The effect of setup uncertainty on normal tissue sparing with IMRT for head-and-neck cancer

A prospective study to assess and quantify the setup errors with cone-beam computed tomography in head-and-neck cancer image-guided radiotherapy treatment

Introduction

This study was done to quantify the translational setup errors with cone-beam computed tomography (CBCT) in the image-guided radiation therapy (IGRT) treatment of head-and-neck cancer (HNC) patients.

Aims

The objective was to quantify the setup errors by CBCT.

Methodology

One hundred patients of HNC were enrolled from March 2020 to March 2021 for IGRT treatment. Pretreatment kV-CBCT images were obtained at the first 3 days of irradiations, and setup error corrections were done in the mediolateral (ML), superior-inferior (SI), and anterior-posterior (AP) directions. Subsequently, a weekly kV-CBCT was repeated for whole duration of radiotherapy for the next 6-7 weeks. Adequacy of planning target volume (PTV) margins was assessed by van Herk's formula.

Results

Total 630 CBCT scans of 100 patients were analyzed. Setup errors greater than 3 mm and 5 mm were seen in 11.4% and 0.31% of the patients, respectively. Systematic errors and random errors before correction in ML, SI, and AP directions were 0.10 cm, 0.11 cm, and 0.12 cm and 0.24 cm, 0.20 cm, and 0.21 cm, respectively. Systematic errors and random errors after correction in ML, SI, and AP directions were 0.06 cm, 0.07 cm, and 0.07 cm and 0.13 cm, 0.10 cm, and 0.12 cm, respectively.

Conclusion

CBCT at the first 3 fractions and then weekly during radiotherapy is effective to detect the setup errors. An isotropic PTV margin of 5 mm over clinical target volume is safe to account for setup errors, however, in the case of close organ at risk, or with IGRT, a PTV margin of 3 mm can be considered.

Impact of respiratory motion on lung dose during total marrow irradiation

We evaluated the impact of respiratory motion on the lung dose during linac-based intensity-modulated total marrow irradiation (IMTMI) using two different approaches: (1) measurement of doses within the lungs of an anthropomorphic phantom using thermoluminescent detectors (TLDs) and (2) treatment delivery measurements using ArcCHECK where gamma passing rates (GPRs) and the mean lung doses were calculated and compared with and without motion. In the first approach, respiratory motions were simulated using a programmable motion platform by using typical published peak-to-peak motion amplitudes of 5, 8, and 12 mm in the craniocaudal (CC) direction, denoted here as M1, M2, and M3, respectively, with 2 mm in both anteroposterior (AP) and lateral (LAT) directions. TLDs were placed in five selected locations in the lungs of a RANDO phantom. Average TLD measurements obtained with motion were normalized to those obtained with static phantom delivery. The mean dose ratios were 1.01 (0.98–1.03), 1.04 (1.01–1.09), and 1.08 (1.04–1.12) for respiratory motions M1, M2, and M3, respectively. To determine the impact of directional respiratory motion, we repeated the experiment with 5-, 8-, and 12-mm motion in the CC direction only. The differences in average TLD doses were less than 1% when compared with the M1, M2, and M3 motions indicating a minimal impact from CC motion on lung dose during IMTMI. In the second experimental approach, we evaluated extreme respiratory motion 15 mm excursion in only the CC direction. We placed an ArcCHECK device on a commercial motion platform and delivered the clinical IMTMI plans of five patients. We compared, with and without motion, the dose volume histograms (DVHs) and mean lung dose calculated with the ArcCHECK-3DVH tool as well as GPR with 3%, 5%, and 10% dose agreements and a 3-mm constant distance to agreement (DTA). GPR differed by 11.1 ± 2.1%, 3.8 ± 1.5%, and 0.1 ± 0.2% with dose agreement criteria of 3%, 5%, and 10%, respectively. This indicates that respiratory motion impacts dose distribution in small and isolated parts of the lungs. More importantly, the impact of respiratory motion on the mean lung dose, a critical indicator for toxicity in IMTMI, was not statistically significant (p > 0.05) based on the Student’s t-test. We conclude that most patients treated with IMTMI will have negligible dose uncertainty due to respiratory motion. This is particularly reassuring as lung toxicity is the main concern for future IMTMI dose escalation studies.

Development of a customisable 3D-printed intra-oral stent for head-and-neck radiotherapy

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Advanced radiotherapy techniques have improved head-and-neck treatments.

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More improvements are possible with intra-oral stents stabilising sensitive anatomy.

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MRI imaging shows new modular 3D printed stents provide stable displacement.

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Modular stents achieve positive outcomes within standard treatment workflow.

Intra-oral stents (including mouth-pieces and bite blocks) can be used to displace adjacent non-involved oral tissue and reduce radiation side effects from radiotherapy treatments for head-and-neck cancer. In this study, a modular and customisable 3D printed intra-oral stent was designed, fabricated and evaluated, to utilise the advantages of the 3D printing process without the interruption of clinical workflow associated with printing time. The stent design used a central mouth-opening and tongue-depressing main piece, with optional cheek displacement pieces in three different sizes, plus an anchor point for moulding silicone to fit individual patients’ teeth. A magnetic resonance imaging (MRI) study of one healthy participant demonstrated the tissue displacement effects of the stent, while providing a best-case indication of its comfort.

Predictive dose accumulation for HN adaptive radiotherapy

During radiation therapy (RT) of head and neck (HN) cancer, the shape and volume of the parotid glands (PG) may change significantly, resulting in clinically relevant deviations of delivered dose from the planning dose. Early and accurate longitudinal prediction of PG anatomical changes during the RT can be valuable to inform decisions on plan adaptation. We developed a deep neural network for longitudinal predictions using the displacement fields (DFs) between the planning computed tomography (pCT) and weekly cone beam computed tomography (CBCT). Sixty-three HN patients treated with volumetric modulated arc were retrospectively studied. We calculated DFs between pCT and week 1-3 CBCT by B-spline and Demon deformable image registration (DIR). The resultant DFs were subsequently used as input to our novel network to predict the week 4 to 6 DFs for generating predicted weekly PG contours and weekly dose distributions. For evaluation, we measured dice similarity (DICE), and the uncertainty of accumulated dose. Moreover, we compared the detection accuracies of candidates for adaptive radiotherapy (ART) when the trigger criteria were mean dose difference more than 10%, 7.5%, and 5%, respectively. The DICE of ipsilateral/contralateral PG at week 4 to 6 using the prediction model trained with B-spline were 0.81 [Formula: see text] 0.07/0.81 [Formula: see text] 0.04 (week 4), 0.79 [Formula: see text] 0.06/0.81 [Formula: see text] 0.05 (week 5) and 0.78 [Formula: see text] 0.06/0.82 [Formula: see text] (week 6). The DICE with the Demons model were 0.78 [Formula: see text] 0.08/0.82 [Formula: see text] 0.03 (week 4), 0.77 [Formula: see text] 0.07/0.82 [Formula: see text] 0.04 (week 5) and 0.75 [Formula: see text] 0.07/0.82 [Formula: see text] 0.02 (week 6). The dose volume histogram (DVH) analysis with the predicted accumulated dose showed the feasibility of predicting dose uncertainty due to the PG anatomical changes. The AUC of ART candidate detection with our predictive model was over 0.90. In conclusion, the proposed network was able to predict future anatomical changes and dose uncertainty of PGs with clinically acceptable accuracy, and hence can be readily integrated into the ART workflow.

