Cone-Beam CT Assessment of Interfraction and Intrafraction Setup Error of Two Head-and-Neck Cancer Thermoplastic Masks

Randomized self-controlled study comparing open-face vs. closed immobilization masks in fractionated cranial radiotherapy

PURPOSE

To compare patient discomfort and immobilisation performance of open-face and closed immobilization masks in cranial radiotherapy.

MATERIAL AND METHODS

This was a single-center randomized self-controlled clinical trial. At CT simulation, an open-face and closed mask was made for each patient and treatment plans with identical dose prescription were generated for each mask. Patients were randomised to start treatment with an open-face or closed mask. Masks were switched halfway through the treatment course; every patient was their own control. Patients self-reported discomfort, anxiety and pain using the visual analogue scale (VAS). Inter- and intrafraction set-up variability was measured with planar kV imaging and a surface guided radiotherapy (SGRT) system for the open-face masks.

RESULTS

30 patients with primary or metastatic brain tumors were randomized - 29 completed radiotherapy to a median total dose of 54 Gy (range 30-60 Gy). Mean discomfort VAS score was significantly lower with open-face masks (0.5, standard deviation 1.0) vs. closed masks (3.3, standard deviation 2.9), P < 0.0001. Anxiety and pain VAS scores were significantly lower with open-face masks (P < 0.0001). Closed masks caused more discomfort in infraorbital (P < 0.001) and maxillary (P = 0.02) areas. Two patients and 27 patients preferred closed or open-face masks, respectively. Interfraction longitudinal shifts and roll and yaw rotations were significantly smaller and lateral shifts were significantly larger with closed masks in combination with the laser system (P < 0.05) compared to open masks in combination with a SGRT system. Intrafraction variability did not differ between the masks.

CONCLUSIONS

Open-face masks are associated with decreased patient discomfort without compromising patient positioning and immobilisation accuracy.

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.

Clinical value of styrofoam fixation in intracranial tumor radiotherapy

Objective

To analyze the application value of two postural fixation techniques.(styrofoam combined with head mask and fixed headrest combined with head mask) in intracranial tumor radiotherapy via cone beam computed tomography (CBCT).

Methods

This study included 104 patients with intracranial tumors undergoing radiotherapy. The patients were divided into two groups: Group A (54 cases with styrofoam fixation) and Group B (50 cases with fixed headrest fixation). The positional deviation in 3D space between the two groups was compared using CBCT. The set-up errors were expressed as median (25th percentile, 75th percentile)or M(p25, p75) since the set-up errors in all directions were not normally distributed,The Mann-Whitney U test was performed.

Results

The age and gender of patients in the two groups were not significantly different. The set-up errors of A in lateral (X), longitudinal (Y), vertical (Z), and yaw(Rtn) axes were 1.0 (0,1) mm, 1.0 (0,1) mm, 1.0 (0,2) mm, and 0.4 (0.1, 0.8) degrees, respectively while the set-up errors of B were 1.0 (0,1) mm, 1.0 (1,2) mm, 1.0 (0,2) mm, and 0.5 (0.15,0.9) degrees, respectively. Moreover, patients in the styrofoam group had significantly smaller set-up errors in the Y-axis than patients in the headrest group (p=0.001). However, set-up errors in the X, Z, and Rtn axes were not significantly different between the two groups. The expansion boundaries of the target area in the X, Y, and Z directions were 1.77 mm, 2.45 mm, and 2.47 mm, respectively. The outer expansion boundaries of the headrest group were 2.03 mm, 3.88 mm, and 2.57 mm in X, Y, and Z directions, respectively. The set-up times of groups A and B were (32.71 ± 5.21) seconds and (46.57 ± 6.68) seconds, respectively (p=0.014). Patients in group A had significantly better comfort satisfaction than patients in group B (p=0.001).

Conclusion

Styrofoam plus head thermoplastic mask body fixation technique has a higher positional accuracy in intracranial tumor radiotherapy than headrest plus head thermoplastic mask fixation. Besides, styrofoam plus head thermoplastic mask body fixation technique is associated with improved positioning efficiency, and better comfort than headrest plus head thermoplastic mask fixation, and thus can be effectively applied for intracranial tumor radiotherapy positioning.

Setup error assessment based on “Sphere-Mask” Optical Positioning System: Results from a multicenter study

Background

The setup accuracy plays an extremely important role in the local control of tumors. The purpose of this study is to verify the feasibility of "Sphere-Mask" Optical Positioning System (S-M_OPS) for fast and accurate setup.

Methods

From 2016 to 2021, we used S-M_OPS to supervise 15441 fractions in 1981patients (with the cancer in intracalvarium, nasopharynx, esophagus, lung, liver, abdomen or cervix) undergoing intensity-modulated radiation therapy (IMRT), and recorded the data such as registration time and mask deformation. Then, we used S-M_OPS, laser line and cone beam computed tomography (CBCT) for co-setup in 277 fractions, and recorded laser line-guided setup errors and S-M_OPS-guided setup errors with CBCT-guided setup result as the standard.

Results

S-M_OPS supervision results: The average time for laser line-guided setup was 31.75s. 12.8% of the reference points had an average deviation of more than 2 mm and 5.2% of the reference points had an average deviation of more than 3 mm. Co-setup results: The average time for S-M_OPS-guided setup was 7.47s, and average time for CBCT-guided setup was 228.84s (including time for CBCT scan and manual verification). In the LAT (left/right), VRT (superior/inferior) and LNG (anterior/posterior) directions, laser line-guided setup errors (mean±SD) were -0.21±3.13mm, 1.02±2.76mm and 2.22±4.26mm respectively; the 95% confidence intervals (95% CIs) of laser line-guided setup errors were -6.35 to 5.93mm, -4.39 to 6.43mm and -6.14 to 10.58mm respectively; S-M_OPS-guided setup errors were 0.12±1.91mm, 1.02±1.81mm and -0.10±2.25mm respectively; the 95% CIs of S-M_OPS-guided setup errors were -3.86 to 3.62mm, -2.53 to 4.57mm and -4.51 to 4.31mm respectively.

Conclusion

S-M_OPS can greatly improve setup accuracy and stability compared with laser line-guided setup. Furthermore, S-M_OPS can provide comparable setup accuracy to CBCT in less setup time.

Evaluation of novel 3D-printed and conventional thermoplastic stereotactic high-precision patient fixation masks for radiotherapy

Purpose

For stereotactic radiation therapy of intracranial malignancies, a patient’s head needs to be immobilized with high accuracy. Fixation devices such as invasive stereotactic head frames or non-invasive thermoplastic mask systems are often used. However, especially stereotactic high-precision masks often cause discomfort for patients due to a long manufacturing time during which the patient is required to lie still and because the face is covered, including the mouth, nose, eyes, and ears. To avoid these issues, the target was to develop a non-invasive 3D-printable mask system with at least the accuracy of the high-precision masks, for producing masks which can be manufactured in the absence of patients and which allow the eyes, mouth, and nose to be uncovered during therapy.

Methods

For four volunteers, a personalized 3D-printed mask based on magnetic resonance imaging (MRI) data was designed and manufactured using fused filament fabrication (FFF). Additionally, for each of the volunteers, a conventional thermoplastic stereotactic high-precision mask from Brainlab AG (Munich, Germany) was fabricated. The intra-fractional fixation accuracy for each mask and volunteer was evaluated using the motion-correction algorithm of functional MRI measurements with and without guided motion.

Results

The average values for the translations and rotations of the volunteers’ heads lie in the range between ±1 mm and ±1° for both masks. Interestingly, the standard deviations and the relative and absolute 3D displacements are lower for the 3D-printed masks compared to the Brainlab masks.

Conclusion

It could be shown that the intra-fractional fixation accuracy of the 3D-printed masks was higher than for the conventional stereotactic high-precision masks.

Human-level comparable control volume mapping with a deep unsupervised-learning model for image-guided radiation therapy

PURPOSE

To develop a deep unsupervised learning method with control volume (CV) mapping from patient positioning daily CT (dCT) to planning computed tomography (pCT) for precise patient positioning.

