Intrafractional Uncertainties Research Articles

Intrafractional stability of MR-guided online adaptive SBRT for prostate cancer

BackgroundMR-guided online adaptive stereotactic body radiation therapy (SBRT) for prostate cancer aims to reduce toxicity by full compensation of interfractional uncertainties. However, the process of online adaptation currently takes approximately 45 min during which intrafractional movements remain unaccounted for. This study aims to analyze the dosimetric benefit of online adaptation and to evaluate its robustness over the duration of one treatment fraction.MethodsBaseline MR-scans at a MR-linear accelerator were acquired for ten healthy male volunteers for generation of mock-prostate SBRT plans with a dose prescription of 5 × 7.25 Gy. On a separate day, online MR-guided adaptation (ViewRay® MRIdian) was performed, and thereafter MR images were acquired every 15 min for 1 h to assess the stability of the adapted plan.ResultsA dosimetric benefit of online MR-guided adaptive re-planning was observed in 90% of volunteers. The median D95CTV- and D95PTV-coverage was improved from 34.8 to 35.5 Gy and from 30.7 to 34.6 Gy, respectively. Improved target coverage was not associated with higher dose to the organs at risk, most importantly the rectum (median D1ccrectum baseline plan vs. adapted plan 33.3 Gy vs. 32.3 Gy). The benefit of online adaptation remained stable over 45 min for all volunteers. However, at 60 min, CTV-coverage was below a threshold of 32.5 Gy in 30% of volunteers (30.6 Gy, 32.0 Gy, 32.3 Gy).ConclusionThe dosimetric benefit of MR-guided online adaptation for prostate SBRT was robust over 45 min in all volunteers. However, intrafractional uncertainties became dosimetrically relevant at 60 min and we therefore recommend verification imaging before delivery of MR-guided online adapted SBRT.

PO-1751 Inter- and intra-fractional uncertainties in deep inspiration breath-hold (DIBH) breast treatment

PO-1751 Inter- and intra-fractional uncertainties in deep inspiration breath-hold (DIBH) breast treatment

PD-0942 Intrafraction error analysis of homemade mouth-bite masks in linac-based SRS for brain metastasis

Purpose or Objective To assess the intrafraction accuracy of a frameless stereotactic linac-based radiosurgery (SRS) for brain metastases (BM) using a homemade mouth-bite thermoplastic mask in combination with cone-beam computed tomography (CBCT) and six-degrees of freedom (6-DOF) couchtop. Materials and Methods Covid-19 restrictions resulted in significant delays in the supply of the dedicated devices for BM SRS. A frameless approach using a homemade mouth-bite thermoplastic mask was implemented to offer BM SRS in a period of limited mobility (figure 1). All patients were immobilized and treated at the same institution with 6 MV FFF VMAT radiosurgery, with a 2 mm isotropic expansion from the GTV to the PTV. Before treatment delivery, patients underwent a low-dose CBCT to check position accuracy. Through image co-registration, translational (x, y, z) and rotational errors (roll, pitch, and yaw) were determined. The 6-DOF couchtop was used to automatically relocate the patient with sub-millimetric precision. Immediately after irradiation, patients underwent a second CBCT to verify any translational or rotational errors occurring during therapy (intrafraction motion). $Φg Results From February 2020 to January 2021 sixteen patients (29 lesions) received BM SRS (14-21 Gy). The whole procedure, from the first CBCT scan to the end of irradiation and subsequent CBCT, required a median time of 9 minutes [6-13]. Mean translational error was 0.0 ± 0.3 mm [-0.7;0.9] in lateral direction, and -0.1 ± 0.5 mm [-1.4;1.0] in longitudinal direction. A 2.2 mm maximum shift was recorded on the vertical axis, nevertheless the mean translation error was 0.1 ± 0.6 mm. Roll, pitch and yaw registered a mean value of -0.1 ± 0.3° [-0.8;0.2], 0.0 ± 0.2° [-0.8;0.2], and -0.1 ± 0.3° [-0.8;0.4], respectively. The results are summarized in figure 2. Conclusion This study demonstrated that homemade mouth-bite thermoplastic masks combined with CBCT, 6-DOF couchtop, and fast FFF treatment delivery allow minimal intra-fraction uncertainties in BM SRS, confirming the accuracy of PTV margin definition. The results are comparable to the declared specifics of SRS dedicated commercial frameless mask, while improving patient's comfort compared to the invasive fixed frame. However, the moulding procedure requires improved RTT skills and expertise. Our findings show that SRS can be safely delivered in radiotherapy units without dedicated expensive installations, such as CyberKnife and Gamma Knife, thus reducing patient mobility according to Covid-19 restrictions.