Design, Fabrication, and Validation of a Polymethyl Methacrylate Head Phantom for Dosimetric Verification of Cranial Radiotherapy Treatment Plans

Purpose:

The present study aims to design and fabricate a novel, versatile, and cost-effective Polymethyl Methacrylate (PMMA) head phantom for the dosimetric pretreatment verification of radiotherapy (RT) treatment plans.

Materials and Methods:

The head phantom designing involves slice-wise modeling of an adult head using PMMA. The phantom has provisions to hold detectors such as ionization chambers of different sizes, Gafchromic films, gel dosimeter, and optically stimulated luminescence dosimeter. For the point dose verification purpose, 15 volumetric modulated arc therapy patient plans were selected, and doses were measured using a CC13 ionization chamber. The percentage gamma passing rate was calculated for acceptance criteria 3%/3 mm and 2%/2 mm using OmniPro I’mRT film QA software, and Gafchromic EBT3 films were used for 2D planar dose verification.

Results:

Treatment planning system calculated, and the measured point doses showed a percentage deviation ranged from 0.26 to 1.92. The planar dose fluence measurements, for set acceptance criteria of 3%/3 mm and 2%/2 mm, percentages of points having gamma value <1 were in the range of 99.17 ± 0.25 to 99.88 ± 0.15 and 93.16 ± 0.38 to 98.89 ± 0.23, respectively. Measured dose verification indices were within the acceptable limit.

Conclusions:

The dosimetric study reveals that head phantom can be used for routine pretreatment verification for the cranial RT, especially for stereotactic radiosurgery/RT as a part of patient-specific quality assurance. The presently fabricated and validated phantom is novel, versatile, and cost-effective, and many institutes can afford it.

Dose accumulation to assess the validity of treatment plans with reduced margins in radiotherapy of head and neck cancer

BACKGROUND AND PURPOSE

Literature has reported reduced treatment toxicity in head-and-neck radiotherapy (HNRT) when reducing the planning target volume (PTV) margin from 5 to 3 mm but loco-regional control was not always preserved. This study used deformable image registration (DIR)-facilitated dose accumulation to assess clinical target volume (CTV) coverage in the presence of anatomical changes.

MATERIALS AND METHODS

VMAT plans for 12 patients were optimized using 3 or 5 mm PTV and planning risk volume (PRV) margins. The planning computed tomography (pCT) scan was registered to each daily cone beam CT (CBCT) using DIR. The inverse registration was used to reconstruct and accumulate dose (D acc ). CTV coverage was assessed using the dose-volume histogram (DVH) metric D 99 % acc and by individual voxel analysis. Both approaches included an uncertainty estimate using the 95% level of confidence.

RESULTS

D 99 % acc was less than 95% of the prescribed doseD presc for three cases including only one case where this was at the 95% level of confidence. However for many patients, the accumulated dose included a substantial volume of voxels receiving less than 95%D presc independent of margin expansion, which predominantly occurred in the subdermal region. Loss in target coverage was very patient specific but tightness of target volume coverage at planning was a common factor leading to underdosage.

CONCLUSION

This study agrees with previous literature that PTV/PRV margin reduction did not significantly reduce CTV coverage during treatment, but also highlighted that tight coverage of target volumes at planning increases the risk of clinically unacceptable dose delivery. Patient-specific verification of dose delivery to assess the dose delivered to each voxel is recommended.

Multi-institutional evaluation of MVCT guided patient registration and dosimetric precision in total marrow irradiation: A global health initiative by the international consortium of total marrow irradiation

PURPOSE

Total marrow irradiation (TMI) is a highly conformal treatment of the human skeleton structure requiring a high degree of precision and accuracy for treatment delivery. Although many centers worldwide initiated clinical studies using TMI, currently there is no standard for pretreatment patient setup. To this end, the accuracy of different patient setups was measured using pretreatment imaging. Their impact on dose delivery was assessed for multiple institutions.

METHODS AND MATERIALS

Whole body imaging (WBI) or partial body imaging (PBI) was performed using pretreatment megavoltage computed tomography (MVCT) in a helical Tomotherapy machine. Rigid registration of MVCT and planning kilovoltage computed tomography images were performed to measure setup error and its effect on dose distribution. The entire skeleton was considered the planning target volume (PTV) with five sub regions: head/neck (HN), spine, shoulder and clavicle (SC), and one avoidance structure, the lungs. Sixty-eight total patients (>300 images) across six institutions were analyzed.

RESULTS

Patient setup techniques differed between centers, creating variations in dose delivery. Registration accuracy varied by anatomical region and by imaging technique, with the lowest to the highest degree of pretreatment rigid shifts in the following order: spine, pelvis, HN, SC, and lungs. Mean fractional dose was affected in regions of high registration mismatch, in particular the lungs.

CONCLUSIONS

MVCT imaging and whole body patient immobilization was essential for assessing treatment setup, allowing for the complete analysis of 3D dose distribution in the PTV and lungs (or avoidance structures).

Optical Surface Scanning for Patient Positioning in Radiation Therapy: A Prospective Analysis of 1902 Fractions

Purpose/Objective:

Reproducible patient positioning remains one of the major challenges in modern radiation therapy. Recently, optical surface scanners have been introduced into clinical practice in addition to well-established positioning systems, such as room laser and skin marks. The aim of this prospective study was to evaluate setup errors of the optical surface scanner Catalyst HD (C-RAD AB) in different anatomic regions.

Material/Methods:

Between October 2016 and June 2017 a total of 1902 treatment sessions in 110 patients were evaluated. The workflow of this study included conventional setup procedures using laser-based positioning with skin marks and an additional registration of the 3-dimensional (3D) deviations detected by the Catalyst system. The deviations of the surface-based method were then compared to the corrections of cone beam computed tomography alignment which was considered as gold standard. A practical Catalyst setup error was calculated between the translational deviations of the surface scanner and the laser positioning. Two one-sided t tests for equivalence were used for statistical analysis.

Results:

Data analysis revealed total deviations of 0.09 mm ± 2.03 mm for the lateral axis, 0.07 mm ± 3.21 mm for the longitudinal axis, and 0.44 mm ± 3.08 mm vertical axis for the Catalyst system, compared to −0.06 ± 3.54 mm lateral, 0.53 ± 3.47 mm longitudinal, and 0.19 ± 3.49 mm vertical for the laser positioning compared to cone beam computed tomography. The lowest positional deviations were found in the cranial region, and larger deviations occurred in the thoracic and abdominal sites. A statistical comparison using 2 one-sided t tests showed a general concordance of the 2 methods (P ≤ 0.036), excluding the vertical direction of the abdominal region (P = 0.198).

Conclusion:

The optical surface scanner Catalyst HD is a reliable and feasible patient positioning system without any additional radiation exposure. From the head to the thoracic and abdominal region, a decrease in accuracy was observed within a comparable range for Catalyst and laser-assisted positioning.

Feasibility of Intensity-Modulated Radiation Therapy in the Treatment of Advanced Cervical Chordoma

Aims and Background

Postoperative radiation is often given in cases of cervical chordoma because of the high incidence of local recurrence. The tumor mass usually surrounds the spinal cord and infiltrates vertebral bone. A combined technique using protons or electrons to boost the initial photon fields is generally applied. We evaluated the use of dynamic intensity-modulated radiation therapy as an alternative technique for treating advanced cervical chordoma.

Methods and Study Design

A female patient with incomplete resection of a vertebral chordoma surrounding C2-C3 was irradiated with a total dose of 58 Gy (ICRU point) in 2 Gy daily fractions for 29 days between December 2001 and January 2002. Beam arrangement consisted of seven 6 MV non-opposed coplanar fields. Pretreatment quality assurance included checking of the absolute dose at reference points and 2D dose map analysis. Treatment was delivered with a 120-leaf collimator in sliding window mode. To verify the daily setup, portal images at 0° and 90° were compared with the simulation images before treatment delivery (manual matching) and after treatment delivery (automatic anatomy matching).