METHODS

We propose an unsupervised learning framework, which maps CVs from dCT to pCT to automatically generate the couch shifts, including translation and rotation dimensions. The network inputs are dCT, pCT and CV positions in the pCT. The output is the transformation parameter of the dCT used to setup the head and neck cancer (HNC) patients. The network is trained to maximize image similarity between the CV in the pCT and the CV in the dCT. A total of 554 CT scans from 158 HNC patients were used for the evaluation of the proposed model. At different points in time, each patient had many CT scans. Couch shifts are calculated for the testing by averaging the translation and rotation from the CVs. The ground-truth of the shifts come from bone landmarks determined by an experienced radiation oncologist.

RESULTS

The system positioning errors of translation and rotation are less than 0.47 mm and 0.17°, respectively. The random positioning errors of translation and rotation are less than 1.13 mm and 0.29°, respectively. The proposed method enhanced the proportion of cases registered within a preset tolerance (2.0 mm/1.0°) from 66.67% to 90.91% as compared to standard registrations.

CONCLUSIONS

We proposed a deep unsupervised learning architecture for patient positioning with inclusion of CVs mapping, which weights the CVs regions differently to mitigate any potential adverse influence of image artifacts on the registration. Our experimental results show that the proposed method achieved efficient and effective HNC patient positioning.

Impact of Cold Weather on Setup Errors in Radiotherapy

Objective

To investigate the influence of cold weather on setup errors of patients with chest and pelvic disease in radiotherapy.

Methods

The image-guided data of the patients were collected from the Radiotherapy Center of Cancer Hospital Affiliated to Guangxi Medical University from October 2020 to February 2021. During this period, the cold weather days were December 15, 16, and 17, 2020, and January 7 and 8, 2021. For body fixation in radiotherapy, an integrated plate and a thermoplastic mold were employed in 18 patients with chest disease, while an integrated plate and a vacuum pad were applied in 19 patients with pelvic disease. All patients underwent cone beam computed tomography (CBCT) scans in the first five treatments and once a week thereafter. The obtained data were registered to the planning CT image to get the setup errors of the patient in the translational direction including X, Y, and Z axes and rotational direction including R _X , R _Y , and R _Z . Then, the Mann-Whitney U test was performed. The expansion boundary values of the chest and pelvis were calculated according to the formula M _PTV=2.5∑+0.7δ.

Results

A total of 286 eligible results of CBCT scans were collected. There were 138 chest CBCT scans, including 26 taken in cold weather and 112 in usual weather, and 148 pelvic CBCT scans, including 33 taken in cold weather and 115 in usual weather. The X-, Y-, and Z-axis translational setup errors of patients with chest disease in the cold weather group were 0.16 (0.06, 0.32) cm, 0.25 (0.17, 0.52) cm, and 0.35 (0.21, 0.47) cm, respectively, and those in the usual weather group were 0.14 (0.08, 0.29) cm, 0.23 (0.13, 0.37) cm, and 0.18 (0.1, 0.35) cm, respectively. The results indicated that there was a statistical difference in the Z-axis translational error between the cold weather group and the usual weather group (U = 935.5; p=0.005 < 0.05), while there was no statistical difference in the rotational error between the two groups. The external boundary values of X, Y, and Z axes in the cold weather group were 0.57 cm, 0.92 cm, and 0.99 cm, respectively, and those in the usual weather group were 0.57 cm, 0.78 cm, and 0.68 cm, respectively. There was no significant difference in the translational and rotational errors of patients with pelvic disease between the cold weather group and the usual weather group (p < 0.05). The external boundary values of X, Y, and Z axes were 0.63 cm, 0.79 cm, and 0.68 cm in the cold weather group and 0.61 cm, 0.79 cm, and 0.61 cm in the usual weather group, respectively.

Conclusion

The setup error of patients undergoing radiotherapy with their bodies fixed by an integrated plate and a thermoplastic mold was greater in cold weather than in usual weather, especially in the ventrodorsal direction.

Development of a new poly-ε-caprolactone with low melting point for creating a thermoset mask used in radiation therapy

This study aimed to develop a poly-ε-caprolactone (PCL) material that has a low melting point while maintaining the deformation ability. The new PCL (abbreviated as 4b45/2b20) was fabricated by mixing two types of PCL with different molecular weights, numbers of branches, and physical properties. To investigate the melting point, crystallization temperature, elastic modulus, and elongation at break for 4b45/2b20 and three commercially available masks, differential scanning calorimetry and tensile tests were performed. The melting point of 4b45/2b20 was 46.0 °C, and that of the commercially available masks was approximately 56.0 °C (55.7 °C–56.5 °C). The elastic modulus at 60 °C of 4b45/2b20 was significantly lower than the commercially available masks (1.1 ± 0.3 MPa and 46.3 ± 5.4 MPa, p = 0.0357). In addition, the elongation at break of 4b45/2b20 were significantly larger than the commercially available masks (275.2 ± 25.0% and 216.0 ± 15.2%, p = 0.0347). The crystallization temperature of 4b45/2b20 (22.1 °C) was clinically acceptable and no significant difference was found in the elastic modulus at 23 °C (253.7 ± 24.3 MPa and 282.0 ± 44.3 MPa, p = 0.4). As a shape memory-based thermoset material, 4b45/2b20 has a low melting point and large deformation ability. In addition, the crystallization temperature and strength are within the clinically acceptable standards. Because masks made using the new PCL material are formed with less pressure on the face than commercially available masks, it is a promising material for making a radiotherapy mask that can reduce the burden on patients.

Accuracy of dose calculation on iterative CBCT for head and neck radiotherapy

PURPOSE

To evaluate the feasibility of the use of iterative cone-beam computed tomography (CBCT) for dose calculation in the head and neck region.

METHODS

This study includes phantom and clinical studies. All acquired CBCT images were reconstructed with Feldkamp-Davis-Kress algorithm-based CBCT (FDK-CBCT) and iterative CBCT (iCBCT) algorithm. The Hounsfield unit (HU) consistency between the head and body phantoms was determined in both reconstruction techniques. Volumetric modulated arc therapy (VMAT) plans were generated for 16 head and neck patients on a planning CT scan, and the doses were recalculated on FDK-CBCT and iCBCT with Anisotropic Analytical Algorithm (AAA) and Acuros XB (AXB). As a comparison of the accuracy of dose calculations, the absolute dosimetric difference and 1%/1 mm gamma passing rate analysis were analyzed.

RESULTS

The difference in the mean HU values between the head and body phantoms was larger for FDK-CBCT (max value: 449.1 HU) than iCBCT (260.0 HU). The median dosimetric difference from the planning CT were <1.0% for both FDK-CBCT and iCBCT but smaller differences were found with iCBCT (planning target volume D_50%: 0.38% (0.15-0.59%) for FDK-CBCT, 0.28% (0.13-0.49%) for iCBCT, AAA; 0.14% (0.04-0.19%) for FDK-CBCT, 0.07% (0.02-0.20%) for iCBCT). The mean gamma passing rate was significantly better in iCBCT than FDK-CBCT (AAA: 98.7% for FDK-CBCT, 99.4% for iCBCT; AXB: 96.8% for FDK_CBCT, 97.5% for iCBCT).

CONCLUSION

The iCBCT-based dose calculation in VMAT for head and neck cancer was accurate compared to FDK-CBCT.

Evaluating the positional uncertainty of intrafraction, adjacent fields, and daily setup with the BrainLAB ExacTrac system in patients who are receiving craniospinal irradiation

Purpose

To investigate the daily setup, interfraction motion, variability in the junction areas, and dosimetric effect in craniospinal irradiation (CSI) patients.

Methods

Fifteen CSI patients who had undergone split‐field IMRT were followed in the study. Previous, middle, and posttreatment, each target volume position was evaluated using the ExacTrac system. Interfraction and intrafraction motions, the margin of the junction in adjacent targets volumes, and the dosimetric effect of the longitudinal residual error were analyzed.

Results

The lowest attainment rate within the tolerance of the initial setup error was 66.79% in six directions. The values of the initial error were within 15 mm (SD 4.5 mm) in the translation direction and 5° (SD 1.3°) in the rotation direction after the transposition of the treatment isocenter. With the guidance of the ExacTrac system, the interfraction and intrafraction residual errors were almost within the tolerance after correction, the margin of CTV‐to‐PCTV was in the range of target expansion criteria. The residual longitudinal errors resulted in only slight changes in the mean doses of PGTV and PCTV, while the maximum dose of the spinal cord increased by 16.1%. The patients did not exhibit any side‐effects by the overall treatment during the follow‐up period.

