The purpose of this study is to validate a previously developed algorithm for alerting clinicians when to consider re-CT simulation due to changes in the patient's anatomy during radiation therapy of head and neck cancer. Cone beam computed tomography (CBCT) data were collected prospectively for 77 patients. Each CBCT was mathematically compared to a reference CBCT using the gamma index. We defined the match quality parameter (MQP) as an indicator of CBCT image similarity, where a negative MQP value indicates a poorer CBCT match than the match between the first two CBCT acquired during treatment. If three consecutive MQP values were below a chosen threshold, an "alert" is triggered to indicate action required, for example, possible re-CT simulation. The timing of image review requests made by the radiation therapists and any re-CT/re-plan decisions were documented for each patient's treatment course. The MQP for each patient (including any re-plans) was calculated in a manner that was blinded from the clinical process. The MQP as a function of fraction number was compared to actual clinical decisions in the treatment progress to evaluate alert system performance. There was a total of 93 plans (including re-plans) with 34 positives (action required) and 59 negatives (no action required). The sensitivity of the alert system was 0.76 and the false positive rate was 0.37. Only 1 case out of the 34 positive cases would have been missed by both the alert system and our clinical process. Despite the false negatives and false positives, analysis of the timing of alert triggers showed that the alert system could have resulted in seven fewer clinical misses. The alert system has the potential to be a valuable tool to complement human judgment and to provide a quality assurance safeguard to help improve the delivery of radiation treatment of head and neck cancer.

Purpose:This article presents a fast algorithm for comparing 3‐D anatomy from Cone‐Beam CT (CBCT) imaging using the gamma comparison index and to demonstrate how this can be used to flag patients for possible re‐planning of treatment.Methods:CBCT scans acquired on a Varian linear accelerator during treatment were used as input to the gamma comparator using thresholds of 5 mm distance to agreement and 30 Hounsfield Unit CT number difference. The fraction 1 CBCT study was initially used as the reference. Should there be a re‐plan during treatment, the reference resets to the CBCT study acquired on the day 1 of the re‐plan. Histograms of failing pixels (γ > 1) were generated from each 3‐D gamma map. An indicator of anatomy congruence, the match quality parameter (MQP), was derived from failed pixel histograms using the 90th percentile gamma value. The MQP was plotted versus fraction number and related to actual repeat computed tomography (re‐CT) order dates as decided by a radiation oncologist. From this, decision criteria were derived for the algorithm to “trigger” re‐CT consideration and predictive power was scored using receiver‐operator characteristic (ROC) analysis.Results:The MQP plot generally showed that the on‐line match from CBCT image guidance deteriorated as the treatment progressed due to weight loss and tumor regression. The optimized MQP criteria for triggering re‐CT consideration demonstrated high sensitivity and specificity, consistent with actual re‐CT order dates within ± 3 fractions. Out of 20 patients that were actually re‐planned, the algorithm failed to trigger a re‐CT recommendation only twice and this was caused by CBCT ring artifacts.Conclusion:We have demonstrated that gamma comparisons can be used to evaluate CBCT‐acquired anatomy pairs and, from this, an algorithm can be “trained” to flag patients for possible re‐planning in a manner consistent with local radiation oncology practice.

Accurate knowledge of ionizing radiation dose from cone-beam CT (CBCT) imaging in radiotherapy is important to allow concomitant risks to be estimated and for justification of imaging exposures. This study uses a Monte Carlo CBCT model to calculate imaging dose for a wide range of imaging protocols for male and female patients. The Elekta XVI CBCT system was modeled using GATE and simulated doses were validated against measurements in a water tank and thorax phantom. Imaging dose was simulated in the male and female ICRP voxel phantoms for a variety of anatomical sites and imager settings (different collimators, filters, full and partial rotation). The resulting dose distributions were used to calculate effective doses for each scan protocol. The Monte Carlo simulated doses agree with validation measurements within 5% and 10% for water tank and thorax phantom respectively. Effective dose for head CBCT scans was generally lower for scans centred on the pituitary than the larynx (0.03 mSv versus 0.06 mSv for male ICRP phantom). Pelvis CBCT scan effective dose was higher for the female than male phantom (5.11 mSv versus 2.80 mSv for M15 collimator scan), principally due to the higher dose received by gonads for the female scan. Medium field of view thorax scan effective doses ranged from 1.38–3.19 mSv depending on scan length and phantom sex. Effective dose for half rotation thorax scans with offset isocentre varied by almost a factor of three depending on laterality of the isocentre, patient sex and imaged field length. The CBCT imaging doses simulated here reveal large variations in dose depending on imaging isocentre T E Marchant and K D Joshi Monte Carlo study of CBCT dose Printed in the UK 13 JRPREA © 2016 IOP Publishing Ltd 37 J. Radiol. Prot.