Results and Conclusions

The mean dose to the planning target volume (PTV) was 57.6 ± 2.1 Gy covering 95% of the PTV per 95% isodose. The minimum dose to the PTV (D99) was 53.6 Gy in the overlapping area between the PTV and the spinal cord planning organ at risk volume (PRV). The maximum dose to the spinal cord was 42.2 Gy and to the spinal cord PRV (8 mm margin) 53.7 Gy. The mean dose to the parotid glands was 37.4 Gy (homolateral gland) and 19.5 Gy (contralateral gland). Average deviation in setup was -1.1 ± 2.5 mm (anterior-posterior), 2.4 ±1.3 mm (latero-lateral), 0.7 ± 0.9 mm (craniocaudal) and -0.43 ± 1° (rotation).

Conclusions

In the treatment of chordomas surrounding the spinal cord, intensity-modulated radiotherapy can provide high dose homogeneity and PTV coverage. Frequent digital portal image-based setup control is able to reduce random positioning errors for head and neck cancer patients immobilized with conventional thermoplastic masks.

Organ-specific modulation complexity score for the evaluation of dose delivery

The purpose of this study was to correlate the modulation complexity score (MCS) with organ location and to predict potential dose errors for organs before beam delivery for intensity-modulated radiation therapy (IMRT) dosimetry. Sixteen head and neck cancer patients treated with IMRT were selected. Distribution of the relative dose error on each beam was performed using forward projection to the planned dose to compute the predicted dose after doing per-beam quality assurance. Original organ-specific modulation complexity score (oMCS) was created based on a modified MLC, which depended on organ location. First, MCS was calculated based on the change in leaf position between adjacent MLC leaves. Second, the segment edge map (SEM) calculated from the intensity map for each beam was applied to the calculation volume. The oMCS with segment edge (oMCSedge) was derived from the product of oMCS and SEM. The correlation between the dose errors (planned and predicted) and oMCSedge values was evaluated for the target and organs at risk. We have also expanded the original MCS concept to oMCSedge including the organ location. We observed a moderate correlation between the dose errors and oMCSedge for all organs and volumes of interest except the gross tumor volume, brain stem, and spinal cord. In other organs, a moderate improvement in sensitivity was observed on the SEM, which was correlated with dose errors. Although the implementation of oMCSedge would be impractical for normal clinical settings, it is expected that oMCSedge would help a treatment planner to judge whether or not the treatment plan would be acceptably delivered.

Three-dimensional dose prediction and validation with the radiobiological gamma index based on a relative seriality model for head-and-neck IMRT

This study proposes a quality assurance (QA) method incorporating radiobiological factors based on the QUANTEC-determined tumor control probability and the normal tissue complication probability (NTCP) of head-and-neck intensity-modulated radiation therapy (HN-IMRT). Per-beam measurements were conducted for 20 cases using a 2D detector array. Three-dimensional predicted dose distributions within targets and organs at risk were reconstructed based on the per-beam QA results derived from differences between planned and measured doses. Under the predicted dose distributions, the differences between the physical and radiobiological gamma indices (PGI and RGI, respectively) based on the relative seriality (RS) model were evaluated. The NTCP values in the RS and Niemierko models were compared. The dose covers 98% (D98%) of the clinical target volume (CTV) decreased by 3.2% (P < 0.001), and the mean dose of the ipsilateral parotid increased by 6.3% (P < 0.001) compared with the original dose. RGI passing rates in the CTV and brain stem were greater than PGI ones by 5.8% (P < 0.001) and 2.0% (P < 0.001), respectively. The RS model's average NTCP values for the ipsilateral and contralateral parotids under the original dose were smaller than those of the Niemierko model by 9.0% (P < 0.001) and 7.0% (P < 0.001), respectively. The 3D predicted dose evaluation with RGI based on the RS model was introduced for QA of HN-IMRT, leading to dose evaluation for each organ with consideration of the radiobiological effect. This method constitutes a rational way to perform QA of HN-IMRT in clinical practice.

Setup errors in patients with head-neck cancer (HNC), treated using the Intensity Modulated Radiation Therapy (IMRT) technique: how it influences the customised immobilisation systems, patient's pain and anxiety

BACKGROUND

In patients with head-neck cancer treated with IMRT, immobility of the upper part of the body during radiation is maintained by means of customised immobilisation devices. The main purpose of this study was to determine how the procedures for preparation of customised immobilisation systems and the patients characteristics influence the extent of setup errors.

METHODS

A longitudinal, prospective study involving 29 patients treated with IMRT. Data were collected before CT simulation and during all the treatment sessions (528 setup errors analysed overall); the correlation with possible risk factors for setup errors was explored using a linear mixed model.

RESULTS

Setup errors were not influenced by the patient's anxiety and pain. Temporary removal of the thermoplastic mask before carrying out the CT simulation shows statistically borderline, clinically relevant, increase of setup errors (+24.7%, 95% CI: -0.5% - 55.8%). Moreover, a unit increase of radiation therapists who model the customised thermoplastic mask is associated to a -18% (-29.2% - -4.9%) reduction of the errors. The setup error is influenced by the patient's physical features; in particular, it increases both in patients in whom the treatment position is obtained with 'Shoulder down' (+27.9%, 2.2% - 59.7%) and in patients with 'Scoliosis/kyphosis' problems (+65.4%, 2.3% - 164.2%). Using a 'Small size standard plus customized neck support device' is associated to a -52.3% (-73.7% - -11.2%) reduction. The increase in number of radiation therapists encountered during the entire treatment cycle does not show associations. Increase in the body mass index is associated with a slight reduction in setup error by (-2.8%, -5% - -0.7%).

CONCLUSION

The position of the patient obtained by forcing the shoulders downwards, clinically significant scoliosis or kyphosis and the reduction of the number of radiation therapists who model the thermoplastic mask are found to be statistically significant risk factors that can cause an increase in setup errors, while the use of 'Small size' neck support device and patient BMI can diminish them.

Robustness quantification methods comparison in volumetric modulated arc therapy to treat head and neck cancer

BACKGROUND

To compare plan robustness of volumetric modulated arc therapy (VMAT) with intensity modulated radiation therapy (IMRT) and to compare the effectiveness of 3 plan robustness quantification methods.

METHODS AND MATERIALS

The VMAT and IMRT plans were created for 9 head and neck cancer patients. For each plan, 6 new perturbed dose distributions were computed using ±3 mm setup deviations along each of the 3 orientations. Worst-case analysis (WCA), dose-volume histogram (DVH) band (DVHB), and root-mean-square dose-volume histogram (RVH) were used to quantify plan robustness. In WCA, a shaded area in the DVH plot bounded by the DVHs from the lowest and highest dose per voxel was displayed. In DVHB, we displayed the envelope of all DVHs in band graphs of all the 7 dose distributions. The RVH represents the relative volume on the vertical axis and the root-mean-square-dose on the horizontal axis. The width from the first 2 methods at different target DVH indices (such as D95% and D5%) and the area under the RVH curve for the target were used to indicate plan robustness. Results were compared using Wilcoxon signed-rank test.

RESULTS

The DVHB showed that the width at D95% of IMRT was larger than that of VMAT (unit Gy) (1.59 vs 1.18) and the width at D5% of IMRT was comparable to that of VMAT (0.59 vs 0.54). The WCA showed similar results between IMRT and VMAT plans (D95%: 3.28 vs 3.00; D5%: 1.68 vs 1.95). The RVH showed the area under the RVH curve of IMRT was comparable to that of VMAT (1.13 vs 1.15). No statistical significance was found in plan robustness between IMRT and VMAT.