Conclusions

Position correction is necessary after setup and the transposition of the treatment isocenter. Intra‐fraction motion in the lateral direction should be monitored throughout treatment. The position errors in junction areas are almost within the tolerance after correction. The patients did not exhibit any side‐effects by the overall treatment.

A comparative study between open-face and closed-face masks for head and neck cancer (HNC) in radiation therapy

Aim

To determine the setup reproducibility in the radiation treatment of Head and Neck (HN) patients using open face head and shoulder masks (OHSM) with customized headrest (CHR) versus standard closed head and shoulder masks (CHSM) and to determine the patient's level of comfort and satisfaction for both masks.

Methods

Forty patients were prospectively randomized into two groups using simple random sampling. Group 1 was assigned with CHSMs, immobilized with a standard HR (SHR) while Group 2 was assigned with OHSMs, and immobilized with CHR. Cone beam computed tomography (CBCT) was taken the first 3 days, followed by weekly CBCT (prior treatment) with results registered to the planning CT to determine translational and rotational inter-fraction shifts and to verify accuracy. Mean (M) and standard deviation (SD) of the systematic and random setup errors of the 2 arms in the translational and rotational directions were analyzed, using Independent t-test and Mann-Whitney U test. Patient comfort was measured using a Likert questionnaire.

Results

The vertical, lateral, longitudinal and Z/roll rotational shifts were not significantly different between the two masks. X/yaw and Y/pitch rotational shifts were significantly greater in Group 2 versus Group 1, for both systematic (p = 0.009 and 0.046, respectively) and random settings (p = 0.016 and 0.020) but still within three degrees. Patients reported higher neck and shoulder comfort (p = 0.020) and overall satisfaction (p = 0.026) using the OHSM with the CHR versus the CHSM with the SHR during CT simulation.

Conclusion

Open masks provide comparable yet comfortable immobilization to closed masks for HN radiotherapy.

Prospective observational study to estimate set-up errors and optimise PTV margins in patients undergoing IMRT for head and neck cancers from a Government cancer centre of Eastern India

Background:

The head and neck cancers as a whole are the most common cancers among males in India. Technological advancements have led to an improvement in radiation therapy (RT) techniques with subsequent reduction in normal tissue complications. To correct patient set-up errors, an off-line correction method like no action level (NAL) protocol may be used as a preferred protocol particularly for a busy department. The objectives of the study were to measure the translational set-up errors using kV cone-beam computed tomography (CBCT) in patients undergoing intensity modulated radiotherapy (IMRT) in head and neck cancers and also to optimise clinical target volume (CTV) to planning target volume (PTV) margin using NAL protocol.

Material and methods:

On the first 5 days of RT, patient’s position was verified by kV-CBCT and then weekly during the course of treatment. The comparison between the reference and kV-CBCT images was performed, and the shifts measured and recorded. The mean error from the initial five consecutive fractions was corrected on the sixth daily fraction. Displacements in all the directions were measured. The population systematic and random errors were determined and used to estimate PTV margins according to the van Herk formula.

Results:

A total of 322 images were analysed. Before correction, 15, 12 and 9% patients had systematic error ≥3 mm on X, Y and Z axes, but after correction this was reduced to 9, 0 and 0%. The total percentage of patients whose set-up margin was ≥5 mm before correction was 5, 6·25, 3·75%, but after correction it reduced to 1·88, 0, and 0·63%. The margins of total population were reduced to 63, 65 and 56% after correction on X, Y and Z axes, respectively.

Conclusion:

A simple off-line NAL protocol can correct the set-up errors without daily on-line imaging in patients undergoing IMRT and hence acting as a resource sparing alternative. Five millimetre margin to CTVs was adequate and safe to overcome the problem of set-up errors in head and neck IMRT.

Effect of Surgical Mask on Setup Error in Head and Neck Radiotherapy

PURPOSE

With the widespread prevalence of Corona Virus Disease 2019 (COVID-19), cancer patients are suggested to wear a surgical mask during radiation treatment. In this study, cone beam CT (CBCT) was used to investigate the effect of surgical mask on setup errors in head and neck radiotherapy.

METHODS

A total of 91 patients with head and neck tumors were selected. CBCT was performed to localize target volume after patient set up. The images obtained by CBCT before treatment were automatically registered with CT images and manually fine-tuned. The setup errors of patients in 6 directions of Vrt, Lng, Lat, Pitch, Roll and Rotation were recorded. The patients were divided into groups according to whether they wore the surgical mask, the type of immobilization mask used and the location of the isocenter. The setup errors of patients were calculated. A t-test was performed to detect whether it was statistically significant.

RESULTS

In the 4 groups, the standard deviation in the directions of Lng and Pitch of the with surgical mask group were all higher than that in the without surgical mask group. In the head-neck-shoulder mask group, the mean in the Lng direction of the with surgical mask group was larger than that of the without surgical mask group. In the lateral isocenter group, the mean in the Lng and Pitch directions of the with surgical mask group were larger than that of the without surgical mask group. The t-test results showed that there was significant difference in the setup error between the 2 groups (p = 0.043 and p = 0.013, respectively) only in the Lng and Pitch directions of the head-neck-shoulder mask group. In addition, the setup error of 6 patients with immobilization open masks exhibited no distinguished difference from that of the patients with regular immobilization masks.

CONCLUSION

In the head and neck radiotherapy patients, the setup error was affected by wearing surgical mask. It is recommended that the immobilization open mask should be used when the patient cannot finish the whole treatment with a surgical mask.

Volumetric modulated arc therapy treatment planning based on virtual monochromatic images for head and neck cancer: effect of the contrast-enhanced agent on dose distribution

Virtual monochromatic images (VMIs) at a lower energy level can improve image quality but the computed tomography (CT) number of iodine contained in the contrast‐enhanced agent is dramatically increased. We assessed the effect of the use of contrast‐enhanced agent on the dose distributions in volumetric modulated arc therapy (VMAT) planning for head and neck cancer (HNC). Based on the VMIs at 40 keV (VMI_40keV), 60 keV(VMI_60keV), and 77 keV (VMI_77keV) of a tissue characterization phantom, lookup tables (LUTs) were created. VMAT plans were generated for 15 HNC patients based on contrast‐enhanced‐ (CE‐) VMIs at 40‐, 60‐, and 77 keV using the corresponding LUTs, and the doses were recalculated based on the noncontrast‐enhanced‐ (nCE‐) VMIs. For all structures, the difference in CT numbers owing to the contrast‐enhanced agent was prominent as the energy level of the VMI decreased, and the mean differences in CT number between CE‐ and nCE‐VMI was the largest for the clinical target volume (CTV) (125.3, 55.9, and 33.1 HU for VMI_40keV, VMI_60keV, and VMI_77keV, respectively). The mean difference of the dosimetric parameters (D_99%, D_50%, D_1%, D_mean, and D_0.1cc) for CTV and OARs was <1% in the treatment plans based on all VMIs. The maximum difference was observed for CTV in VMI_40keV (2.4%), VMI_60keV (1.9%), and VMI_77keV (1.5%) plans. The effect of the contrast‐enhanced agent was larger in the VMAT plans based on the VMI at a lower energy level for HNC patients. This effect is not desirable in a treatment planning procedure.

Technical Note: Rotational positional error corrected intrafraction set-up margins in stereotactic radiotherapy: A spatial assessment for coplanar and noncoplanar geometry

PURPOSE

The aim of this study is to calculate setup margin based on six-dimensional (6D) corrected residual positional errors from kV cone beam computed tomography (CBCT) and from intrafraction projection kV imaging in coplanar and in noncoplanar couch positions in stereotactic radiotherapy.

METHODS

Six dimensional positional corrections were carried out before patient treatments, using a robotic couch and CBCT matching. A CBCT and stereoscopic ExacTrac image were acquired post-table position correction. Further, a series of intrafraction ExacTrac images were obtained for the variable couch position. Translational and rotational errors were identified as lateral (X), longitudinal (Y), vertical (Z); roll (Ɵ°), pitch (Φ°) and yaw (Ψ°). A total of 699 intrafraction image sets (361 coplanar and 338 noncoplanar) for 51 SRS/SRT patients were analysed. Rotational errors were corrected in terms of translational coordinates. Residual set-up margins were calculated from CBCT shifts. ExacTrac shifts give residual + intrafraction setup margins as a function of coplanar and noncoplanar couch positions.