Purpose:To investigate the clinical utility of on‐line verification of respiratory gated VMAT dosimetry during treatment.Methods:Portal dose images were acquired during treatment in integrated mode on a Varian TrueBeam (v. 1.6) linear accelerator for gated lung and liver patients that used flattening filtered beams. The source to imager distance (SID) was set to 160 cm to ensure imager clearance in case the isocenter was off midline. Note that acquisition of integrated images resulted in no extra dose to the patient. Fraction 1 was taken as baseline and all portal dose images were compared to that of the baseline, where the gamma comparison and dose difference were used to measure day‐to‐day exit dose variation. All images were analyzed in the Portal Dosimetry module of Aria (v. 10). The portal imager on the TrueBeam was calibrated by following the instructions for dosimetry calibration in service mode, where we define 1 calibrated unit (CU) equal to 1 Gy for 10×10 cm field size at 100 cm SID. This reference condition was measured frequently to verify imager calibration.Results:The gamma value (3%, 3 mm, 5% threshold) ranged between 92% and 100% for the lung and liver cases studied. The exit dose can vary by as much as 10% of the maximum dose for an individual fraction. The integrated images combined with the information given by the corresponding on‐line soft tissue matched cone‐beam computed tomography (CBCT) images were useful in explaining dose variation. For gated lung treatment, dose variation was mainly due to the diaphragm position. For gated liver treatment, the dose variation was due to both diaphragm position and weight loss.Conclusion:Integrated images can be useful in verifying dose delivery consistency during respiratory gated VMAT, although the CBCT information is needed to explain dose differences due to anatomical changes.

During volumetric modulated arc therapy (VMAT) of head and neck cancer, some patients lose weight which may result in anatomical deviations from the initial plan. If these deviations are substantial a new treatment plan can be designed for the remainder of treatment (i.e., adaptive planning). Since the adaptive treatment process is resource intensive, one possible approach to streamlining the quality assurance (QA) process is to use the electronic portal imaging device (EPID) to measure the integrated fluence for the adapted plans instead of the currently‐used ArcCHECK device (Sun Nuclear). Although ArcCHECK is recognized as the clinical standard for patient‐specific VMAT plan QA, it has limited length (20 cm) for most head and neck field apertures and has coarser detector spacing than the EPID (10 mm vs. 0.39 mm). In this work we compared measurement of the integrated fluence using the EPID with corresponding measurements from the ArcCHECK device. In the past year nine patients required an adapted plan. Each of the plans (the original and adapted) is composed of two arcs. Routine clinical QA was performed using the ArcCHECK device, and the same plans were delivered to the EPID (individual arcs) in integrated mode. The dose difference between the initial plan and adapted plan was compared for ArcCHECK and EPID. In most cases, it was found that the EPID is more sensitive in detecting plan differences. Therefore, we conclude that EPID provides a viable alternative for QA of the adapted head and neck plans and should be further explored.

Stereotactic Body Radiation Therapy (SBRT) is an option for early stage non-small cell lung cancer treatment. In SBRT treatment, high biological effective dose is delivered to the patient within a small number of fractions. High level of confidence in accuracy is required in the entire treatment procedure, from patient setup, tumour delineation, treatment simulation and planning, to the final dose delivery. SBRT lung treatment utilizes small fields that are incident on large tissue inhomogeneities within the patient. It is difficult for commercially available treatment planning systems (TPS) to model the lack of charged particle equilibrium and the dose near tissue-lung interfaces accurately. The Monte Carlo (MC) technique calculates the dose distribution from the first principles thereby providing a feasible tool for verifying the dose distribution computed from TPS. In this study, we compared the SBRT dose distribution between Eclipse 8.9 and BEAMnrc/DOSXYZnrc for both conformal and RapidArc plans. Calculation results for five clinical SBRT conformal lung plans were compared. Eclipse and MC results for each plan showed good agreement in dose received by organs at risk. MC simulation predicted uniformly hotter or similar PTV coverage for three cases with tumor either small or attached to the chest wall. When tumor is inside lung and at relatively medium to larger size for SBRT, MC predicted lower PTV coverage. The variation in dose coverage may depend on the tumour size and its position within the lung. Dose comparison for RapidArc plans shows similar dependence.