PO-0203 Range of intra-fraction dose uncertainty in cervical cancer HDR brachytherapy.

PO-0203 Range of intra-fraction dose uncertainty in cervical cancer HDR brachytherapy.

Kv Intrafraction Verification for SBRT Amplitude-Gated Rapidarc Treatments: An Initial Experience

To report the workflow and the quality assurance program established for the first kV-IGRT intrafraction verification gated RapidArc (RA) treatment successfully delivered in our institution. One patient with stage I NSCLC was treated using the gating technique on a medical linear accelerator. Prescription dose was 48 Gy in four fractions and one fiducial marker was implanted near the tumor. During simulation a gated CT in a Stable Exhale (SE) phase (breath hold pause = 20 sec) was acquired. A 6 MV FFF plan was generated on the gated CT using four partial arcs; the fiducial and its 3 mm PRV were delineated in the planning CT. During each fraction a gating amplitude window of 2.5 mm was set, a gated CBCT on the SE was acquired and positioning was performed according to a tumor fusion. Two orthogonal kV images were then collected to verify the position of the marker. Triggered kV images were acquired immediately before beam-on to verify the intrafraction fiducial position during treatment and to perform an a posteriori evaluation of the marker displacements in the superior-inferior (SI) direction with respect to DRR images. Pretreatment quality assurance was performed using a Delta4 phantom (static delivery) and with EBT3 Gafchromic films coupled with a Quasar moving phantom (gated delivery) using three breathing pattern (BP): 1) FB: free BP of the patient, 2) EX1: SE obtained by modifying the FB pattern and 3) EX2: a random SE. A film irradiated in static delivery was taken as a reference to evaluate the gating deliveries and compared with TPS dose as additional static QA. Beam-on time was 1.8 ± 0.5 min per arc and 7.2 ± 1.4 min per fraction. The duty cycle averaged over all arcs was 42 ± 3.9 % and 56 ± 17 kV triggered images per fraction were acquired. The SI distance between the marker and its expected position was 1.7 ± 0.4 mm on average and 2.9 ± 0.7 mm at 95th percentile (range: 0 - 4.2 mm). During treatment the fiducial was always displayed inside its PRV, except during the delivery of arc 3 in fraction 3. This is in agreement with shifts evaluation, where we found an average displacement of 3.6 ± 0.3 mm. 2%/2mm gamma agreement for the static delivery was 97.8% and 100% for the Delta4 phantom and films, respectively. The gated deliveries were dosimetrically equivalent to the static procedure, with dose differences at isocenter below 1% and gamma pass-rates above 98.2%. Good agreement was also found with TPS (gamma scores > 98.9%). The system capability to superimpose marker PRV on real-time kV images made the clinicians confident during the delivery and the evaluation of the fiducial shifts has proven to be consistent to what was observed during the treatment. A 3 mm margin was required to account for 95% of the gating intrafraction uncertainties. Gated treatments were delivered to a moving phantom and no significant differences were found with respect to a static delivery. The use of a small gate coupled with a SE BP allows delivering acceptable dose distributions with reasonable duty cycles.

Evaluation of offline adaptive planning techniques in image-guided brachytherapy of cervical cancer.

Modern three‐dimensional image‐guided intracavitary high dose rate (HDR) brachytherapy is often used in combination with external beam radiotherapy (EBRT) to manage cervical cancer. Intrafraction motion of critical organs relative to the HDR applicator in the time between the planning CT and treatment delivery can cause marked deviations between the planned and delivered doses. This study examines offline adaptive planning techniques that may reduce intrafraction uncertainties by shortening the time between the planning CT and treatment delivery. Eight patients who received EBRT followed by HDR boosts were retrospectively reviewed. A CT scan was obtained for each insertion. Four strategies were simulated: (A) plans based on the current treatment day CT; (B) plans based on the first fraction CT; (C) plans based on the CT from the immediately preceding fraction; (D) plans based on the closest anatomically matched previous CT, using all prior plans as a library. Strategies B, C, and D allow plans to be created prior to the treatment day insertion, and then rapidly compared with the new CT. Equivalent doses in 2 Gy for combined EBRT and HDR were compared with online adaptive plans (strategy A) at D 90 and D 98 for the high‐risk CTV (HR‐CTV), and D 2 cc for the bladder, rectum, sigmoid, and bowel. Compared to strategy A, D 90 deviations for the HR‐CTV were −0.5 ± 2.8 Gy, −0.9 ± 1.0 Gy, and −0.7 ± 1.0 Gy for Strategies B, C, and D, respectively. D 2 cc changes for rectum were 2.7 ± 5.6 Gy, 0.6 ± 1.7 Gy, and 1.1 ± 2.4 Gy for Strategies B, C, and D. With the exception of one patient using strategy B, no notable variations for bladder, sigmoid, and bowel were found. Offline adaptive planning techniques can shorten time between CT and treatment delivery from hours to minutes, with minimal loss of dosimetric accuracy, greatly reducing the chance of intrafraction motion.