RESULTS

The average residual positional error obtained from CBCT in X, Y, Z, Ɵ, Φ, Ψ were 0.1 ± 0.4 mm, 0.0 ± 0.6 mm, 0.0 ± 0.5 mm, 0.2 ± 0.8°, 0.1 ± 0.6° and -0.1 ± 0.7° respectively. For ExacTrac, the shits were -0.5 ± 0.9 mm, -0.0 ± 1mm, -0.6 ± 1.0mm, 0.4 ± 0.9°, -0.2 ± 0.6°, and -0.0 ± 0.8°. CBCT calculated linear setup margins in X, Y, Z direction were 0.5, 1.2, and 1 mm respectively. ExacTrac yielded coplanar and noncoplanar linear setup margins were 1.2, 1.3, 1.5, 1.4, 1.5, and 2.1 mm respectively.

CONCLUSION

CBCT-based gross residual set-up margin is equal to 1 mm. ExacTrac calculated residual plus intrafraction setup margin falls within a 2 mm range; attributed to intrafraction patient movement, table position inaccuracies, and poor image fusion in noncoplanar geometry. There could be variations in the required additional margin between centers and between machines, which require further studies.

Performance of the cone beam computed tomography-based patient positioning system on the Gamma Knife Icon™

PURPOSE

Cone beam computed tomography (CBCT) imaging has been implemented on the Leksell Gamma Knife^® Icon™ for assessing patient positioning in mask-based Gamma Knife radiosurgery. The purpose of this study was to evaluate the performance of the CBCT-based patient positioning system as a tool for frameless Gamma Knife radiosurgery.

METHODS

Daily quality assurance (QA) CBCT precision test results from a 12-month period were analyzed for the geometric accuracy and the stability of the imager. The performance of the image acquisition module and the image registration algorithm was evaluated using an anthropomorphic head phantom (CIRS Inc., Norfolk, VA) and a XYZR axis manual positioning stage (TOAUTO Inc., Guangdong, China). The head phantom was fixed on a mask adaptor and manually translated in the X, Y, Z directions or rotated around the X, Y, Z axes in the range of ±10 mm or ±10º. A CBCT scan was performed after each manual position setup followed by an image registration to the reference scan. To assess the overall setup uncertainties in fractionated treatment, two cylindrical Presage phantoms (Heuris Inc., Skillman, NJ) of 15 cm diameter and 10 cm height were irradiated with identical prescription dose and shot placement following standard mask-based treatment workflow according to two different fraction schedules: a single fraction treatment of 7.5 Gy and a 5-fraction treatment with 1.5 Gy per fraction.

RESULTS

The averaged vector deviations of the four marks from their preset values are 0.087, 0.085, 0.095, and 0.079 mm from the 212 daily QA tests. The averaged displacements in the X, Y, Z coordinates and the pitch, yaw, roll angles from the image registration tests are 0.23, 0.27, 0.14, 0.32º, 0.19º, 0.31º from the manual setup. The corresponding maximum differences are 0.41, 0.33, 0.29 mm, 0.45º, 0.31º, and 0.43º, respectively. Compared to the treatment plan using the 2% & 1 mm criteria, the averaged 2D Gamma passing rate is 98.25% for the measured dose distribution from the Presage phantom with 1-fraction irradiation and 95.12% for the 5-fraction irradiation. The averaged Gamma passing rates are 99.53% and 98.16% for the 1-fraction and 5-fraction irradiations using the 2% & 2 mm criteria.

CONCLUSIONS

The CBCT imager and the image registration algorithm can reproduce phantom position with <0.5 mm/0.5º uncertainty. A systematic contribution from the interfraction phantom repositioning procedure was observed in the Gamma analysis over the irradiated volumes of two end-to-end test phantoms.

Quantifying the dosimetric impact of organ-at-risk delineation variability in head and neck radiation therapy in the context of patient setup uncertainty

The purpose of this study was to quantify the potential dosimetric impact of delineation variability (DV) in head and neck radiation therapy (RT) when inherent patient setup variability (SV) is also considered. The impact of DV was assessed by generating plans with multiple structure sets, cross-evaluating them, including SV, across sets, and determining P _PQM: the probability of achieving organ-specific plan quality metrics (PQM). DV was incorporated by: (1) using multiple organ at risk (OAR) structure sets delineated by independent manual observers; and (2) randomly perturbing manually generated OARs to generate alternatives with varying levels of uncertainty (low, medium, and high DV). For each structure set, independent VMAT plans were auto-generated to meet clinical PQMs. Each plan was cross-evaluated using OARs from multiple structure sets with simulated SV including per-fraction random (σ _s) and per-treatment-course systematic (Σ_s) setup errors. The dosimetric impact of DV was assessed by examining P _PQM with and without SV/DV. Clinically significant differences were defined by those that exceeded differences caused by a  +2% output variation. Without including SV, simulated DV at the medium level reduced P _PQM by an average of 5.5% for all OARs with D _max PQMs. This reduction decreased to 2.8% for SV  =  2 mm and 2.4% for SV  =  4 mm (the average P _PQM reduction due to 2% output errors was 2.7%). For OARs with D _mean PQMs, the average P _PQM reduction was 0.9% for SV  =  0 and  ⩽0.1% for SV  ⩾  2 mm. The effect of DV was larger for OARs that directly abutted a target volume than for those that did not. These trends were also observed with real DV from multi-observer delineations. The dosimetric impact of DV appeared to decrease when random and systematic SV was considered. Sensitivity to DV was affected by OAR objective type (i.e. D _mean versus D _max objectives) as well as distance from the target volume.

Dosimetric influence of pitch in patient positioning for radiotherapy of long treatment volumes; the usefulness of six degree of freedom couch

OBJECTIVE:

Pitch, the rotation around the transversal axis of the patient during radiotherapy has little impact on the dose distribution of small spherical treatment volumes; however it might affect treatment of long volumes requiring a correction with a six degree of freedom couch.

METHODS:

We included 10 patients each with nasopharyngeal carcinoma (NPC) and esophageal cancer, treated with volumetric modulated arc therapy. Pitch was simulated by tilting the planning CT in ventral and dorsal direction by ± 1.5° and ± 3°. Verification plans were calculated on the tilted datasets and were compared to the original plan and the dose constraints of the organs at risk (OAR).

RESULTS:

The deviation in dose to the planning target volume is increasing with the degree of pitch with mean changes of up to 2% for NPC and 1% for esophageal cancer. The most affected OAR in NPC patients are brainstem (max. dose +6.0%) and spinal cord (max. dose +10.0%) when tilted by 3° dorsally and lenses (max. dose +3.3%), oral mucosa (mean dose +2.6%) and parotid glands (mean dose +4.3%) when tilted by 3° ventrally. For esophageal cancer patients, there was no significant change in dose to any OAR. Whereas for esophageal cancer, all tilted treatment plans were still clinically acceptable regarding OAR, 5 NPC plans would no longer be acceptable with a pitch of 1.5° ventral (N = 1), 3° ventral (N = 2) and 3° dorsal (N = 2).

CONCLUSION:

Planning target volume coverage in both tumor entities was only slightly affected, but pitch errors could be relevant for OAR in NPC patients.

ADVANCES IN KNOWLEDGE:

A correction with a six degree of freedom couch is recommended for NPC patients with a pitch mismatch of more than 1.5° to avoid exceeded doses to the OAR.

Analysis of which local set-up errors can be covered by a 5-mm margin for cone beam CT-guided radiotherapy for nasopharyngeal carcinoma

Objective:

To analyse which local set-up errors can be covered by a 5-mm margin for cone beam computed tomography (CBCT)-guided radiotherapy in nasopharyngeal carcinoma (NPC).

Methods:

11 regions of interest (ROIs) were registered for 24 NPC patients, with a total of 323 CBCT scans. According to the registration results, clinical target volume–planning target volume (CTV–PTV)/organs at risk-planning risk volume (OAR-PRV) margin analysis; Pearson correlation analysis; Bland–Altman plots; and a receiver operating characteristic (ROC) analysis were used to investigate which local set-up errors of substructure can be represented by the PTV_ROI.