CONCLUSIONS

The VMAT is comparable to IMRT in terms of plan robustness. For the 3 quantification methods, WCA and DVHB are DVH parameter-dependent, whereas RVH captures the overall effect of uncertainties.

Radiation Oncology--New Approaches in Squamous Cell Cancer of the Head and Neck

The many advances in radiotherapy for squamous cell cancer of the head and neck described in this article will have significant effects on the ultimate outcomes of patients who receive this treatment. The technological and clinical advances should allow one to maintain or improve disease control, while moderating the toxicity associated with head and neck radiation therapy.

Setup uncertainties for inter-fractional head and neck cancer in radiotherapy

Purpose

The aim of this study is to determine the inter-fractional motion of cervical spine in radiotherapy (RT).

Materials and Methods

Eleven localized head and neck cancer patients who were treated from April 2014 to September 2015 were evaluated. Every patient underwent 3 times of computed tomography (CT) simulation with equivalent setting. Left-right (LR, x) and antero-posterior (AP, z) directional shift of cervical spine were evaluated using 33 number of CT image. In regard to random error, geometric changes were evaluated by 22 data set (compared the first obtained CT to second or third CT) by one-sample T test. Systemic error was evaluated by each patients’ data set (11 pairs) by paired T test.

Results

The mean random error of LR and AP translational shift of cervical spine were −0.39 ± 3.24 mm and −0.57 ± 0.99 mm respectively. The mean random error of translational change of AP direction showed statistical significance (p = 0.014). The mean random error of x and z rotational shift were −0.07 ± 0.29° and −0.05 ± 0.35°, respectively. The mean systemic error of translational shift of LR and AP direction were −0.64 ± 2.57 mm and −0.33 ± 1.22 mm, respectively. The mean systemic error of rotational shift of x and z were 0.01 ± 0.18° and −0.27 ± 0.33°, respectively. The mean systemic error of rotational changes of z direction showed statistical difference (p = 0.022).

Conclusions

We have to be aware of the inter-fractional motion of cervical spine in head and neck RT and give enough margins in RT planning.

Dosimetric impact of setup errors in head and neck cancer patients treated by image-guided radiotherapy

To assess and analyze the impact of setup uncertainties on target volume coverage and doses to organs at risk (OAR) in head and neck cancer (HNC) patients treated by image-guided radiotherapy (IGRT). Translational setup errors in 25 HNC patients were observed by kilovoltage cone beam computed tomography (kV CBCT). Two plans were generated. Plan one – the original plan which was the initially optimized and approved plan of the patient. All patients were treated according to their respective approved plans at a defined isocenter. Plan two – the plan sum which was the sum of all plans recalculated at a different isocenter according to setup errors in x, y, and z-direction. Plan sum was created to evaluate doses that would have been received by planning target volume (PTV) and OARs if setup errors were not corrected. These 2 plans were analyzed and compared in terms of target volume coverage and doses to OARs. A total 503 kV CBCT images were acquired for evaluation of setup errors in 25 HNC patients. The systematic (mean) and random errors (standard deviation) combined for 25 patients in x, y, and z directions were 0.15 cm, 0.21 cm, and 0.19 cm and 0.09 cm, 0.12 cm, and 0.09 cm, respectively. The study showed that there was a significant difference in PTV coverage between 2 plans. The doses to various OARs showed a nonsignificant increase in the plan sum. The correction of translational setup errors is essential for IGRT treatment in terms of delivery of planned optimal doses to target volume.

Deformable image registration and interobserver variation in contour propagation for radiation therapy planning

Deformable image registration (DIR) and interobserver variation inevitably introduce uncertainty into the treatment planning process. The purpose of the current work was to measure deformable image registration (DIR) errors and interobserver variability for regions of interest (ROIs) in the head and neck and pelvic regions. Measured uncertainties were combined to examine planning margin adequacy for contours propagated for adaptive therapy and to assess the trade‐off of DIR and interobserver uncertainty in atlas‐based automatic segmentation. Two experienced dosimetrists retrospectively contoured brainstem, spinal cord, anterior oral cavity, larynx, right and left parotids, optic nerves, and eyes on the planning CT (CT1) and attenuation‐correction CT of diagnostic PET/CT (CT2) for 30 patients who received radiation therapy for head and neck cancer. Two senior radiation oncology residents retrospectively contoured prostate, bladder, and rectum on the postseed‐implant CT (CT1) and planning CT (CT2) for 20 patients who received radiation therapy for prostate cancer. Interobserver variation was measured by calculating mean Hausdorff distances between the two observers' contours. CT2 was deformably registered to CT1 via commercially available multipass B‐spline DIR. CT2 contours were propagated and compared with CT1 contours via mean Hausdorff distances. These values were summed in quadrature with interobserver variation for margin analysis and compared with interobserver variation for statistical significance using two‐tailed t‐tests for independent samples (α=0.05). Combined uncertainty ranged from 1.5‐5.8 mm for head and neck structures and 3.1‐3.7 mm for pelvic structures. Conventional 5 mm margins may not be adequate to cover this additional uncertainty. DIR uncertainty was significantly less than interobserver variation for four head and neck and one pelvic ROI. DIR uncertainty was not significantly different than interobserver variation for four head and neck and one pelvic ROI. DIR uncertainty was significantly greater than interobserver variation for two head and neck and one pelvic ROI. The introduction of DIR errors may offset any reduction in interobserver variation by using atlas‐based automatic segmentation.

PACS number(s): 87.57.nj, 87.55.D‐

Positioning reproducibility with and without rotational corrections for 2 head and neck immobilization systems

PURPOSE

The purpose of this study was to evaluate the impact of offline rotational corrections and assess intrafraction motion for head and neck (H&N) cancer patients immobilized with and without a custom neck cushion.

METHODS AND MATERIALS

Fifty H&N cancer patients were immobilized and imaged with pretreatment and posttreatment cone beam computed tomography (CBCT) for each treatment fraction. Of these patients, 25 had a custom neck cushion added to their immobilization. Each CBCT was registered to the simulation computed tomography offline. Registrations were performed with automatching tools and a matching volume of interest that consisted of a 5-mm expansion around the mandible, occipital bone, C1/C2, and C7/T1. To determine positioning accuracy, the registration was inspected to confirm these bony anatomy structures were contained within a 3- or 5-mm expansion of the simulation position. If not, the registration was repeated with rotational corrections included and re-evaluated. For each fraction, intrafraction motion was also quantified through the difference between the pretreatment and posttreatment CBCT registration coordinates.

RESULTS

For translational registrations, the bony anatomy in pretreatment imaging was outside the 3-mm or 5-mm expansion structure, respectively, for 49% and 15% of fractions on average for patients without a custom headrest and for 48% and 13% of fractions on average for patients with a custom headrest. The addition of rotational corrections reduced these numbers to 21% and 4% and to 28% and 6%, respectively. Intrafraction motion was significantly lower for patients immobilized with the addition of a custom neck cushion: 1.0 ± 0.5 mm compared with 1.8 ± 1.6 mm for patients with the standard headrest only (P = .02). This was reflected in posttreatment positioning accuracy, which was significantly reduced in the case of the standard headrest compared with pretreatment imaging (P values of < .001 to .048).

CONCLUSIONS

Rotational corrections significantly improved pretreatment patient positioning accuracy (P < .001). Intrafraction motion was reduced significantly through the addition of a custom neck cushion and resulted in an increase in posttreatment positioning accuracy for these patients.