(OA22) Inter-Fractional Motion in Patients With Intact Cervical Cancer Treated on a MRI Guided External Beam Radiation Therapy System

Treatment verification has improved significantly over the past decades, now with the ability to have on-board MRI guidance. MRI provides superior soft tissue contrast compared to CT-based imaging modalities and provides better delineation of pelvic organs and gross tumor volumes (GTV) for those with cervical cancer. Consensus contouring guidelines for intensity modulated radiation therapy (IMRT) for cervical cancer advise including the whole uterus in the target volume and adding large planning target volume (PTV) margins to account for inter- and intra-fractional uncertainties. The primary objectives of our analysis was to assess the inter-fractional GTV motion for those with an intact cervix using on-board, daily MRI and to assess the relationship between bladder and rectal volumes and GTV motion. We analyzed 125 daily set-up MRI’s from five patients with intact cervical cancer who received MRI-guided pelvic IMRT at our institution between September 2016 and October 2017. The GTV, bladder, uterus, and rectum were contoured on all 125 MRI’s, which included 25 daily set-up MRI’s for each patient. The daily set-up MRI was registered with the planning MRI to assess organ motion. Tumor motion was assessed using GTV centroid displacements baselined to the planning MRI. Tumor volume and organ volumes were determined for each fraction. Then, a linear regression analysis was utilized to evaluate the relationship between rectal and bladder volumes and GTV motion. Patients were advised to have comfortable bladder filling for treatment. The initial GTV’s ranged in size from 31.9 cc to 119.5 cc. All GTV’s decreased in size during the course of external beam radiation, ranging from 34.0% to 85.2% of the initial tumor volumes on the final day of treatment. The ranges of daily centroid GTV displacement for all patients were from 0.08 cm to 2.23 cm, and the median GTV displacement for each patient was 0.69 cm to 1.04 cm. On linear regression analysis, 4 out of 5 patients demonstrated a strong relationship between rectal volume and GTV displacement with 26.4% of these displacements larger than 1 cm. Rectal volumes had the strongest impact on anterior-posterior GTV motion. Rectal volume appeared to better correlate with GTV displacement than bladder volume. However, for large tumors (>75 cc), tumor shrinkage during the course of radiation therapy seemed to have the most significant impact on GTV displacement. GTV shrinkage for large tumors strongly correlated with inferior GTV shifts, which were up to 1.4 cm in magnitude. Inter-fractional target motion is considerable and is affected by a variety of factors, including target volume changes, rectal volume, and, to a lesser degree, bladder volume. Our data suggests GTV shrinkage for large tumors has the greatest impact on GTV displacement. Inclusion of the whole uterus within the target volume and large CTV to PTV expansions remain standard practice. However, since the GTV is easily visible on daily MR imaging, one could think that eventually the target volume may exclude the uterine fundus and may also allow for decreased PTV margins with the ultimate goal of decreasing treatment toxicity.

Three-dimensional intrafractional internal target motions in accelerated partial breast irradiation using three-dimensional conformal external beam radiotherapy

PurposeWe evaluated three-dimensional intrafractional target motion, divided into respiratory-induced motion and baseline drift, in accelerated partial breast irradiation (APBI). MethodsPaired fluoroscopic images were acquired simultaneously using orthogonal kV X-ray imaging systems at pre- and post-treatment for 23 patients who underwent APBI with external beam radiotherapy. The internal target motion was calculated from the surgical clips placed around the tumour cavity. ResultsThe peak-to-peak respiratory-induced motions ranged from 0.6 to 1.5mm in all directions. A systematic baseline drift of 1.5mm towards the posterior direction and a random baseline drift of 0.3mm in the lateral-medial and cranial-caudal directions were observed. The baseline for an outer tumour cavity drifted towards the lateral and posterior directions, and that for an upper tumour cavity drifted towards the cranial direction. Moderate correlations were observed between the posterior baseline drift and the patients’ physical characteristics. The posterior margin for intrafractional uncertainties was larger than 5mm in patients with greater fat thickness due to the baseline drift. ConclusionsThe magnitude of the intrafractional motion was not uniform according to the direction, patients’ physical characteristics, or tumour cavity location due to the baseline drift. Therefore, the intrafractional systematic movement should be properly managed.

