAPY.

ABTRACT- Prostate cancer is second most frequent cancer in men and fifth leading cause of death worldwide. Most radiotherapy patients with prostate cancer are treated with intensity modulated radiotherapy. IMRT requires high spatial accuracy in localizing the target volume and high precision in treatment delivery, because even a small geographic miss can have a large dosimetric effect. However, the patient setup position and anatomy changes daily, particularly in the prostate region, due to rectum and bladder filling. Materials Methods: We analyzed 30 patients with prostate cancer patient treated with IMRT kV CBCT was performed for each patient twice per week during the entire course of treatment. The bladder, rectum, prostate and PTV were contoured on each CBCT scan. Results: Delivered dose of prostate D95% and prostate D100% was 99.15% and 93.66% respectively and mean value of V70 for bladder was 16% and delivered value was 16.45%. Whereas the mean value of V70 of plan for rectum was 18.12% and delivered value was 27.58%. Conclusion: Interfractional variation during prostate radiotherapy can results in substantial difference between planned and delivered doses, particularly in critical structure. IG-IMRT is useful tool in assessing dosimetric changes in critical structure due to interfractional anatomical interfractional anatomical variation in prostate radiotherapy.

Investigation of inter-fraction target motion variations in the context of pencil beam scanned proton therapy in non-small cell lung cancer patients.

PurposeFor locally advanced‐stage non‐small cell lung cancer (NSCLC), inter‐fraction target motion variations during the whole time span of a fractionated treatment course are assessed in a large and representative patient cohort. The primary objective is to develop a suitable motion monitoring strategy for pencil beam scanning proton therapy (PBS‐PT) treatments of NSCLC patients during free breathing.MethodsWeekly 4D computed tomography (4DCT; 41 patients) and daily 4D cone beam computed tomography (4DCBCT; 10 of 41 patients) scans were analyzed for a fully fractionated treatment course. Gross tumor volumes (GTVs) were contoured and the 3D displacement vectors of the centroid positions were compared for all scans. Furthermore, motion amplitude variations in different lung segments were statistically analyzed. The dosimetric impact of target motion variations and target motion assessment was investigated in exemplary patient cases.ResultsThe median observed centroid motion was 3.4 mm (range: 0.2–12.4 mm) with an average variation of 2.2 mm (range: 0.1–8.8 mm). Ten of 32 patients (31.3%) with an initial motion <5 mm increased beyond a 5‐mm motion amplitude during the treatment course. Motion observed in the 4DCBCT scans deviated on average 1.5 mm (range: 0.0–6.0 mm) from the motion observed in the 4DCTs. Larger motion variations for one example patient compromised treatment plan robustness while no dosimetric influence was seen due to motion assessment biases in another example case.ConclusionsTarget motion variations were investigated during the course of radiotherapy for NSCLC patients. Patients with initial GTV motion amplitudes of < 2 mm can be assumed to be stable in motion during the treatment course. For treatments of NSCLC patients who exhibit motion amplitudes of > 2 mm, 4DCBCT should be considered for motion monitoring due to substantial motion variations observed.

Dosimetric effect of MLC speed obtained with machine log files of VMAT delivery

This paper aims to investigate the stability of volumetric modulated arc therapy (VMAT) delivery during treatment using the multi-leaf collimator (MLC) speed recorded in the log files. The mechanical error, such as the position error and speed of the MLC recorded in the log file, affects the dose accuracy of the VMAT delivery. We evaluated the stability of VAMT delivery by determining the relationship between the gamma pass rate (GPR) and MLC speed. The 15 VMAT plans were used to investigate the stability of the beam delivery. The Mobius software calculated the dose distribution by the log files from the planning CT image set. The GPR was evaluated by comparing the reconstructed dose with the planned dose. To validate the gamma analysis calculated from the log file, we compared the results with the GPR measured by the ArcCHECK phantom. A comparison of the GPRs between ArcCHECK and Mobius was performed based on different criteria: 3%/3 mm and 2%/2 mm. Based on the results, we represent that (1) the 2%/2 mm criterion of the Mobius system is comparable to perform the quality assurance (QA) with ArcCHECK using the criterion of 3%/3 mm, and (2) the interfractional variation of the VMAT delivery depends on the distribution of the MLC speed. In this study, we determined that the variation of the GPRs during the whole treatment is less than 2% and the average leaves speed is less than 10 mm/s. These results reveal that the log files are one of the indicators to monitor the variation of VMAT delivery.

Variations in hot spots during intracavitary brachytherapy reduces long-term toxicities associated with image-guided brachytherapy

Rectal associated complications secondary to intracavitary brachytherapy of cervical cancers has long been among the main concerns of practitioners. The heterogeneous doses produced by brachytherapy enhances the probability of hotspots formation and morbidities in organs at the risk (OARs). Although different studies have focused on correlating inter-fraction/application D2cc dose variations with OARs morbidities, few have focused on their spatial location variation. This retrospective study evaluates the relation between inter-fractional variations in localization of rectum hot spots and observed morbidities.