Volumetric modulated arc therapy (VMAT) has recently been used to improve the dose distribution and efficiency of treatment delivery over the standard intensity-modulated radiotherapy (IMRT) technique. This study compares the dosimetry between RapidArc plan and standard IMRT plan for head and neck cancer. Three head and neck patients treated clinically with sliding window intensity-modulated radiotherapy (IMRT) technique at Grand River Regional Cancer Center were selected randomly and re-planned using RapidArc technique with 6 MV photon beams generated by a Varian 21EX linac with 120-leaf multileaf collimator. Three dose prescriptions were used to deliver 70 Gy, 63 Gy and 58.1 Gy to the regions of the primary tumors, intermediate-risk nodes and low-risk nodal level, respectively, in 35 fractions. Dosimetric comparison based on the dose-volume histogram, target coverage, organ at risk (OAR) dose sparing were studied between the RapidArc plan and IMRT plan. RapidArc technique from Varian Medical Systems showed superior target coverage, better OAR sparing, fewer monitor units per fraction with less treatment time over IMRT technique for head and neck cancers. The average homogeneity index, defined as the difference between the percentage dose covering 5% and 95% of the PTV, is 9.5 for RapidArc plan and 10.5 for IMRT plan. All RapidArc plans met the dose objectives for the primary OAR: spinal cord, brainstem, brain etc. Both parotid mean dose and D50% are lower for RapidArc plan than those of the IMRT plan. The technique is currently being used clinically at our cancer center.

One of the greatest challenges in radiation therapy is the ability to deliver a lethal dose of radiation to a tumour while sparing the surrounding normal tissues. In theory, the dose delivered to a tumour during photon-based radiation therapy can be enhanced by loading high atomic number (Z) materials into the tumour, which results in greater photoelectric absorption and hence increased photoelectron fluence within the tumour than in surrounding tissues. The EGSnrcMP Monte Carlo code, together with DOSXYZnrc, a three-dimensional voxel dose calculation module has been used to study the macroscopic dose enhancement factor (MDEF) in a tumour infused with gold nanoparticles at the kilo-voltage energies. We observed that gold nanoparticles infused in a tumour irradiated with kilo-voltage energies has the potential to enhance the tumour dose by a factor ranging from 0.25 to about 5 depending on the mean energy of the beam and the concentration of gold nanoparticles in the tumour. The increase in dose can be attributed to the significant increase in the photoelectron fluence within the tumour loaded with gold particles during the irradiation. Future studies will involve the characterization of the MDEF at megavoltage energies.

Image-guided radiotherapy (IGRT) is becoming the standard treatment for prostate cancer. One approach employs a linear accelerator-mounted, cone-beam computed tomography (CBCT) system to acquire 3D images at treatment. Typically, radiation therapists manually register such images to the planning CT to determine couch shifts, thereby ignoring anatomical rotations and deformations. Furthermore, CBCT systems deliver significant imaging doses over long fractionation schedules. Therefore, we continue developing image guidance (IG) techniques relying on automatic registration and low dose CBCT images. Our “global” method computes couch corrections while the “local” variant provides a deformable transformation that can be used to adapt the original plan. Previous validation with Varian's On-board Imager (OBI v1.3) showed that IG error is maintained despite reducing the mAs to 15% of the standard 1300. Recent improvements in OBI v1.4 result in similar image quality at 680 mAs (“pelvis” mode). Additionally, “pelvis spotlight” mode was introduced with additional lateral collimation and 720 mAs employed over 200 degrees. Retesting showed that global IG error is 3.5 ±1.0 mm irrespective of the OBI version down to 10% of the standard dose. Local IG required 20% of the standard dose to achieve 1.8 ± 0.6 mm accuracy with OBI v1.4 pelvis, while the 2.0 ± 0.5 mm error was maintained down to 15% with OBI v1.3 and v1.4 spotlight. In absolute terms, the dose savings achieved by our IG methods are cumulative with those offered by the upgrade. Our local IG technique has great potential to significantly improve the precision of radiation therapy.