Dose escalation study with respiratory-gated carbon-ion scanning radiotherapy using a simultaneous integrated boost for pancreatic cancer: simulation with four-dimensional computed tomography.

Pancreatic cancer is a difficult to treat disease with a persistently high mortality rate. We evaluated dose distribution simulation with respiratory-gated carbon-ion pencil beam scanning (C-PBS) with a simultaneous integrated boost (SIB) to increase tumour dose, sparing organs at risk (OARs). Using four-dimensional CT data of 12 patients, we delineated gross tumour volume and two clinical target volumes (CTVs). To consider beam range intrafractional uncertainty, we calculated field-specific target volumes, from which two planning target volumes (PTVs) were generated. PTV1 would receive a planned dose of 55.2 Gy [relative biological effectiveness (RBE)-weighted absorbed dose] in 12 fractions, and PTV2 would receive an SIB dose up to 67.2 Gy (RBE). Dose assessments were conducted with regard to the targets and OARs. CTV2 dose covering 95% of the volume (D95%) increased from 50.3 ± 5.1 Gy (RBE) to 62.5 ± 3.5 Gy (RBE) for a planned dose from 55.2 Gy (RBE) to 67.2 Gy (RBE). For 4 of 12 patients with a distance of ≥5 mm between the tumour and the gastrointestinal tract, CTV2 D95% was ≥95% of planned dose at all dose levels. We quantified dose escalation with respiratory-gated C-PBS using SIB for pancreatic cancer and revealed that OAR dose was not affected to the same degree as the tumour dose. Advances in knowledge: A simulation study on respiratory-gated C-PBS with SIB for pancreatic cancer was performed. The results indicated the feasibility of dose escalation for pancreatic cancer, which should be confirmed in clinical trials.

An <i>In-Vivo</i> Study during Combined Intracavitary and Interstitial Brachytherapy of Gynaecological Malignancies Using microMOSFET

Aim: To analyze the inter-fraction, intra-fraction uncertainties and to verify the delivered total dose with planned dose in the combined intracavitary-interstitial brachytherapy of gynaecological cancer patients using microMOSFET in-vivo dosimeter. Materials and Methods: Between May 2014 and March 2016, 22 patients who underwent brachytherapy treatments with an applicator combination of CT/MR compatible tandem, ring and Syed-Neblett template-guided rigid needles were included in this study. Specially designed microMOSFET, after calibration, was used to analyze the variations in dosimetry of combined intracavitary-interstitial application. Results: The standard deviation for Inter-fraction variation among 22 combined intracavitary interstitial applications ranged between 0.86% and 10.92%. When compared with the first fraction dose, the minimum and maximum dose variations were −9.5% and 26.36%, respectively. However, the mean doses varied between −5.95% and 14.49%. Intra-fraction variation, which is the difference of TPS calculated dose with first fraction microMOSFET-measured dose ranges from −6.77% to 8.68%. The variations in the delivered total mean dose in 66 sessions with planned doses were −3.09% to 10.83%. Conclusions: It is found that there was a gradual increase in microMOSFET measured doses as compared to the first fraction with that of subsequent fractions in 19 out of 22 applications. Tumor deformation and edema may be the influencing factors, but the applicator movements played a major role for the variations. We find that the microMOSFET is an easy and reliable system for independent verification of uncertainties during ICBT-ISBT treatments.

Frameless image-guided radiosurgery for trigeminal neuralgia.

Background:Frameless image-guided radiosurgery (IGRS) is a safe and effective noninvasive treatment for trigeminal neuralgia (TN). This study evaluates the use of frameless IGRS to treat patients with refractory TN.Methods:We reviewed the records of 20 patients diagnosed with TN who underwent frameless IGRS treatments between March 2012 and December 2013. Facial pain was graded using the Barrow Neurological Institute (BNI) scoring system. The initial setup uncertainty from simulation to treatment and the patient intrafraction uncertainty were measured. The median follow-up was 32 months.Results:All patients’ pain was BNI Grade IV or V before the frameless IGRS treatment. The mean intrafraction shift was 0.43 mm (0.28–0.76 mm), and the maximum intrafraction shift was 0.95 mm (0.53–1.99 mm). At last follow-up, 8 (40%) patients no longer required medications (BNI 1 or 2), 11 (55%) patients were pain free but required medication (BNI 3), and 1 (5%) patient had no pain relief (BNI 5). Patients who did not have prior surgery had a higher odds ratio for pain relief compared to patients who had prior surgery (14.9, P = 0.0408).Conclusions:Frameless IGRS provides comparable dosimetric and clinical outcomes to frame-based SRS in a noninvasive fashion for patients with medically refractory TN.