Results:

The clinical target volume-PTV/OAR-planning risk volume margins were less than 5 mm for C1_ROI-C4_ROI, mandible (M_ROI), and sphenoid sinus (S_ROI) with respect to PTV_ROI. C1_ROI-C4_ROI, M_ROI, and S_ROI exhibited significant correlations and consistencies in the mediolateral, superior–inferior, and anteroposterior (AP) directions and significant receiver operating characteristic analysis results in the anteroposterior direction.

Conclusion:

Only the upper local set-up error of C1_ROI-C4_ROI, M_ROI, and S_ROI can be covered by a 5-mm margin for CBCT-guided NPC radiotherapy with a large ROI. Using these ROIs as an integral reference ROI is better than individual bony landmark.

Advances in knowledge:

This report is helpful to CBCT registration for NPC radiotherapy in clinical practice.

Set-up errors in head and neck cancer treated with IMRT technique assessed by cone-beam computed tomography: a feasible protocol

PURPOSE

To investigate set-up errors, suggest the adequate planning target volume (PTV) margin and image-guided radiotherapy frequency in head and neck (H&N) cancer treated with intensity-modulated radiotherapy (IMRT) assessed by kV cone-beam computed tomography (CBCT).

METHODS

We analyzed 360 CBCTs in 60 patients with H&N cancer treated with IMRT. The target delineation was contoured according to ICRU62. PTVs were generated by adding a 3-5 mm margin in all directions to the respective clinical target volumes. The kV CBCT images were obtained at first three days of irradiation and weekly thereafter. The overall mean displacement, range, systematic (∑) and random (σ) errors were calculated. Adequate PTV margins were calculated according to the van Herk formula (2.5∑ + 0.7r).

RESULTS

The mean of set-up errors was less than 2 mm in any direction. The overall frequency of set-up displacements greater than 3 mm was 3.9% in medial-lateral (ML) direction, 8% in superior-inferior (SI) direction, and 15.5% in anterior-posterior (AP) direction. The range of translations shifts was 0-9 mm in ML direction, 0-5 mm in SI direction and 0-10 mm in AP direction, respectively. After systematic set-up errors correction, the adequate margin to overcome the problem of set-up errors was found to be less than 3 mm.

CONCLUSION

Image-guided kV CBCT was effective for the evaluation of set-up accuracy in H&N cancer. The kV CBCT at first three fractions and followed-by weekly appears adequate for reducing significantly set-up errors in H&N cancer treated with IMRT technique. Finally, 3-5 mm PTV margins appear adequate and safe to overcome the problem of set-up errors.

3D-Printed masks as a new approach for immobilization in radiotherapy - a study of positioning accuracy

We developed a new approach to produce individual immobilization devices for the head based on MRI data and 3D printing technologies. The purpose of this study was to determine positioning accuracy with healthy volunteers.

3D MRI data of the head were acquired for 8 volunteers. In-house developed software processed the image data to generate a surface mesh model of the immobilization mask. After adding an interface for the couch, the fixation setup was materialized using a 3D printer with acrylonitrile butadiene styrene (ABS). Repeated MRI datasets (n=10) were acquired for all volunteers wearing their masks thus simulating a setup for multiple fractions. Using automatic image-to-image registration, displacements of the head were calculated relative to the first dataset (6 degrees of freedom).

The production process has been described in detail. The absolute lateral (x), vertical (y) and longitudinal (z) translations ranged between −0.7 and 0.5 mm, −1.8 and 1.4 mm, and −1.6 and 2.4 mm, respectively. The absolute rotations for pitch (x), yaw (y) and roll (z) ranged between −0.9 and 0.8°, −0.5 and 1.1°, and −0.6 and 0.8°, respectively. The mean 3D displacement was 0.9 mm with a standard deviation (SD) of the systematic and random error of 0.2 mm and 0.5 mm, respectively.

In conclusion, an almost entirely automated production process of 3D printed immobilization masks for the head derived from MRI data was established. A high level of setup accuracy was demonstrated in a volunteer cohort. Future research will have to focus on workflow optimization and clinical evaluation.

Minimal mask immobilization with optical surface guidance for head and neck radiotherapy

PURPOSE

Full face and neck thermoplastic masks provide standard-of-care immobilization for patients receiving H&N IMRT. However, these masks are uncomfortable and increase skin dose. The purpose of this pilot trial was to investigate the feasibility and setup accuracy of minimal face and neck mask immobilization with optical surface guidance.

METHODS

Twenty patients enrolled onto this IRB-approved protocol. Patients were immobilized with masks securing only forehead and chin. Shoulder movement was restricted by either moldable cushion or hand held strap retractors. Positional information, including isocenter location and CT skin contours, were imported to a commercial surface image guidance system. Patients typically received standard-of-care IMRT to 60-70 Gy in 30-33 fractions. Patients were first set up to surface markings with optical image guidance referenced to regions of interest (ROIs) on simulation CT images. Positioning was confirmed by in-room CBCT. Following six-dimensional robotic couch correction, a new optical real-time surface image was acquired to track intrafraction motion and to serve as a reference surface for setup at the next treatment fraction. Therapists manually recorded total treatment time as well as couch shifts based on kV imaging. Intrafractional ROI motion tracking was automatically recorded by the optical image guidance system. Patient comfort was assessed by self-administered surveys.

RESULTS

Setup error was measured as six-dimensional shifts (vertical/longitudinal/lateral/rotation/pitch/roll). Mean error values were -0.51 ± 2.42 mm, -0.49 ± 3.30 mm, 0.23 ± 2.58 mm, -0.15 ± 1.01o , -0.02 ± 1.19o , and 0.06 ± 1.08o , respectively. Average treatment time was 21.6 ± 8.4 mins). Subjective comfort during surface-guided treatment was confirmed on patient surveys.

CONCLUSION

These pilot results confirm feasibility of minimal mask immobilization combined with commercially available optical image guidance. Patient acceptance of minimal mask immobilization has been encouraging. Follow-up validation, with direct comparison to standard mask immobilization, appears warranted.

Impact of robust treatment planning on single- and multi-field optimized plans for proton beam therapy of unilateral head and neck target volumes

BACKGROUND

Proton beam therapy is promising for the treatment of head and neck cancer (HNC), but it is sensitive to uncertainties in patient positioning and particle range. Studies have shown that the planning target volume (PTV) concept may not be sufficient to ensure robustness of the target coverage. A few planning studies have considered irradiation of unilateral HNC targets with protons, but they have only taken into account the dose on the nominal plan, without considering anatomy changes occurring during the treatment course.

METHODS

Four pencil beam scanning (PBS) proton therapy plans were calculated for 8 HNC patients with unilateral target volumes: single-field (SFO) and multi-field optimized (MFO) plans, either using the PTV concept or clinical target volume (CTV)-based robust optimization. The dose was recalculated on computed tomography (CT) scans acquired during the treatment course. Doses to target volumes and organs at risk (OARs) were compared for the nominal plans, cumulative doses considering anatomical changes, and additional setup and range errors in each fraction. If required, the treatment plan was adapted, and the dose was compared with the non-adapted plan.

RESULTS

All nominal plans fulfilled the clinical specifications for target coverage, but significantly higher doses on the ipsilateral parotid gland were found for both SFO approaches. MFO PTV-based plans had the lowest robustness against range and setup errors. During the treatment course, the influence of the anatomical variation on the dose has shown to be patient specific, mostly independent of the chosen planning approach. Nine plans in four patients required adaptation, which led to a significant improvement of the target coverage and a slight reduction in the OAR dose in comparison to the cumulative dose without adaptation.

CONCLUSIONS

The use of robust MFO optimization is recommended for ensuring plan robustness and reduced doses in the ipsilateral parotid gland. Anatomical changes occurring during the treatment course might degrade the target coverage and increase the dose in the OARs, independent of the chosen planning approach. For some patients, a plan adaptation may be required.

A Pilot Study Evaluating the Effectiveness of Dual-Registration Image-Guided Radiotherapy in Patients with Oropharyngeal Cancer

PURPOSE

The purpose of the article was to determine the impact of Dual Registration (DR) image-guided radiotherapy (IGRT) on clinical judgement and treatment delivery for patients with oropharyngeal cancer before implementation.