Estimation of daily interfractional larynx residual setup error after isocentric alignment for head and neck radiotherapy: quality assurance implications for target volume and organs-at-risk margination using daily CT on- rails imaging

Larynx may alternatively serve as a target or organs at risk (OAR) in head and neck cancer (HNC) image-guided radiotherapy (IGRT). The objective of this study was to estimate IGRT parameters required for larynx positional error independent of isocentric alignment and suggest population-based compensatory margins. Ten HNC patients receiving radiotherapy (RT) with daily CT on-rails imaging were assessed. Seven landmark points were placed on each daily scan. Taking the most superior-anterior point of the C5 vertebra as a reference isocenter for each scan, residual displacement vectors to the other six points were calculated postisocentric alignment. Subsequently, using the first scan as a reference, the magnitude of vector differences for all six points for all scans over the course of treatment was calculated. Residual systematic and random error and the necessary compensatory CTV-to-PTV and OAR-to-PRV margins were calculated, using both observational cohort data and a bootstrap-resampled population estimator. The grand mean displacements for all anatomical points was 5.07 mm, with mean systematic error of 1.1 mm and mean random setup error of 2.63 mm, while bootstrapped POIs grand mean displacement was 5.09 mm, with mean systematic error of 1.23 mm and mean random setup error of 2.61 mm. Required margin for CTV-PTV expansion was 4.6 mm for all cohort points, while the bootstrap estimator of the equivalent margin was 4.9 mm. The calculated OAR-to-PRV expansion for the observed residual setup error was 2.7 mm and bootstrap estimated expansion of 2.9 mm. We conclude that the interfractional larynx setup error is a significant source of RT setup/delivery error in HNC, both when the larynx is considered as a CTV or OAR. We estimate the need for a uniform expansion of 5 mm to compensate for setup error if the larynx is a target, or 3 mm if the larynx is an OAR, when using a nonlaryngeal bony isocenter.

Evaluation of interfraction patient setup errors for image-guided prostate and head-and-neck radiotherapy using kilovoltage cone beam and megavoltage fan beam computed tomography

Purpose

To analyse interfraction setup using two different image guidance modalities for prostate and head-and-neck (H&N) cancer treatment.

Materials and methods

Seventy-two prostate and 60 H&N cancer patients, imaged with kilovoltage cone beam computed tomography (KVCBCT) or megavoltage fan beam computed tomography (MVFBCT), were studied retrospectively. The daily displacements in mediolateral (ML), craniocaudal (CC) and anteroposterior (AP) dimensions were investigated. The setup errors were calculated to determine the clinical target volume to planning target volume (CTV-to-PTV) margins.

Results

Based on 1,606 KVCBCT and 2,054 MVFBCT scans, average interfraction shifts in ML, CC and AP direction for H&N cases were 0·5 ± 1·5, −0·3 ± 2·0, 0·3 ± 1·7 mm using KVCBCT, 0·2 ± 1·9, −0·2 ± 2·4 and 0·0 ± 1·7 mm using MVFBCT. For prostate cases, average interfraction displacements were −0·3 ± 3·9, 0·2 ± 2·4, 0·4 ± 3·8 mm for MVFBCT and −0·2 ± 2·7, −0·6 ± 2·9, −0·5 ± 3·4 mm for KVCBCT. The calculated CTV-to-PTV margins, if determined by image-guided radiotherapy (IGRT) data, were 5·6 mm (H&N) and 7·8 mm (prostate) for MVFBCT, compared with 4·8 mm and 7·2 mm for KVCBCT. We observed no statistically significant difference in daily repositioning using KVCBCT and MVFBCT in early, middle and late stages of the treatment course.

Conclusion

In the absence of IGRT, the CTV-to-PTV margin determined using IGRT data, may be varied for different imaging modalities for prostate and H&N irradiation.

Application of an independent dose calculation software for estimating the impact of inter-fractional setup shifts in Helical Tomotherapy treatments

The purpose of this study is to validate the capability of in-house independent point dose calculation software to be used as a second check for Helical Tomotherapy treatment plans. The software performed its calculations in homogenous conditions (using the Cheese phantom, which is a cylindrical phantom with radius 15 cm and length 18 cm) using a factor-based algorithm. Fifty patients, who were treated for pelvic (10), prostate (14), lung (10), head & neck (12) and brain (4) cancers, were used. Based on the individual patient kVCT images and the pretreatment MVCT images for each treatment fraction, the corresponding daily patient setup shifts in the IEC-X, IEC-Y, and IEC-Z directions were registered. For each patient, the registered fractional setup shifts were grouped into systematic and random shifts. The average systematic dosimetric variations showed small dose deviation for the different cancer types (1.0%-3.0%) compared to the planned dose. Of the fifty patients, only three had percent differences larger than 5%. The average random dosimetric variations showed relatively small dose deviations (0.2%-1.1%) compared to the planned dose. None of the patients had percent differences larger than 5%. By examining the individual fractions of each patient, it is observed that only in 31 out of 1358 fractions the percent differences exceeded the border of 5%. These results indicate that the overall dosimetric impact from systematic and random variations is small and that the software is a capable platform for independent point dose validation for the Helical Tomotherapy modality.

Dosimetry investigation of MOSFET for clinical IMRT dose verification

In IMRT, patient-specific dose verification is followed regularly at each centre. Simple and efficient dosimetry techniques play a very important role in routine clinical dosimetry QA. The MOSFET dosimeter offers several advantages over the conventional dosimeters such as its small detector size, immediate readout, immediate reuse, multiple point dose measurements. To use the MOSFET as routine clinical dosimetry system for pre-treatment dose verification in IMRT, a comprehensive set of experiments has been conducted, to investigate its linearity, reproducibility, dose rate effect and angular dependence for 6 MV x-ray beam. The MOSFETs shows a linear response with linearity coefficient of 0.992 for a dose range of 35 cGy to 427 cGy. The reproducibility of the MOSFET was measured by irradiating the MOSFET for ten consecutive irradiations in the dose range of 35 cGy to 427 cGy. The measured reproducibility of MOSFET was found to be within 4% up to 70 cGy and within 1.4% above 70 cGy. The dose rate effect on the MOSFET was investigated in the dose rate range 100 MU/min to 600 MU/min. The response of the MOSFET varies from -1.7% to 2.1%. The angular responses of the MOSFETs were measured at 10 degrees intervals from 90 to 270 degrees in an anticlockwise direction and normalized at gantry angle zero and it was found to be in the range of 0.98 ± 0.014 to 1.01 ± 0.014. The MOSFETs were calibrated in a phantom which was later used for IMRT verification. The measured calibration coefficients were found to be 1 mV/cGy and 2.995 mV/cGy in standard and high sensitivity mode respectively. The MOSFETs were used for pre-treatment dose verification in IMRT. Nine dosimeters were used for each patient to measure the dose in different plane. The average variation between calculated and measured dose at any location was within 3%. Dose verification using MOSFET and IMRT phantom was found to quick and efficient and well suited for a busy radiotherapy department.