Proton pencil beam scanning for mediastinal lymphoma: treatment planning and robustness assessment

ABSTRACTBackground: Modern radiotherapy (RT) for lymphoma is highly personalized. While advanced imaging is largely employed to define limited treatment volumes, the use of proton pencil beam scanning (PBS) for highly conformal lymphoma RT is still in its infancy. Here, we assess the dosimetric benefits and feasibility of PBS for mediastinal lymphoma (ML).Materials and methods: Ten patients were planned using PBS for involved-site RT. The initial plans were calculated on the average four-dimensional computed tomography (4D-CT). PBS plans were compared with 3D conformal radiotherapy (3D-CRT), intensity-modulated radiotherapy (IMRT), and proton double scattering (DS). In order to evaluate the feasibility of PBS and the plan robustness against inter- and intra-fractional uncertainties, the 4D dose was calculated on initial and verification CTs. The deviation of planned dose from delivered dose was measured. The same proton beamline was used for all patients, while another beamline with larger spots was employed for patients with large motion perpendicular to the beam.Results: PBS provided the lowest mean lung dose (MLD) and mean heart dose (MHD) for all patients in comparison with 3D-CRT, IMRT, and DS. For eight patients, internal target volume (ITV) D98% was degraded by <3%; and the MLD and MHD deviated by <10% of prescription over the course of treatment when the PBS field was painted twice in each session. For one patient with target motion perpendicular to the beam (>5 mm), the degradation of ITV D98% was 9%, which was effectively mitigated by employing large spots. One patient exhibited large dose degradation due to pericardial effusion, which required replanning across all modalities.Conclusions: This study demonstrates that PBS plans significantly reduce MLD and MHD relative to 3D-CRT, IMRT, and DS and identifies requirements for robust free-breathing ML PBS treatments, showing that PBS plan robustness can be maintained with repainting and/or large spots.

Improved setup and positioning accuracy using a three-point customized cushion/mask/bite-block immobilization system for stereotactic reirradiation of head and neck cancer.

The purpose of this study was to investigate the setup and positioning uncertainty of a custom cushion/mask/bite‐block (CMB) immobilization system and determine PTV margin for image‐guided head and neck stereotactic ablative radiotherapy (HN‐SABR). We analyzed 105 treatment sessions among 21 patients treated with HN‐SABR for recurrent head and neck cancers using a custom CMB immobilization system. Initial patient setup was performed using the ExacTrac infrared (IR) tracking system and initial setup errors were based on comparison of ExacTrac IR tracking system to corrected online ExacTrac X‐rays images registered to treatment plans. Residual setup errors were determined using repeat verification X‐ray. The online ExacTrac corrections were compared to cone‐beam CT (CBCT) before treatment to assess agreement. Intrafractional positioning errors were determined using prebeam X‐rays. The systematic and random errors were analyzed. The initial translational setup errors were −0.8±1.3 mm, −0.8±1.6 mm, and 0.3±1.9 mm in AP, CC, and LR directions, respectively, with a three‐dimensional (3D) vector of 2.7±1.4 mm. The initial rotational errors were up to 2.4° if 6D couch is not available. CBCT agreed with ExacTrac X‐ray images to within 2 mm and 2.5°. The intrafractional uncertainties were 0.1±0.6 mm, 0.1±0.6 mm, and 0.2±0.5 mm in AP, CC, and LR directions, respectively, and 0.0∘±0.5°, 0.0∘±0.6°, and −0.1∘±0.4∘ in yaw, roll, and pitch direction, respectively. The translational vector was 0.9±0.6 mm. The calculated PTV margins mPTV(90,95) were within 1.6 mm when using image guidance for online setup correction. The use of image guidance for online setup correction, in combination with our customized CMB device, highly restricted target motion during treatments and provided robust immobilization to ensure minimum dose of 95% to target volume with 2.0 mm PTV margin for HN‐SABR.PACS number(s): 87.55.ne

The Use of Cone Beam Computed Tomography for Image Guided Gamma Knife Stereotactic Radiosurgery: Initial Clinical Evaluation