B Schaly1, V Varchena2, P Au3, G Pang3,41Grand River Regional Cancer Centre, Kitchener, ON, Canada; 2CIRS Inc., Norfolk, VA, USA; 3Odette Cancer Centre, Toronto, ON, Canada; 4Departments of Radiation Oncology and Medical Biophysics, University of Toronto, Toronto, ON, Canada Abstract: Soft-tissue imaging in the treatment room is one of the main challenges faced today in high precision radiotherapy. The objective of this work is to evaluate a new anthropomorphic male pelvic phantom (CIRS Inc., Norfolk, VA, USA) that can be used in a radiotherapy department to assess the ability of an X-ray imaging system for imaging soft-tissue targets in the treatment room. To this end, we evaluated the tissue-equivalency of the phantom materials in terms of the linear attenuation and energy absorption coefficients. X-ray computed tomography (CT) images of the phantom were also obtained and compared with that of patients. Our results demonstrated that the male pelvic phantom is a good representation of actual prostate cancer patients and can be a valuable tool for image-guided radiotherapy. Keywords: image-guided radiotherapy, X-ray imaging, anthropomorphic phantomPACS numbers: 87.56.Fc, 87.59.bd, 87.85.Lf

Currently, we are using Varian's On Board Imager to acquire cone beam computed tomography (CBCT) images prior to prostate intensity modulated radiotherapy (IMRT). The images are used to determine the post-setup prostate position, and thus, to shift the couch towards the situation assumed during treatment planning. This “manual forward” image guided radiotherapy (IGRT) technique is time consuming and requires significant imaging dose over the course of treatment. Therefore, we present two low-dose IGRT methods that are entirely automatic. The dose is reduced by lowering the mAs during CBCT acquisition at the cost of increased noise in the final images. In the “auto forward” IGRT technique, the CBCT image is registered to the planning CT and the result is used to shift the patient on the couch. The “auto reverse” technique involves non-rigid registration of the planning CT to the CBCT. Both techniques were evaluated using a retrospective analysis of data from ten patients, including one planning CT and five CBCT images acquired at evenly spaced fractions per patient. At the standard imaging dose, IGRT error was 4.3 ± 1.6, 3.9 ± 1.2, 3.8 ± 1.8, and 3.1 ± 1.1 mm for no correction, manual forward, auto forward, and auto reverse techniques, respectively. Errors exceeded seven mm in 4% of the fractions when employing the auto reverse technique, highlighting a possible need for manual surveillance. The imaging dose could be reduced to 20% without an appreciable loss in accuracy of the automatic methods.

Linac-mounted cone beam computed tomography (CBCT) using Varian's On Board Imager (OBI) currently delivers significant imaging dose and lacks automatic methods for clinical target volume (CTV) registration. In this work, we address these two issues to enable frequent treatment corrections during a course of prostate intensity modulated radiation therapy (IMRT). The process starts by acquiring a low dose (low mAs) CBCT image after patient setup. The image is then used in one of two automatic image guidance strategies. The "global" technique provides the couch corrections necessary to improve patient setup by registering the CBCT to the planning CT. The "local" method involves non-rigid registration of the planning CT to the CBCT followed by automatic treatment re-optimization using the deformed planning CT and contours. Thus, the global method attempts to correct patient setup to match the planned treatment, while the local method corrects the treatment to match the patient setup. Both techniques were evaluated using images of an anthropomorphic male pelvis phantom. Global image guidance resulted in a registration error of 3.6 ± 1.3 mm (imaging dose independent) and high treatment doses to the bladder and rectum for large magnitude motion. The local technique always resulted in clinically acceptable treatment doses due to a reduced registration error of 2.3 ± 0.8 mm, obtained at 15% of the OBI's default dose (125 kVp, 2 mAs per projection). These preliminary results show that our automatic local image guidance technique reduces imaging dose and is sufficiently accurate and robust for application in prostate IMRT.