Dosimetric influences of rotational setup errors on head and neck carcinoma intensity-modulated radiation therapy treatments

The purpose of this work is to investigate the dosimetric influence of the residual rotational setup errors on head and neck carcinoma (HNC) intensity-modulated radiation therapy (IMRT) with routine 3 translational setup corrections and the adequacy of this routine correction. A total of 66 kV cone beam computed tomography (CBCT) image sets were acquired on the first day of treatment and weekly thereafter for 10 patients with HNC and were registered with the corresponding planning CT images, using 2 3-dimensional (3D) rigid registration methods. Method 1 determines the translational setup errors only, and method 2 determines 6-degree (6D) setup errors, i.e., both rotational and translational setup errors. The 6D setup errors determined by method 2 were simulated in the treatment planning system and were then corrected using the corresponding translational data determined by method 1. For each patient, dose distributions for 6 to 7 fractions with various setup uncertainties were generated, and a plan sum was created to determine the total dose distribution through an entire course and was compared with the original treatment plan. The average rotational setup errors were 0.7°± 1.0°, 0.1°±1.9°, and 0.3°±0.7° around left-right (LR), anterior-posterior (AP), and superior-inferior (SI) axes, respectively. With translational corrections determined by method 1 alone, the dose deviation could be large from fraction to fraction. For a certain fraction, the decrease in prescription dose coverage (Vp) and the dose that covers 95% of target volume (D95) could be up to 15.8% and 13.2% for planning target volume (PTV), and the decrease in Vp and the dose that covers 98% of target volume (D98) could be up to 9.8% and 5.5% for the clinical target volume (CTV). However, for the entire treatment course, for PTV, the plan sum showed that the average Vp was decreased by 4.2% and D95 was decreased by 1.2 Gy for the first phase of IMRT with a prescription dose of 50 Gy. For CTV, the plan sum showed that the average Vp was decreased by 0.8% and D98, relative to prescription dose, was not decreased. Among these 10 patients, the plan sum showed that the dose to 1-cm(3) spinal cord (D(1 cm(3))) increased no more than 1 Gy for 7 patients and more than 2 Gy for 2 patients. The average increase in D(1 cm(3)) was 1.2 Gy. The study shows that, with translational setup error correction, the overall CTV Vp has a minor decrease with a 5-mm margin from CTV to PTV. For the spinal cord, a noticeable dose increase was observed for some patients. So to decide whether the routine clinical translational setup error correction is adequate for this HNC IMRT technique, the dosimetric influence of rotational setup errors should be evaluated carefully from case to case when organs at risk are in close proximity to the target.

Variable planning margin approach to account for locoregional variations in setup uncertainties

PURPOSE

To develop a method for creating variable planning margins around a clinical treatment volume (CTV) and to evaluate its application in head and neck cancer radiotherapy in accounting for locoregional variations of nonrigid setup uncertainties.

METHODS

Ten computed tomography (CT) images (with a resolution of 0.68 × 0.68 × 2.5 mm(3)) of a head and neck cancer patient were acquired from the first two weeks of treatment for this study. Five rigid structures (the C2, C5, and caudal C7 vertebrae, mandible, and jugular notch) were used as the landmarks for creating variable local margins. At different CTV locations, local margins were calculated as the weighted average of margins determined at different landmark points from previous studies. The weight was determined by a Gaussian falloff function of the distance between the current location and each landmark point. The CTV delineated on the planning CT image, spanning from the upper portion of the mouth to the lower part of the neck, was expanded to form the planning treatment volume (PTV) with either variable or the conventional constant margins. To evaluate the target coverage, the original planning CTV was deformably mapped to each daily treatment CT using a deformable image registration method. We examined the overlap of the deformed CTV and the rigidly aligned PTV for each margin design strategy and compared the efficacy of the variable margin with the constant margin approach.

RESULTS

For the variable margin approach with a baseline C2 margin of 2.5 mm in the left-right, anterior-posterior, and superior-inferior directions, an average of 99.2% of the CTV was within the PTV, and for the approach with a constant 2.5 mm margin, an average of 97.9% of the CTV was within the PTV. With a baseline margin of 2.0 mm, the variable margin approach had an average coverage of 97.8%, similar to that of the constant 2.5 mm margin approach. However, its average nonoverlapped PTV proportion was 32.4%, smaller than that of the constant 2.5 mm margin approach (33.7%). Paired t-tests of computations from the ten treatment fractions showed no significant difference in CTV coverage for the variable margin approach with a baseline of 2.0 mm and the constant 2.5 margin approach (p = 0.054), but the nonoverlapped PTV proportion was significantly smaller for the variable margin approach with a baseline of 2.0 mm than for the constant 2.5 mm margin approach (p < 0.0001). The CTV coverage with the variable margin approach was also significantly higher than with the constant margin approach in the lower neck area, where a larger setup error normally occurs.

CONCLUSIONS

We implemented a variable margin approach to account for locoregional variations of setup uncertainties for head and neck cancer radiotherapy, and demonstrated the effectiveness of this approach when compared with the conventional global constant margin expansion approach, where the treatment target spreads out to a broad region. As variable margin data become available and more clinical studies are performed, this approach could be applicable to other treatment sites as well.

Interobserver variation in parotid gland delineation: a study of its impact on intensity-modulated radiotherapy solutions with a systematic review of the literature

OBJECTIVES

This study evaluates the interobserver variation in parotid gland delineation and its impact on intensity-modulated radiotherapy (IMRT) solutions.

METHODS

The CT volumetric data sets of 10 patients with oropharyngeal squamous cell carcinoma who had been treated with parotid-sparing IMRT were used. Four radiation oncologists and three radiologists delineated the parotid gland that had been spared using IMRT. The dose-volume histogram (DVH) for each study contour was calculated using the IMRT plan actually delivered for that patient. This was compared with the original DVH obtained when the plan was used clinically.

RESULTS

70 study contours were analysed. The mean parotid dose achieved during the actual treatment was within 10% of 24 Gy for all cases. Using the study contours, the mean parotid dose obtained was within 10% of 24 Gy for only 53% of volumes by radiation oncologists and 55% of volumes by radiologists. The parotid DVHs of 46% of the study contours were sufficiently different from those used clinically, such that a different IMRT plan would have been produced.

CONCLUSION

Interobserver variation in parotid gland delineation is significant. Further studies are required to determine ways of improving the interobserver consistency in parotid gland definition.

Helical tomotherapy setup variations in canine nasal tumor patients immobilized with a bite block

The purpose of our study was to compare setup variation in four degrees of freedom (vertical, longitudinal, lateral, and roll) between canine nasal tumor patients immobilized with a mattress and bite block, versus a mattress alone. Our secondary aim was to define a clinical target volume (CTV) to planning target volume (PTV) expansion margin based on our mean systematic error values associated with nasal tumor patients immobilized by a mattress and bite block. We evaluated six parameters for setup corrections: systematic error, random error, patient-patient variation in systematic errors, the magnitude of patient-specific random errors (root mean square [RMS]), distance error, and the variation of setup corrections from zero shift. The variations in all parameters were statistically smaller in the group immobilized by a mattress and bite block. The mean setup corrections in the mattress and bite block group ranged from 0.91 mm to 1.59 mm for the translational errors and 0.5°. Although most veterinary radiation facilities do not have access to Image-guided radiotherapy (IGRT), we identified a need for more rigid fixation, established the value of adding IGRT to veterinary radiation therapy, and define the CTV-PTV setup error margin for canine nasal tumor patients immobilized in a mattress and bite block.

The Effect of Registration Volume Extent on Residual Errors Assessed Using Cone-Beam Computed Tomography in Radiation Treatment of Head and Neck Cancer

PURPOSE

The objective of this study was to investigate the effect of the varying extent of cone-beam computed tomography (CBCT) registration volumes (RVs) on setup errors for head and neck (H&N) radiotherapy.

METHODS AND MATERIALS

Daily CBCT images for 31 patients receiving H&N intensity-modulated radiotherapy (IMRT) were reviewed. Registrations using anatomically defined RVs with a fixed superior border at base of sella and varying inferior extent were used retrospectively to evaluate patient setup. The inferior extent was defined as the number of cervical bodies included, from none (C0) to six (C6). The frequency of residual displacements at four landmarks (clivus, vertebral bodies C5-C6, manubrium-sterni, and anterior body of mandible) was assessed.