The present study used cone beam computed tomography (CBCT) to measure the inter- and intrafraction uncertainties for intracranial stereotactic radiosurgery (SRS) using the Leksell Gamma Knife (GK). Using a novel CBCT system adapted to the GK radiosurgery treatment unit, CBCT images were acquired immediately before and after treatment for each treatment session within the context of a research ethics board-approved prospective clinical trial. Patients were immobilized in the Leksell coordinate frame (LCF) for both volumetric CBCT imaging and GK-SRS delivery. The relative displacement of the patient's skull to the stereotactic reference (interfraction motion) was measured for each CBCT scan. Differences between the pre- and post-treatment CBCT scans were used to determine the intrafraction motion. We analyzed 20 pre- and 17 post-treatment CBCT scans in 20 LCF patients treated with SRS. The mean translational pretreatment setup error ± standard deviation in the left-right, anteroposterior, and craniocaudal directions was -0.19±0.32, 0.06±0.27, and -0.23±0.2mm, with a maximum of -0.74, -0.53, and -0.68mm, respectively. After an average time between the pre- and post-treatment CBCT scans of 82minutes (range 27-170), the mean intrafraction error±standard deviation for the LCF was -0.03±0.05, -0.03±0.18, and -0.03±0.12mm in the left-right, anteroposterior, and craniocaudual direction, respectively. Using CBCT on a prototype image guided GK Perfexion unit, we were able to measure the inter- and intrafraction positional changes for GK-SRS using the invasive frame. In the era of image guided radiation therapy, the use of CBCT image guidance for both frame- and non-frame-based immobilization systems could serve as a useful quality assurance tool. Our preliminary measurements can guide the application of achievable thresholds for inter- and intrafraction discrepancy when moving to a frameless approach.

Inter- and intrafractional dose uncertainty in hypofractionated Gamma Knife radiosurgery.

The purpose of this study is to evaluate inter‐ and intrafractional dose variations resulting from head position deviations for patients treated with the Extend relocatable frame system utilized in hypofractionated Gamma Knife radiosurgery (GKRS). While previous reports characterized the residual setup and intrafraction uncertainties of the system, the dosimetric consequences have not been investigated. A digital gauge was used to measure the head position of 16 consecutive Extend patients (62 fractions) at the time of simulation, before each fraction, and immediately following each fraction. Vector interfraction (difference between simulation and prefraction positions) and intrafraction (difference between postfraction and prefraction positions) shifts in patient position were calculated. Planned dose distributions were shifted by the offset to determine the time‐of‐treatment dose. Variations in mean and maximum target and organ at risk (OAR) doses as a function of positional shift were evaluated. The mean vector interfraction shift was 0.64 mm (Standard Deviation (SD): 0.25 mm, maximum: 1.17 mm). The mean intrafraction shift was 0.39 mm (SD: 0.25 mm, maximum: 1.44 mm). The mean variation in mean target dose was 0.66% (SD: 1.15%, maximum: 5.77%) for interfraction shifts and 0.26% (SD: 0.34%, maximum: 1.85%) for intrafraction shifts. The mean variation in maximum dose to OARs was 7.15% (SD: 5.73%, maximum: 30.59%) for interfraction shifts and 4.07% (SD: 4.22%, maximum: 17.04%) for intrafraction shifts. Linear fitting of the mean variation in maximum dose to OARs as a function of position yielded dose deviations of 10.58%/mm for interfractional shifts and 7.69%/mm for intrafractional shifts. Positional uncertainties when performing hypofractionated Gamma Knife radiosurgery with the Extend system are small and comparable to frame‐based uncertainties (<1 mm). However, the steep dose gradient characteristics of GKRS mean that the dosimetric consequences of positional uncertainties should be considered as part of treatment planning. These dose uncertainties should be evaluated in the context of tumor response and OAR tolerance for hypofractionated treatment scenarios where any increase in dose may be tempered by the increased protection hypofractionation provides to normal tissue.PACS number(s): 87.52.‐g

Dosimetric effect of intra-fractional and inter-fractional target motion in lung cancer radiotherapy techniques