METHODS

Ninety cone beam computed tomography images from 10 retrospective patients were matched using standard clipbox registration (SCR) and DR. Three IGRT specialist radiographers performed all registrations and evaluated by intraclass correlation to determine inter-rater agreement, Bland-Altman with 95% limits of agreement to determine differences between SCR and DR procedures, changes in clinical judgment, time taken to perform registrations, and radiographer satisfaction.

RESULTS

Inter-rater agreement between radiographers using both SCR and DR was high (0.867 and 0.917, P ≤ .0001). The 95% limits of agreement between SCR and DR procedures in the mediolateral, cranial-caudal, and ventrodorsal translational directions were -6.40 to +4.91, -7.49 to +6.05, and -7.00 to +5.44 mm, respectively. The mediolateral direction demonstrated significant proportional bias (P ≤ .001) suggesting non-agreement between SCR and DR. Eighty percent of DR matches resulted in a change in clinical judgement to ensure maximum target coverage. Mean registration times for SCR and DR were 94 and 115 seconds, respectively, and radiographers found DR feasible and satisfactory.

CONCLUSION

The standard method using SCR in patients with oropharyngeal cancer underestimates the deviation in the lower neck. In these patients, DR is an effective IGRT tool to ensure target coverage of the inferior neck nodes and has demonstrated acceptability to radiotherapy clinical practice.

Superiority in Robustness of Multifield Optimization Over Single-Field Optimization for Pencil-Beam Proton Therapy for Oropharynx Carcinoma: An Enhanced Robustness Analysis

PURPOSE

To compare the difference in robustness of single-field optimized (SFO) and robust multifield optimized (rMFO) proton plans for oropharynx carcinoma patients by an improved robustness analysis.

METHODS AND MATERIALS

We generated rMFO proton plans for 11 patients with oropharynx carcinoma treated with SFO intensity modulated proton therapy with simultaneous integrated boost prescription. Doses from both planning approaches were compared for the initial plans and the worst cases from 20 optimization scenarios of setup errors and range uncertainties. Expected average dose distributions per range uncertainty were obtained by weighting the contributions from the respective scenarios with their expected setup error probability, and the spread of dose parameters for different range uncertainties were quantified. Using boundary dose distributions created from 56 combined setup error and range uncertainty scenarios and considering the vanishing influence of setup errors after 30 fractions, we approximated realistic worst-case values for the total treatment course. Error bar metrics derived from these boundary doses are reported for the clinical target volumes (CTVs) and organs at risk (OARs).

RESULTS

The rMFO plans showed improved CTV coverage and homogeneity while simultaneously reducing the average mean dose to the constrictor muscles, larynx, and ipsilateral middle ear by 5.6 Gy, 2.0 Gy, and 3.9 Gy, respectively. We observed slightly larger differences during robustness evaluation, as well as a significantly higher average brainstem maximum and ipsilateral parotid mean dose for SFO plans. For rMFO plans, the range uncertainty-related spread in OAR dose parameters and many error bar metrics were found to be superior. The SFO plans showed a lower global maximum dose for single-scenario worst cases and a slightly lower mean oral cavity dose throughout.

CONCLUSIONS

An enhanced robustness analysis has been proposed and implemented into clinical systems. The benefit of better CTV coverage and OAR dose sparing in oropharynx carcinoma patients by rMFO compared with SFO proton plans is preserved in a robustness analysis with consideration of setup error and range uncertainty.

Utility of intraoral stents in external beam radiotherapy for head and neck cancer

AIM

This study aimed to assess the utility and stability of intraoral stent during intensity-modulated radiation therapy (IMRT).

BACKGROUND

The benefits of intraoral stents in radiotherapy are unclear.

MATERIALS AND METHODS

We analyzed 386 setup errors in 12 patients who received IMRT for head and neck cancers without intraoral stents (intraoral stent [-]) and 183 setup errors in 6 patients who received IMRT with intraoral stents (intraoral stent [+]). All patients were matched according to the immobilization method (masks and boards). Setup errors were measured as the distance from the initial setup based on the marking on the skin and mask to the corrected position based on bone matching on cone beam computed tomography.

RESULTS

The mean interfractional setup errors in the right-left, craniocaudal, anterior-posterior (AP), and three-dimensional (3D) directions were -0.33, 0.08, -0.25, and 2.75 mm in the intraoral stent (-) group and -0.37, 0.24, -0.63, and 2.42 mm in the intraoral stent (+) group, respectively (P = 0.50, 0.65, 0.01, and 0.02, respectively). The systematic errors for the same directions were 0.89, 1.46, 1.15, and 0.88 mm in the intraoral stent (-) group and 0.62, 1.69, 0.68, and 0.56 mm in the intraoral stents (+) group, respectively. The random errors were 1.43, 1.43, 1.44, and 1.22 mm in the intraoral stent (-) group and 1.06, 1.11, 1.05, and 0.92 mm in the intraoral stents (+) group, respectively.

CONCLUSION

Setup errors can be significantly reduced in the AP and 3D-directions by using intraoral stents.

Slant board immobilisation of head-and-neck radiotherapy patients who cannot tolerate a flat position

Purpose

Patients treated with intensity-modulated radiation therapy (IMRT) for head-and-neck cancer are often positioned supine on a carbon fibre board to which a thermoplastic mask is attached to immobilise the head and shoulders. For patients unable to tolerate a supine position, we developed a tilting board that accommodates a full-scale head-and-shoulder mask.

Materials and methods

Phantom measurements were obtained to confirm the dosimetric accuracy of our treatment planning system when using this board. A patient was simulated in the flat and tilted positions on the board. The two corresponding treatment plans were evaluated by comparing the target coverage and doses with organs at risk. The patient’s intra-fraction motion was quantified during his tilted treatments.

Results

Phantom measurements confirmed the accuracy of the dosimetric calculations. The tilted plan met dosimetric standards for clinical acceptability. The intra-fraction motion of the patient in the tilted position was >3 mm in any direction.

Conclusions

The tilting board met clinical requirements for IMRT planning and delivery. Full-scale head-and-shoulder immobilisation was achieved in a more tolerable tilted position.

Dosimetric impacts on skin toxicity for patients using topical agents and dressings during radiotherapy

Background and purpose

Skin care practices for radiotherapy patients are complicated by dosimetric concerns. This study measures the effect on skin dose of various topical agents and dressings.

Materials and methods

Superficial doses were measured under 17 topical agents and dressings and three clinical materials for reference. Dose was measured using a MOSFET detector under a 1 mm polymethyl methacrylate slab, with 6 MV photon beams at 100 cm source to surface distance.

Results

Relative skin dose under reference materials was 128% (thermoplastic mask), 158% (5 mm bolus) and 171% (10 mm bolus). Under a realistic application of topical agent (0·5 mm), relative skin doses were 106–111%. All dry dressings yielded relative dose of ≤111%; two wet dressings yielded higher relative doses (133 and 141%).

Conclusions

Under clinically relevant conditions, no cream, gel or dry dressing increased the skin dose beyond that seen with a thermoplastic mask. Dressings soaked with water produced less skin dose than 5 mm bolus. This may be unacceptable if wet dressings are in place for the majority of the treatment course. Our results suggest that skin care practices should not be limited by dosimetric concerns when using a 6 MV photon beam.

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

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

PACS number(s): 87.55.ne

Analysis of precision in tumor tracking based on optical positioning system during radiotherapy

Tumor tracking is performed during patient set-up and monitoring of respiratory motion in radiotherapy. In the clinical setting, there are several types of equipment for this set-up such as the Electronic Portal imaging Device (EPID) and Cone Beam CT (CBCT). Technically, an optical positioning system tracks the difference between the infra ball reflected from body and machine isocenter. Our objective is to compare the clinical positioning error of patient setup between Cone Beam CT (CBCT) with the Optical Positioning System (OPS), and to evaluate the traditional positioning systems and OPS based on our proposed approach of patient positioning. In our experiments, a phantom was used, and we measured its setup errors in three directions. Specifically, the deviations in the left-to-right (LR), anterior-to-posterior (AP) and inferior-to-superior (IS) directions were measured by vernier caliper on a graph paper using the Varian Linear accelerator. Then, we verified the accuracy of OPS based on this experimental study. In order to verify the accuracy of phantom experiment, 40 patients were selected in our radiotherapy experiment. To illustrate the precise of optical positioning system, we designed clinical trials using EPID. From our radiotherapy procedure, we can conclude that OPS has higher precise than conventional positioning methods, and is a comparatively fast and efficient positioning method with respect to the CBCT guidance system.