The use of on-line kilovoltage cone-beam computed tomography (CBCT) is increasing as part of the current evolution of image-guided radiotherapy. At our institution, we use Varian's On Board Imager® (OBI) mainly for imaging prostate cancer patients. Since daily CBCT can add significant dose, we have performed a comprehensive set of dose measurements using acrylic cylindrical phantoms using ionization chambers as well as skin dose measurements using Thermoluminescence Dosimeters (TLD). The TLD were calibrated under chosen reference conditions (10×10 cm2 field size at 100 cm SSD) using the OBI beam with bowtie filters (125 kVp, 6.0 mm Al). For the patients, TLD were placed on the anterior, left and right lateral locations to give the peripheral dose. The CBCT dose values (in units of mGy/100mAs) were then used to model the central (phantom only) and peripheral dose (phantom and patients) as a function of equivalent diameter, deq , using exponential functions. Dose values measured in Rando phantom as well as published dose values agreed with the model quite well for deq ⩾ 24 cm (body), but there was higher variation in CBCT dose for deq ⩽ 18 cm (head). Hence, it is recommended that skin dose be measured for head scans to validate the estimate. This method provides a quick estimate of CBCT dose so that a decision can be made whether to incorporate it into the treatment prescription. Also, knowledge of CBCT dose as a function of patient size may enable reduction of the total mAs for smaller body scans.

Purpose: To evaluate various image‐guidance technologies and their potential impact on the outcome of hypofractionated prostate cancer radiation therapy to mitigate against geometric uncertainties. Method and Materials: Five prostate cancer patients were analyzed. All patients were planned twice with an 18MV six‐field conformal technique with a 10 and 5 mm margin sizes, with various prescription doses (35 to 70 Gy) of equal late complications (assuming normal tissue α/β = 3 Gy). The various target localization techniques simulated were (1) laser alignment to external tattoo marks, (2) alignment to bony landmarks with daily portal images, (3) alignment to the clinical target volume (CTV) with daily CT imaging, and (4) the repeat of technique (3) with daily monitor unit updates to account for patient shape changes. The impact of uncertainty in the assumed α/β value for the prostate was also assessed. Results: For all treatment schedules simulated, technique (4) achieved the ideal condition most closely (i.e., ΔTCP = TCPtechnique − TCPplan ≈ 0 %), then followed by (3), (2) and (1). As the number of fractions decreased (i.e., increasingly hypofractionated), the ΔTCP generally decreased for all techniques. Because the hypofractionated schedules were designed to keep late complications constant, the NTCP values were also relatively constant for all treatment schedules. Generally, the average NTCP values were lower than the plan for all techniques. However, the most effective way to reduce NTCP was to reduce the margin size from 10 to 5 mm. Overall, the uncertainty in α/β values had a far more influence on the outcome of the hypofractionated treatment than would the geometric uncertainties. Conclusion: This study suggests that, although the impact of geometric uncertainties increases as the number of fractions decrease, the reduction in TCP due to the uncertainties does not significantly offset the expected gain in TCP by hypofractionation.

Computed tomography (CT)‐based image guidance has the feature of including the tumour as well as normal tissue in the image localization process during radiation treatment. We propose to use a new anthropomorphic male pelvic phantom (CIRS Inc.) to test the feasibility of using a linear‐accelerator‐based megavoltage (MV) cone‐beam CT system for imaging the prostate. The objective of this work is to verify the tissue‐equivalency of the phantom to determine whether it is representative of prostate cancer patients. We evaluated the phantom in two ways. (1) The linear attenuation coefficient was determined by measuring the photon transmission through uniform samples of the phantom materials (e.g., prostate, muscle, bladder, etc.) using (2) The phantom material CT numbers (kVCT) were compared to CT number data acquired from randomly selected planning CT scans of prostate cancer patients at our institution. The measured linear attenuation coefficients agreed to ∼ 1% of the manufacturer's specifications, while the reproducibility of the measurement was ∼ 1%. The kVCT numbers in the phantom were also in good agreement with the manufacturer's specifications and the patient data. However, the bony structures are comprised of solid average bone equivalent material (cortical and trabecular bone), which caused artifacts within some regions of the kVCT images. However, MVCT imaging is expected to suppress these artifacts. These findings indicate that the phantom is representative of prostate cancer patients and will be a valuable tool in investigating the feasibility of MV cone‐beam CT.This work was partially supported by Siemens Medical Solution USA, Inc.