Purpose: The purpose of present study was to experimentally evaluate the dosimetric uncertainties in 3-dimensional conformal radiotherapy (3DCRT), dynamic intensity modulated radiotherapy (D-IMRT), step-shoot (SS-IMRT), and volumetric modulated arc therapy (VMAT) treatment delivery techniques due to intra- and inter-fractional target motion. Methods: A previously treated lung patient was selected for this study and was replanned for 60 Gy in 30 fractions using four techniques (3DCRT, D-IMRT, SS-IMRT, and VMAT). These plans were delivered in a clinical linear accelerator equipped with HexaPOD™ evo RT System. The target dose of static QUASAR phantom was calculated that served as reference dose to the target. The QUASAR respiratory body phantom along with patients breathing wave form and HexaPOD™ evo RT System was used to simulate the intra-fraction and inter-fraction motions. Dose measurements were done by applying the intra-fractional and inter-fractional motions in all the four treatment delivery techniques. Results: The maximum percentage deviation in a single field was -4.3%, 10.4%, and -12.2% for 3DCRT, D-IMRT and SS-IMRT deliveries, respectively. Similarly, the deviation for a single fraction was -1.51%, -1.88%, -2.22%, and -3.03% for 3DCRT, D-IMRT, SS-IMRT and VMAT deliveries, respectively. Conclusion: The impact of inter-fractional and intra-fractional uncertainties calculated as deviation between dynamic and static condition dose was large in some fractions, however average deviation calculated for thirty fractions was well within 0.5% in all the four techniques. Therefore, inter- and intra-fractional uncertainties could be concern in fewer fraction treatments such as stereotactic body radiation therapy, and should be used in conjunction with intra- and inter-fractional motion management techniques.

Geometric uncertainties in voluntary deep inspiration breath hold radiotherapy for locally advanced lung cancer

Background and purposeDeep inspiration breath hold (DIBH) increases lung volume and can potentially reduce treatment-related toxicity in locally advanced lung cancer. We estimated geometric uncertainties in visually guided voluntary DIBH and derived the appropriate treatment margins for different image-guidance strategies. Material and methodsSeventeen patients were included prospectively. An optical marker-based respiratory monitoring with visual guidance enabled comfortable DIBHs, adjusted to each patient’s performance. All patients had three consecutive DIBH CTs at each of the treatment fractions 2, 16 and 31. DIBH reproducibility was evaluated as inter- and intra-fractional variations in lung volume, tumour position and differential motion between primary tumour and mediastinal lymph nodes. ResultsLung volume increased by median 60% in DIBH. Inter- and intra-fractional lung volume variations were median 2.1% and 1.1%, respectively.Inter- and intra-fractional uncertainties in 3D tumour position were 4.8±2.8mm and 1.7±1.4mm (mean±SD). Inter- and intra-fractional differential motion was 4.8±3.3mm and 0.0±1.1mm. ConclusionsFor single targets, visually guided voluntary DIBH radiotherapy is highly reproducible provided an image-guidance strategy with tumour registration is performed. If the primary tumour is separated from the mediastinal lymph nodes, inter-fractional differential motion remains a challenge and margins must be adapted to reflect the image registration strategy.

Quantification of intra-fraction changes during radiotherapy of cervical cancer assessed with pre- and post-fraction Cone Beam CT scans

Background and purposeWith the introduction of Intensity Modulated Radiotherapy (IMRT) and image-guided plan-of-the-day strategies, the treatment of cervical cancer has become more sensitive to intra-fraction uncertainties. In this study we quantified intra-fraction changes in cervix-uterus shape, bladder and rectum filling, and patient setup using pre- and post-fraction CBCT scans. Materials and methodsA total of 632 CBCT scans were analyzed for 16 patients with large tip-of-uterus displacement (>2.5cm) measured in an empty and full bladder CT scan. In all scans, the bladder, cervix-uterus, and rectum were delineated. For rectum and bladder, intra-fraction volume changes were assessed. Systematic cervix-uterus intra-fraction displacements were obtained by non-rigidly aligning the pre-fraction cervix-uterus to that in the post-fraction CBCT. Intra-fraction patient setup changes were obtained by rigidly aligning pre- and post-CBCTs using the bony anatomy. ResultsThe mean time between pre- and post-fraction CBCT scan was 20.8min. The group-mean intra-fraction displacements averaged over the cervix-uterus were 0.1±1.4/1.8±1.5/−2.8±1.8 (LR/CC/AP) mm. The group-mean 5th and 95th percentile intra-fraction displacements were −2.3,2.1/−0.8,4.9/−5.8,0.5 (LR/CC/AP) mm. There was a significant correlation between bladder inflow rate and cervix-uterus motion (r=0.6 and p<0.01). Intra-fraction changes in patient setup were 1.3/0.4/0.6 and 1.4/1.0/1.1mm (LR/CC/AP), for systematic and random changes, respectively. ConclusionIntra-fraction cervix-uterus motion can be considerable and should be taken into account using appropriate PTV margins.

Single-isocenter hybrid IMRT plans versus two-isocenter conventional plans and impact of intrafraction motion for the treatment of breast cancer with supraclavicular lymph nodes involvement.