RESULTS

Expansion of the RVs inferiorly reduced the occurrence of residual displacements for the C5-C6 vertebral bodies (from 57% to 93% of fractions with residual displacements ≤ 3 mm) and increased the rate of simultaneous positioning of C5-C6 and clivus (from 41% to 76%). Maximum residual displacements for mandible (48%-64% ≤ 3 mm) and manubrium (73%-81% ≤ 3 mm) varied somewhat by the inferior extent of the RV. Residual displacements for clivus were small (88%-96% ≤ 3 mm) in all cases. Random and systematic errors were clinically acceptable for a 5-mm planning margin around the clinical targets.

CONCLUSIONS

In conclusion, expansion of the RV inferiorly to include C6 will improve the positioning of structures in the C5-C6 region (adjacent nodal zones 3 and 4) without compromising clival positioning. Insufficient inferior extent of the RV reduces reliability of low neck positioning. Substantial variability can occur for structures not included in the RV. Based on these data, we use the C6 RV except in cases with planning concerns outside this volume.

Adaptive radiation therapy for head and neck cancer-can an old goal evolve into a new standard?

Current head and neck intensity-modulated radiotherapy (IMRT) techniques cause significant toxicity. This may be explained in part by the fact that IMRT cannot compensate for changes in the location of disease and normal anatomy during treatment, leading to exposure of at-risk bystander tissues to higher-than-anticipated doses. Adaptive radiotherapy (ART) is a novel approach to correct for daily tumor and normal tissue variations through online or offline modification of original IMRT target volumes and plans. ART has been discussed on a conceptual level for many years, but technical limitations have hampered its integration into routine care. In this paper, we review the key anatomic, dosimetric, and treatment delivery issues at play in current investigational development of head and neck ART. We also describe pilot findings from initial clinical deployment of head and neck ART, as well as emerging pathways of future research.

Dosimetric impact of daily setup variations during treatment of canine nasal tumors using intensity-modulated radiation therapy

Intensity-modulated radiation therapy (IMRT) can be employed to yield precise dose distributions that tightly conform to targets and reduce high doses to normal structures by generating steep dose gradients. Because of these sharp gradients, daily setup variations may have an adverse effect on clinical outcome such that an adjacent normal structure may be overdosed and/or the target may be underdosed. This study provides a detailed analysis of the impact of daily setup variations on optimized IMRT canine nasal tumor treatment plans when variations are not accounted for due to the lack of image guidance. Setup histories of ten patients with nasal tumors previously treated using helical tomotherapy were replanned retrospectively to study the impact of daily setup variations on IMRT dose distributions. Daily setup shifts were applied to IMRT plans on a fraction-by-fraction basis. Using mattress immobilization and laser alignment, mean setup error magnitude in any single dimension was at least 2.5 mm (0-10.0 mm). With inclusions of all three translational coordinates, mean composite offset vector was 5.9 +/- 3.3 mm. Due to variations, a loss of equivalent uniform dose for target volumes of up to 5.6% was noted which corresponded to a potential loss in tumor control probability of 39.5%. Overdosing of eyes and brain was noted by increases in mean normalized total dose and highest normalized dose given to 2% of the volume. Findings suggest that successful implementation of canine nasal IMRT requires daily image guidance to ensure accurate delivery of precise IMRT distributions when non-rigid immobilization techniques are utilized. Unrecognized geographical misses may result in tumor recurrence and/or radiation toxicities to the eyes and brain.

Evaluation of the radiobiological impact of anatomic modifications during radiation therapy for head and neck cancer: can we simply summate the dose?

BACKGROUND AND PURPOSE

Adaptive strategies in radiotherapy (RT) require the knowledge of the total dose given to every organ of the body. Because of anatomical changes and setup errors non-rigid registration is necessary to map the different dose fractions to a common reference. This study evaluates practically if the accumulation of all of these registered dose fractions must take radiobiology into account in a classical clinical setting.

MATERIALS AND METHODS

Ten patients with head and neck tumors treated by chemo-RT were used. Contrast-enhanced CT scans were acquired prior and during RT following delivery of mean doses of 14.2, 24.5, 35.0 and 44.9 Gy and the planned pre-treatment helical tomotherapy sinograms were applied on the per-treatment CTs to create a series of per-treatment dose distributions corresponding to each per-treatment CT image. In order to calculate the cumulative dose distribution, the per-treatment dose maps were non-rigidly deformed by using the deformation map computed by a non-rigid registration. The deformed dose maps were then summed in two ways: one while taking radiobiology into account and one without. These two strategies were compared using clinical surrogates in the target volumes (TV) and in surrounding organs at risk (OAR).

RESULTS

The differences between the strategies, while statistically significant (p<0.05), are clinically irrelevant. In the OARs, the mean differences stay in the 0.01-0.07 Gy range for the total dose. In the targets, all mean differences stay in the 0.001-0.012 Gy range. However, some local high difference spots appear leading to punctual errors as high as 2.5 Gy.

CONCLUSION

If using current radiotherapy practices and clinical recommendations based on dose surrogates computed globally on OARs and TVs, one does not need to take radiobiological effects into account while accumulating total dose as these lead to very small differences compared to a simple accumulation technique consisting of a linear sum of the dose fractions. However, care must be taken if other adaptive strategies, based on local rather than global information, are used.

Dosimetric assessment of rigid setup error by CBCT for HN-IMRT

Dose distributions in HN-IMRT are complex and may be sensitive to the treatment uncertainties. The goals of this study were to evaluate: 1) dose differences between plan and actual delivery and implications on margin requirement for HN-IMRT with rigid setup errors; 2) dose distribution complexity on setup error sensitivity; and 3) agreement between average dose and cumulative dose in fractionated radiotherapy. Rigid setup errors for HN-IMRT patients were measured using cone-beam CT (CBCT) for 30 patients and 896 fractions. These were applied to plans for 12HN patients who underwent simultaneous integrated boost (SIB) IMRT treatment. Dose distributions were recalculated at each fraction and summed into cumulative dose. Measured setup errors were scaled by factors of 2-4 to investigate margin adequacy. Two plans, direct machine parameter optimization (DMPO) and fluence only (FO), were available for each patient to represent plans of different complexity. Normalized dosimetric indices, conformity index (CI) and conformation number (CN) were used in the evaluation. It was found that current 5 mm margins are more than adequate to compensate for rigid setup errors, and that standard margin recipes overestimate margins for rigid setup error in SIB HN-IMRT because of differences in acceptance criteria used in margin evaluation. The CTV-to-PTV margins can be effectively reduced to 1.9 mm and 1.5 mm for CTV1 and CTV2. Plans of higher complexity and sharper dose gradients are more sensitive to setup error and require larger margins. The CI and CN are not recommended for cumulative dose evaluation because of inconsistent definition of target volumes used. For fractionated radiotherapy in HN-IMRT, the average fractional dose does not represent the true cumulative dose received by the patient through voxel-by-voxel summation, primarily due to the setup error characteristics, where the random component is larger than systematic and different target regions get underdosed at each fraction.