Quantification and Assessment of Interfraction Setup Errors Based on Cone Beam CT and Determination of Safety Margins for Radiotherapy

Introduction

To quantify interfraction patient setup-errors for radiotherapy based on cone-beam computed tomography and suggest safety margins accordingly.

Material and Methods

Positioning vectors of pre-treatment cone-beam computed tomography for different treatment sites were collected (n = 9504). For each patient group the total average and standard deviation were calculated and the overall mean, systematic and random errors as well as safety margins were determined.

Results

The systematic (and random errors) in the superior-inferior, left-right and anterior-posterior directions were: for prostate, 2.5(3.0), 2.6(3.9) and 2.9(3.9)mm; for prostate bed, 1.7(2.0), 2.2(3.6) and 2.6(3.1)mm; for cervix, 2.8(3.4), 2.3(4.6) and 3.2(3.9)mm; for rectum, 1.6(3.1), 2.1(2.9) and 2.5(3.8)mm; for anal, 1.7(3.7), 2.1(5.1) and 2.5(4.8)mm; for head and neck, 1.9(2.3), 1.4(2.0) and 1.7(2.2)mm; for brain, 1.0(1.5), 1.1(1.4) and 1.0(1.1)mm; and for mediastinum, 3.3(4.6), 2.6(3.7) and 3.5(4.0)mm. The CTV-to-PTV margins had the smallest value for brain (3.6, 3.7 and 3.3mm) and the largest for mediastinum (11.5, 9.1 and 11.6mm). For pelvic treatments the means (and standard deviations) were 7.3 (1.6), 8.5 (0.8) and 9.6 (0.8)mm.

Conclusions

Systematic and random setup-errors were smaller than 5mm. The largest errors were found for organs with higher motion probability. The suggested safety margins were comparable to published values in previous but often smaller studies.

Setup error analysis in helical tomotherapy based image-guided radiation therapy treatments

The adequacy of setup margins for various sites in patients treated with helical tomotherapy was investigated. A total of 102 patients were investigated. The breakdown of the patients were as follows: Twenty-five patients each in brain, head and neck (H and N), and pelvis, while 12 patients in lung and 15 in craniospinal irradiation (CSI). Patients were immobilized on the institutional protocol. Altogether 2686 megavoltage computed tomography images were analyzed with 672, 747, 622, 333, and 312 fractions, respectively, from brain, H and N, pelvis, lung, and CSI. Overall systematic and random errors were calculated in three translational and three rotational directions. Setup margins were evaluated using van Herk formula. The calculated margins were compared with the margins in the clinical use for various directions and sites. We found that the clinical isotropic margin of 3 mm was adequate for brain patients. However, in the longitudinal direction it was found to be out of margin by 0.7 mm. In H and N, the calculated margins were well within the isotropic margin of 5 mm which is in clinical use. In pelvis, the calculated margin was within the limits, 8.3 mm versus 10 mm only in longitudinal direction, however, in vertical and lateral directions the calculated margins were out of clinical margins 11 mm versus 10 mm, and 8.7 mm versus 7.0, mm respectively. In lung, all the calculated margins were well within the margins used clinically. In CSI, the variation was found in the middle spine in the longitudinal direction. The clinical margins used in our hospital are adequate enough for sites H and N, lung, and brain, however, for CSI and pelvis the margins were found to be out of clinical margins.

Dosimetric benefit to organs at risk following margin reductions in nasopharyngeal carcinoma treated with intensity-modulated radiation therapy

Introduction

It is important to decrease the radiation exposure of normal tissue in intensity-modulated radiation therapy (IMRT). Minimizing planning target volume (PTV) margins with more precise target localization techniques can achieve this goal. This study aimed to quantify the extent to which organs at risk (OARs) are spared when using reduced margins in the treatment of nasopharyngeal carcinoma (NPC).

Methods

Two IMRT plans were regenerated for 40 patients with NPC based on two PTV margins, which were reduced or unchanged following cone beam computed tomography online correction. The reduced-margin plan was optimized based on maximal dose reduction to OARs without compromising target coverage. Dosimetric comparisons were evaluated in terms of target coverage and OAR sparing.

Results

Improvements in target coverage occurred with margin reduction, and significant improvements in dosimetric parameters were observed for all OARs (P < 0.05) except for the right optic nerve, chiasm, and lens. Doses to OARs decreased at a rate of 1.5% to 7.7%. Sparing of the left parotid and right parotid, where the mean dose (D_mean) decreased at a rate of 7.1% and 7.7%, respectively, was greater than the sparing of other OARs.

Conclusions

Significant improvements in OAR sparing were observed with margin reduction, in addition to improvement in target coverage. The parotids benefited most from the online imaging-guided approach.

Inter- and Intrafraction Target Motion in Highly Focused Single Vocal Cord Irradiation of T1a Larynx Cancer Patients

PURPOSE

The purpose of this study was to verify clinical target volume--planning target volume (CTV-PTV) margins in single vocal cord irradiation (SVCI) of T1a larynx tumors and characterize inter- and intrafraction target motion.

METHODS AND MATERIALS

For 42 patients, a single vocal cord was irradiated using intensity modulated radiation therapy at a total dose of 58.1 Gy (16 fractions × 3.63 Gy). A daily cone beam computed tomography (CBCT) scan was performed to online correct the setup of the thyroid cartilage after patient positioning with in-room lasers (interfraction motion correction). To monitor intrafraction motion, CBCT scans were also acquired just after patient repositioning and after dose delivery. A mixed online-offline setup correction protocol ("O2 protocol") was designed to compensate for both inter- and intrafraction motion.

RESULTS

Observed interfraction, systematic (Σ), and random (σ) setup errors in left-right (LR), craniocaudal (CC), and anteroposterior (AP) directions were 0.9, 2.0, and 1.1 mm and 1.0, 1.6, and 1.0 mm, respectively. After correction of these errors, the following intrafraction movements derived from the CBCT acquired after dose delivery were: Σ = 0.4, 1.3, and 0.7 mm, and σ = 0.8, 1.4, and 0.8 mm. More than half of the patients showed a systematic non-zero intrafraction shift in target position, (ie, the mean intrafraction displacement over the treatment fractions was statistically significantly different from zero; P<.05). With the applied CTV-PTV margins (for most patients 3, 5, and 3 mm in LR, CC, and AP directions, respectively), the minimum CTV dose, estimated from the target displacements observed in the last CBCT, was at least 94% of the prescribed dose for all patients and more than 98% for most patients (37 of 42). The proposed O2 protocol could effectively reduce the systematic intrafraction errors observed after dose delivery to almost zero (Σ = 0.1, 0.2, 0.2 mm).

CONCLUSIONS

With adequate image guidance and CTV-PTV margins in LR, CC, and AP directions of 3, 5, and 3 mm, respectively, excellent target coverage in SVCI could be ensured.

Applications of linac-mounted kilovoltage Cone-beam Computed Tomography in modern radiation therapy: A review

The use of Cone-beam Computed Tomography (CBCT) in radiotherapy is increasing due to the widespread implementation of kilovoltage systems on the currently available linear accelerators. Cone beam CT acts as an effective Image-Guided Radiotherapy (IGRT) tool for the verification of patient position. It also opens up the possibility of real-time re-optimization of treatment plans for Adaptive Radiotherapy (ART). This paper reviews the most prominent applications of CBCT (linac-mounted) in radiation therapy, focusing on CBCT-based planning and dose calculation studies. This is followed by a concise review of the main issues associated with CBCT, such as imaging artifacts, dose and image quality. It explores how medical physicists and oncologists can best apply CBCT for therapeutic applications.

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.

A randomized controlled trial of lorazepam to reduce liver motion in patients receiving upper abdominal radiation therapy

PURPOSE

Reduction of respiratory motion is desirable to reduce the volume of normal tissues irradiated, to improve concordance of planned and delivered doses, and to improve image guided radiation therapy (IGRT). We hypothesized that pretreatment lorazepam would lead to a measurable reduction of liver motion.