The purpose of this study was to compare the single‐isocenter, four‐field hybrid IMRT with the two‐isocenter techniques to treat the whole breast and supraclavicular fields and to investigate the intrafraction motions in both techniques in the superior direction. Fifteen breast cancer patients who underwent lumpectomy and adjuvant radiation to the whole breast and supraclavicular (SCV) fossa at our institution were selected for this study. Two planning techniques were compared for the treatment of the breast and SCV lymph nodes. The patients were divided into three subgroups according to the whole breast volume. For the two‐isocenter technique, conventional wedged or field‐within‐a‐field tangents (FIF) were used to match with the same anterior field for the SCV region. For the single‐isocenter technique, four‐field hybrid IMRT was used for the tangent fields matched with a half blocked anterior field for the SCV region. To simulate the intrafraction uncertainties in the longitudinal direction for both techniques, the treatment isocenters were shifted by 1 mm and 2 mm in the superior direction. The average breast clinical tumor volume (CTV) receiving 100% (V100%) of the prescription dose (50 Gy) was 99.3%±0.5% and 96.4%±1.2% for the for two‐isocenter and single‐isocenter plans (p<0.05), respectively. The breast CTV receiving 95% of the prescription dose (V95%) was close to 100% in both techniques. The average breast CTV receiving 105% (V105%) of the prescription dose was 32.4%±19.3% and 23.8%±13.3% (p=0.08). The percentage volume of the breast CTV receiving 110% of the dose was 0.4%±1.2% in the two‐isocentric technique vs. 0.1%±0.2% in the single‐isocentric technique. The average uniformity index was 0.91±0.02 vs. 0.91±0.01 in both techniques (p=0.04), but had no clinical impact. The percentage volume of the contralateral breast receiving a dose of 1 Gy was less than 2.3% in small breast patients and insignificant for medium and large breast sizes. The percentage of the total lung volume receiving g>20 Gy (V20Gy) and the heart receiving >30 Gy (V30Gy) were 13.6% vs. 14.3% (p=0.03) and 1.25% vs. 1.2% (p=0.62), respectively. Shifting the treatment isocenter by 1 mm and 2 mm superiorly showed that the average maximum dose to 1 cc of the breast volume was 55.5±1.8 Gy and 58.6±4.3 Gy in the two‐isocentric technique vs. 56.4±2.1 Gy and 59.1±5.1 Gy in the single‐isocentric technique (p=0.46, 0.87), respectively. The single‐isocenter technique using four‐field hybrid IMRT approach resulted in comparable plan quality as the two‐isocentric technique. The single‐isocenter technique is more sensitive to intrafraction motion in the superior direction compared to the two‐isocentric technique. The advantages of the single‐isocenter include elimination of isocentric errors due to couch and collimator rotations and reduction in treatment time. This study supports consideration of a single‐isocenter four‐field hybrid IMRT technique for patients undergoing breast and supraclavicular nodal irradiation.PACS number: 87.55.D, 87.55.de, 87.55.dk,

Inter- and intrafractional setup errors and baseline shifts of fiducial markers in patients with liver tumors receiving free-breathing postoperative radiation analyzed by cone-beam computed tomography.

This study was to evaluate the interfractional and intrafractional setup errors and baseline shifts of golden fiducial markers in patients receiving postoperative radiotherapy (RT) using cone‐beam computed tomography (CBCT) in order to calculate PTV margins for patients with liver cancer. Twelve patients with liver tumors underwent postoperative RT. CBCT images were acquired before and after the treatment. Off‐line vertebral body match and fiducial marker match were used, respectively. The results of vertebral body match represented the setup errors of the patients, while the results of fiducial marker match represented the absolute position errors of the target volume. Baseline shifts of the target volume were calculated as the absolute target position errors minus setup errors. A total of 12 patients with 214 acquisitions of CBCTs were analyzed. Both Σ and σ of setup errors and baseline shifts in left–right (L/R), superior–inferior (S/I), and anterior–posterior(A/P) directions were calculated, including interfractional and intrafractional uncertainties. Planning target volume (PTV) margins were calculated according to margin=2.5Σ+0.7σ. Margins of 1.8 mm, 3.8 mm, and 1.4 mm in L/R, S/I, and A/P directions are needed to compensate intrafractional errors when daily online CBCT correction is used. When CBCT correction with no action level (NAL) protocol is used, PTV margin should be 2.6 mm, 5.9 mm, and 2.6 mm in L/R, S/I, and A/P directions. Margins of 5.5 mm, 14.6 mm, and 7.2 mm were needed to compensate the baseline shifts when electronic portal imaging devices (EPID) or CBCT with bone match is used for online correction of setup error.PACS number: 87.55.‐x