The goal of this work was to evaluate the efficacy of various image-guided adaptive radiation therapy (IGART) techniques to deliver and escalate dose to the prostate in the presence of geometric uncertainties. Five prostate patients with 15-16 treatment CT studies each were retrospectively analyzed. All patients were planned with an 18 MV, six-field conformal technique with a 10 mm margin size and an initial prescription of 70 Gy in 35 fractions. The adaptive strategy employed in this work for patient-specific dose escalation was to increase the prescription dose in 2 Gy-per-fraction increments until the rectum normal tissue complication probability (NTCP) reached a level equal to that of the nominal plan NTCP (i.e., iso-NTCP dose escalation). The various target localization techniques simulated were: (1) daily laser-guided alignment to skin tattoo marks that represents treatment without image-guidance, (2) alignment to bony landmarks with daily portal images, and (3) alignment to the clinical target volume (CTV) with daily CT images. Techniques (1) and (3) were resimulated with a reduced margin size of 5 mm to investigate further dose escalation. When delivering the original clinical prescription dose of 70 Gy in 35 fractions, the "CTV registration" technique yielded the highest tumor control probability (TCP) most frequently, followed by the "bone registration" and "tattoo registration" techniques. However, the differences in TCP among the three techniques were minor when the margin size was 10 mm (⩽1.1%). Reducing the margin size to 5 mm significantly degraded the TCP values of the "tattoo registration" technique in two of the five patients, where a large difference was found compared to the other techniques (⩽11.8%). The "CTV registration" technique, however, did maintain similar TCP values compared to their 10 mm margin counterpart. In terms of normal tissue sparing, the technique producing the lowest NTCP varied from patient to patient. Reducing the margin size seemed the only sure way to reduce the NTCP significantly, irrespective of the IGART technique employed. In escalating the dose with the iso-NTCP constraint, the largest average gain in dose was observed with the "tattoo registration" technique, followed by the "CTV registration" and "bone registration" techniques. This is attributed to the fact that in three of the five patients, the "tattoo registration" technique yielded the lowest NTCP, hence a greater window of opportunity to escalate the dose was possible with this technique. However, the variation among the five patients was also largest with the "tattoo registration" technique where, in the case of one patient, the required dose actually needed to be below the original prescription dose of 70 Gy to satisfy the iso-NTCP constraint. This was not the case with the "CTV registration" technique where positive and similar dose escalation was allowed on all five patients. Based on these data, an attractive dose escalation strategy may be to implement the "CTV registration" technique (for consistent dosimetric coverage) for daily target localization in combination with a margin reduction (for increased normal tissue sparing).

Purpose: To evaluate the efficacy of various image‐guided target localization techniques to assist in the safe dose escalation for prostate cancer radiation therapy in the presence of geometric uncertainties. Method and Materials: Five prostate cancer patients were analyzed, retrospectively. All patients were planned with an 18MV six‐field conformal technique with a 10 mm margin and an initial prescription dose of 70 Gy in 35 fractions. For the dose escalation study, the prescription dose was increased from 50 Gy in 2 Gy increments until the rectum normal tissue complication probability (NTCP) reached the level equal to that of the plan NTCP (i.e., iso‐NTCP). The target localization techniques simulated were (1) laser alignment to external tattoo marks, (2) alignment to bony landmarks with daily portal images, and (3) alignment to the clinical target volume (CTV) with daily CT imaging. Techniques (1) and (3) were re‐simulated with a reduced margin of 5 mm to investigate further dose escalation. Results: The selection of the target localization technique was less critical on the treatment outcome when 10 mm margin was used. Reducing the margin size from 10 to 5 mm effectively decreased the NTCP by ∼60 % for all patients. For iso‐NTCP dose escalation, to our surprise, the average dose and TCP gain for the five patients were largest with the external tattoo technique (6.4 Gy and 7.5 %, respectively) followed by the daily CTV and the bony landmarks. The variability in gain across the five patients, however, was also largest with the tattoo registration technique and smallest with the daily CTV technique. Conclusion: Based on these data, the best dose escalation strategy is to combine margin reduction (for increased normal tissue sparing) with the use of the daily CTV technique to localize the target volume (for consistent dosimetric coverage and escalation).