METHODS AND MATERIALS

Thirty-three patients receiving upper abdominal IGRT were recruited to a double-blinded randomized controlled crossover trial. Patients were randomized to 1 of 2 study arms: arm 1 received lorazepam 2 mg by mouth on day 1, followed by placebo 4 to 8 days later; arm 2 received placebo on day 1, followed by lorazepam 4 to 8 days later. After tablet ingestion and daily radiation therapy, amplitude of liver motion was measured on both study days. The primary outcomes were reduction in craniocaudal (CC) liver motion using 4-dimensional kV cone beam computed tomography (CBCT) and the proportion of patients with liver motion ≤5 mm. Secondary endpoints included motion measured with cine magnetic resonance imaging and kV fluoroscopy.

RESULTS

Mean relative and absolute reduction in CC amplitude with lorazepam was 21% and 2.5 mm respectively (95% confidence interval [CI] 1.1-3.9, P=.001), as assessed with CBCT. Reduction in CC amplitude to ≤5 mm residual liver motion was seen in 13% (95% CI 1%-25%) of patients receiving lorazepam (vs 10% receiving placebo, P=NS); 65% (95% CI 48%-81%) had reduction in residual CC liver motion to ≤10 mm (vs 52% with placebo, P=NS). Patients with large respiratory movement and patients who took lorazepam ≥60 minutes before imaging had greater reductions in liver CC motion. Mean reductions in liver CC amplitude on magnetic resonance imaging and fluoroscopy were nonsignificant.

CONCLUSIONS

Lorazepam reduces liver motion in the CC direction; however, average magnitude of reduction is small, and most patients have residual motion >5 mm.

Migration from full-head mask to "open-face" mask for immobilization of patients with head and neck cancer

To provide an alternative device for immobilization of the head while easing claustrophobia and improving comfort, an “open‐face” thermoplastic mask was evaluated using video‐based optical surface imaging (OSI) and kilovoltage (kV) X‐ray radiography. A three‐point thermoplastic head mask with a precut opening and reinforced strips was developed. After molding, it provided sufficient visible facial area as the region of interest for OSI. Using real‐time OSI, the head motion of ten volunteers in the new mask was evaluated during mask locking and 15 minutes lying on the treatment couch. Using a nose mark with reference to room lasers, forced head movement in open‐face and full‐head masks (with a nose hole) was compared. Five patients with claustrophobia were immobilized with open‐face masks, set up using OSI and kV, and treated in 121 fractions, in which 61 fractions were monitored during treatment using real‐time OSI. With the open‐face mask, head motion was found to be 1.0 ± 0.6 mm and 0.4° ± 0.2° in volunteers during the experiment, and 0.8 ± 0.3 mm and 0.4° ± 0.2° in patients during treatment. These agree with patient motion calculated from pre‐/post‐treatment OSI and kV data using different anatomical landmarks. In volunteers, the head shift induced by mask‐locking was 2.3 ± 1.7 mm and 1.8° ± 0.6°, and the range of forced movements in the open‐face and full‐head masks were found to be similar. Most (80%) of the volunteers preferred the open‐face mask to the full‐head mask, while claustrophobic patients could only tolerate the open‐face mask. The open‐face mask is characterized for its immobilization capability and can immobilize patients sufficiently (< 2 mm) during radiotherapy. It provides a clinical solution to the immobilization of patients with head and neck (HN) cancer undergoing radiotherapy, and is particularly beneficial for claustrophobic patients. This new open‐face mask is readily adopted in radiotherapy clinic as a superior alternative to the standard full‐head mask.

PACS numbers: 87.19.xj, 87.63.L‐, 87.59.‐e, 87.55.tg, 87.55.‐x

Analysis of inter- and intrafraction accuracy of a commercial thermoplastic mask system used for image-guided particle radiation therapy

The present paper reports and discusses the results concerning both the inter- and intrafraction accuracy achievable combining the immobilization system employed in patients with head-and-neck, brain and skull base tumors with image guidance at our particle therapy center. Moreover, we investigated the influence of intrafraction time on positioning displacements. A total of 41 patients treated between January and July 2011 represented the study population. All the patients were immobilized with a tailored commercial thermoplastic head mask with standard head-neck rest (HeadSTEP(®), IT-V). Patient treatment position was verified by two orthogonal kilovoltage images acquired through a ceiling imaging robot (Siemens, Erlangen, Germany). The analysis of the applied daily corrections during the first treatment week before and after treatment delivery allowed the evaluation of the interfraction and intrafraction reproducibility of the thermoplastic mask, respectively. Concerning interfraction reproducibility, translational and rotational systematic errors (Σs) were ≤ 2.2 mm and 0.9º, respectively; translational and rotational random errors (σs) were ≤ 1.6 mm and 0.6º, respectively. Regarding the intrafraction accuracy translational and rotational Σs were ≤ 0.4 mm and 0.4º, respectively; translational and rotational σs were ≤ 0.5 mm and 0.3º, respectively. Concerning the time-intrafraction displacements correlation Pearson coefficient was 0.5 for treatment fractions with time between position checks less than or equal to median value, and 0.2 for those with time between position controls longer than the median figure. These results suggest that intrafractional patient motion is smaller than interfractional patient motion. Moreover, we can state that application of different imaging verification protocols translate into a relevant difference of accuracy for the same immobilization device. The magnitude of intrafraction displacements correlates with the time for short treatment sessions or during the early phase of long treatment delivery.

Evaluating the relevance of dosimetric considerations to patient instructions regarding skin care during radiation therapy

Introduction

Patient teaching in radiation therapy may include restrictions on applying skin products owing to concerns that the presence of such materials may increase skin dose. These restrictions may create unnecessarily complicated and conflicting self-care instructions.

Purpose

To determine what thickness of skin product is necessary to produce a clinically meaningful dose increase to the skin, and provide recommendations for evidence-based patient instructions.

Methods

Dosimetric measurements and Monte Carlo simulations were used to calculate skin dose under 0–1·5 mm thicknesses of two common classes of skin product for a variety of treatment geometries. The thickness of product required to produce a clinically significant dose increase to the skin was determined.

Results

The thickness of product required to create a clinically meaningful dose increase was >0·7 mm for 10 × 10 cm2 fields and >1·5 mm for 1 × 1 cm2 fields. A typical application of product would be only 0·3 mm.

Conclusion

It seems unrealistic to anticipate patients using sufficiently large quantities of skin product to be of clinical concern. We therefore recommend that there are no dosimetric reasons to restrict the use of these types of skin products during radiation therapy for common treatment scenarios.

Quality Improvement Investigation for Head and Neck Stabilization in Radiotherapy Using Setup Tattoos

PURPOSE

Highly complex planning techniques and delivery methods in the treatment of head and neck cancer require an advanced level of accuracy and reproducibility.

AIM

To determine if the addition of tattoos placed on the chest inferior to the CIVCO Vac-Lok stabilization system improves accuracy and reproducibility of patient set up.

METHODS

Eighteen patients with head and neck cancer were studied. Nine underwent radical treatment using the routine CIVCO stabilization system. The second group of nine used the same stabilization device but were positioned daily with the use of tattoos. Daily orthogonal kilovoltage setup images were used to calculate setup errors. Displacements in the left/right (Lt/Rt), superior/inferior (Sup/Inf), and anterior/posterior (Ant/Post) directions were determined as well as pitch and yaw rotational errors.

RESULTS

Five hundred and twenty-three image pairs were analysed. Clinically significant differences were found in yaw error, Lt/Rt displacement, and Sup/Inf displacement in the tattooed patients. The median (interquartile range) absolute yaw error was larger for patients without tattoos: 1.4° (1.4° to 2.1°) compared to 0.8° (0.8° to 1.4°) for patients with tattoos. The percentage of both Sup/Inf and Lt/Rt errors >3 mm was also greater for patients without tattoos: 23.7% of Sup/Inf errors were >3 mm compared with 17.3% for patients with tattoos, and 22.3% of Lt/Rt errors were >3 mm compared with 10.0% for patients with tattoos.

CONCLUSION

The addition of chest tattoos resulted in clinically relevant improvements in Lt/Rt and Sup/Inf translational displacements and variations in yaw for head and neck cancer patients.