Acrylonitrile Butadiene Styrene (ABS) plastic based low cost tissue equivalent phantom for verification dosimetry in IMRT

A novel IMRT phantom was designed and fabricated using Acrylonitrile Butadiene Styrene (ABS) plastic. Physical properties of ABS plastic related to radiation interaction and dosimetry were compared with commonly available phantom materials for dose measurements in radiotherapy. The ABS IMRT phantom has provisions to hold various types of detectors such as ion chambers, radiographic/radiochromic films, TLDs, MOSFETs, and gel dosimeters. The measurements related to pretreatment dose verification in IMRT of carcinoma prostate were carried out using ABS and Scanditronix‐Wellhofer RW3 IMRT phantoms for five different cases. Point dose data were acquired using ionization chamber and TLD discs, while Gafchromic EBT and radiographic EDR2 films were used for generating 2D dose distributions. Treatment planning system (TPS) calculated and measured doses in ABS plastic and RW3 IMRT phantom were in agreement within ± 2%. The dose values at a point in a given patient acquired using ABS and RW3 phantoms were found comparable within 1%. Fluence maps and dose distributions of these patients generated by TPS and measured in ABS IMRT phantom were also found comparable both numerically and spatially. This study indicates that ABS plastic IMRT phantom is a tissue‐equivalent phantom and, dosimetrically, it is similar to solid/plastic water IMRT phantoms. Although this material is demonstrated for IMRT dose verification, it can also be used as a tissue‐equivalent phantom material for other dosimetry purposes in radiotherapy.

PACS number: 87.53Kn, 87.55Qr, 87.53Bn and 87.55Km

Megavoltage versus kilovoltage image guidance for efficiency and accuracy in head and neck IMRT

Accurate patient positioning is vitally important in intensity modulated radiation therapy (IMRT) for head and neck (H&N) cancer. The introduction of kilovoltage (kV) on-board imaging (OBI) at our centre was anticipated to improve the accuracy and efficiency of H&N IMRT patient position verification over traditional megavoltage (MV) electronic portal imaging (EPI). This study compares these imaging systems with a phantom accuracy study and retrospective analysis of imaging workload in H&N IMRT at our centre. Six therapists performed online evaluation of phantom images, and residual positional errors for each system were recorded. The largest residual error was 1 mm for OBI and 3 mm for EPI. The estimated improvement in residual error in OBI over EPI was 0.57 mm (95% confidence interval 0.33–0.81 mm), suggesting treatment staff would be better able to detect set-up deviations with OBI. Electronic treatment records of 20 H&N IMRT patients (10 verified daily with MV EPI and 10 with kV OBI) were analysed. Mean imaging session duration was 9.51 min for EPI and 9.76 min for OBI. Analysis found no evidence of an effect on duration due to the imaging system used for this subset of patients (p = 0.664).

Obtaining normal tissue constraints using intensity modulated radiotherapy (IMRT) in patients with oral cavity, oropharyngeal, and laryngeal carcinoma

The purpose of this study was to evaluate normal tissue dose constraints while maintaining planning target volume (PTV) prescription without reducing PTV margins. Sixteen patients with oral cavity carcinoma (group I), 27 patients with oropharyngeal carcinoma (group II), and 28 patients with laryngeal carcinoma (group III) were reviewed. Parotid constraints were a mean dose to either parotid < 26 Gy (PP1), 50% of either parotid < 30 Gy (PP2), or 20 cc of total parotid < 20 Gy (PP3). Treatment was intensity modulated radiation therapy (IMRT) with simultaneous integrated boost (SIB). All patients met constraints for cord and brain stem. The mandibular constraints were met in 66%, 29%, and 57% of patients with oral, oropharyngeal, and laryngeal cancers, respectively. Mean dose of 26 Gy (PP1) was achieved in 44%, 41%, and 38% of oral, oropharyngeal, and laryngeal patients. PP2 (parotid constraint of 30 Gy to less than 50% of one parotid) was the easiest to achieve (group I, II, and III: 82%, 76%, and 78%, respectively). PP3 (20 cc of total parotid < 20 Gy) was difficult, and was achieved in 25%, 17%, and 35% of oral, oropharyngeal, and laryngeal patients, respectively. Mean parotid dose of 26 Gy was met 40% of the time. However, a combination of constraints allowed for sparing of the parotid based on different criteria and was met in high numbers. This was accomplished without reducing PTV-parotid overlap. What dose constraint best correlates with subjective and objective functional outcomes remains a focus for future study.

Impact of residual setup error on parotid gland dose in intensity-modulated radiation therapy with or without planning organ-at-risk margin

PURPOSE

To estimate the dosimetric impact of residual setup errors on parotid sparing in head-and-neck (H&N) intensity-modulated treatments and to evaluate the effect of employing an PRV (planning organ-at-risk volume) margin for the parotid gland.

PATIENTS AND METHODS

Ten patients treated for H&N cancer were considered. A nine-beam intensity-modulated radiotherapy (IMRT) was planned for each patient. A second optimization was performed prescribing dose constraint to the PRV of the parotid gland. Systematic setup errors of 2 mm, 3 mm, and 5 mm were simulated. The dose-volume histograms of the shifted and reference plans were compared with regard to mean parotid gland dose (MPD), normal-tissue complication probability (NTCP), and coverage of the clinical target volume (V95% and equivalent uniform dose [EUD]); the sensitivity of parotid sparing on setup error was evaluated with a probability-based approach.

RESULTS

MPD increased by 3.4%/mm and 3.0%/mm for displacements in the craniocaudal and lateral direction and by 0.7%/ mm for displacements in the anterior-posterior direction. The probability to irradiate the parotid with a mean dose > 30 Gy was > 50%, for setup errors in cranial and lateral direction and < 10% in the anterior-posterior direction. The addition of a PRV margin improved parotid sparing, with a relative reduction in NTCP of 14%. The PRV margin compensates for setup errors of 3 mm and 5 mm (MPD < or = 30 Gy in 87% and 60% of cases), without affecting clinical target volume coverage (V95% and EUD variations < 1% and < 1 Gy).

CONCLUSION

The parotid gland is more sensitive to craniocaudal and lateral displacements. A setup error of 2 mm guarantees an MPD < or = 30 Gy in most cases, without adding a PRV margin. If greater displacements are expected/accepted, an adequate PRV margin could be used to meet the clinical parotid gland constraint of 30 Gy, without affecting target volume coverage.

Interfractional set-up errors evaluation by daily electronic portal imaging of IMRT in head and neck cancer patients

INTRODUCTION

Interfractional set-up errors were assessed from daily portal images (PI) registration for head and neck cancer patients. We aimed to evaluate whether a daily PI is worthwhile and we derived the Planning Target Volume (PTV) margins from the estimation of systematic and random errors.

MATERIAL AND METHODS

Twenty patients were treated in supine position with a fixed 5-point mask immobilisation system and head-and-knee supports. DRRs (Digitally Reconstructed Radiograph) were obtained from the planning CT-scan and considered the reference images to be compared with two orthogonal PI by matching bone anatomy landmarks. A total of 567 PI were done. For the set-up errors analysis, we determined the systematic, random, and overall standard deviations (SD), as well as the overall means in three directions (cranio caudal CC, medio lateral ML and anterior posterior AP). PTV-margins were calculated according to three methods. Differences of SD regarding the overall displacements among portals performed every day and each 2, 3, or 4 days were tested.

RESULTS

The systematic set-up errors were less than 1 mm in the three directions whereas the random set-up errors were around 2 mm. PTV margins varied from 3 to 4 mm in the 3 directions. Corrections were significant in the CC direction only, in which the set-up error increased significantly when the scenario of one PI every 3 fractions was adopted.

CONCLUSIONS

It is of practical importance to apply on-line protocols with contouring of the bony landmarks on the PI in order to decrease the systematic mean error in this patient group. This study suggested that a PI in AP and ML directions once a week and every two days in the CC direction would be adequate to overcome the problem of set-up errors.