Mask dependency of the lacrimal gland dose under whole brain radiotherapy when the six-degrees of freedom couch is not available.

BACKGROUND: Dry eye syndrome has been recently reported in patients who underwent whole brain radiotherapy (WBRT). WBRT based on a couch with three-degrees of freedom (3D) can occasionally be performed in which the rotational head motion is not corrected. This study assessed the dependency of the rotational errors on the mask and the dose variation of the lens and lacrimal gland in WBRT patients. METHODS: Translational and rotational setup errors were obtained at the first treatment with cone-beam CT (CBCT) for patients under WBRT and frameless stereotactic radiosurgery (SRS) (n = 20 each) immobilized using a conventional WB mask and an SRS mask with a bite block, respectively. For the CT sets of SRS cases, WBRT plans were generated for the study. To simulate the rotational error, rotated CT images were created with each rotational error, on which initial WBRT plans were copied and doses were recalculated. The lens and lacrimal gland doses with and without rotation errors were compared. RESULTS: Despite similar translational setup errors for the two masks, the SRS mask showed a dramatic reduction in rotational errors compared to those of the WB mask. The errors varied within -2.9° to 2.9° and -1.2° to 0.7° for the WB and SRS masks, respectively. Accordingly, the SRS mask confined the change in the maximum lens dose, mean dose of the lacrimal gland, and lacrimal volume receiving 15 Gy to one-third of those using the WB mask. CONCLUSION: When the six-degrees of freedom (6D) couch is not available, the frameless SRS mask is beneficial to WBRT for the faithful treatment as it was planned.

Enhancement of megavoltage electronic portal images for markerless tumor tracking.

PURPOSE: The poor quality of megavoltage (MV) images from electronic portal imaging device (EPID) hinders visual verification of tumor targeting accuracy particularly during markerless tumor tracking. The aim of this study was to investigate the effect of a few representative image processing treatments on visual verification and detection capability of tumors under auto tracking. METHODS: Images of QC-3 quality phantom, a single patient's setup image, and cine images of two-lung cancer patients were acquired. Three image processing methods were individually employed to the same original images. For each deblurring, contrast enhancement, and denoising, a total variation deconvolution, contrast-limited adaptive histogram equalization (CLAHE), and median filter were adopted, respectively. To study the effect of image enhancement on tumor auto-detection, a tumor tracking algorithm was adopted in which the tumor position was determined as the minimum point of the mean of the sum of squared pixel differences (MSSD) between two images. The detectability and accuracy were compared. RESULTS: Deblurring of a quality phantom image yielded sharper edges, while the contrast-enhanced image was more readable with improved structural differentiation. Meanwhile, the denoising operation resulted in noise reduction, however, at the cost of sharpness. Based on comparison of pixel value profiles, contrast enhancement outperformed others in image perception. During the tracking experiment, only contrast enhancement resulted in tumor detection in all images using our tracking algorithm. Deblurring failed to determine the target position in two frames out of a total of 75 images. For original and denoised set, target location was not determined for the same five images. Meanwhile, deblurred image showed increased detection accuracy compared with the original set. The denoised image resulted in decreased accuracy. In the case of contrast-improved set, the tracking accuracy was nearly maintained as that of the original image. CONCLUSIONS: Considering the effect of each processing on tumor tracking and the visual perception in a limited time, contrast enhancement would be the first consideration to visually verify the tracking accuracy of tumors on MV EPID without sacrificing tumor detectability and detection accuracy.

Association Between Thyroid Radiation Dose and Hypothyroidism in Breast Cancer Patients Undergoing Volumetric Modulated Arc Therapy for Regional Nodal Irradiation.

BACKGROUND/AIM: To investigate the association between the thyroid dysfunction and thyroid radiation dose in regional nodal irradiation (RNI) using volumetric modulated arc therapy (VMAT) for breast cancer. PATIENTS AND METHODS: We reviewed medical data of 67 patients with breast cancer who underwent curative surgery followed by adjuvant radiotherapy, including RNI using VMAT, between 2018 and 2021. All patients had normal thyroid functional test results, including thyroid stimulating hormone (TSH), T3, and free-T4. We defined subclinical hypothyroidism as increased TSH with or without decreased levels of free-T4 and T3 after the completion of VMAT. We calculated dose-volume histogram parameters (DVHPs), including the mean dose and relative thyroid volume receiving at least 10, 20, 30, and 40 Gy. RESULTS: The median follow-up time was 23.2 months. The 3-year locoregional failure-free survival, progression-free survival, and overall survival rates were 96.3%, 94.7%, and 96.2%, respectively. The mean thyroid dose was 21.4 Gy (range=11.5-29.4 Gy). Subclinical hypothyroidism was noted in 14 patients (20.9%) and the median time to the event was 4.1 months. Among the DVHPs, the relative volume receiving ≥20 Gy (V20Gy) was associated with subclinical hypothyroidism. The 2-year rates of subclinical hypothyroidism were 24.8% and 59.1% in patients with V20Gy ≤46.3% and >46.3%, respectively. CONCLUSION: A significant proportion of patients with breast cancer developed subclinical hypothyroidism after undergoing VMAT for RNI. Our findings highlight the importance of considering the thyroid as an organ at risk for VMAT planning, and suggest that V20Gy could be a useful dose-volume constraint.

Variations in dose distribution and optical properties of Gafchromic(TM) EBT2 film according to scanning mode.

PURPOSE: The authors aim was to investigate the effects of using transmission and reflection scanning modes, the film orientation during scanning, and ambient room light on a dosimetry system based on the Gafchromic(TM) EBT2 film model. METHODS: For calibration, the films were cut to 3 × 3 cm(2) and irradiated from 20 to 700 cGy at the depth of maximum dose using 6 and 10 MV photon beams in a 10 × 10 cm(2) field size. Absolute dose calibration of the linear accelerator was done according to the TRS398 protocol. An FG65-G ionization chamber was used to monitor the dose while irradiating the films in solid water. The film pieces were scanned with an EPSON Expression 1680 Pro flatbed scanner in transmission and reflection modes. Authors investigated the effect of orientation on films and examined the optical properties of EBT2 film using an ellipsometer and an ultraviolet (UV)/visible spectrometer to explain the dosimetric dependence of the film on orientation during the scanning process. To investigate the effect of ambient room light, films were preirradiated in 6 and 10 MV photon beams with intensity-modulated radiotherapy (IMRT) quality assurance (QA) plans, and then exposed to room light, either directly for 2 days in a workroom or for 2 months in a film box. Gamma index pass criteria of (3%, 3 mm) were used. RESULTS: The dose response curves based on net optical density (NOD) indicated that the reflection scanning mode can provide a better dose sensitivity than the transmission scanning mode, whereas the standard deviation of the dose is greater in reflection mode than in transmission mode. When the film was rotated 90° from the portrait orientation, the average dose of the EBT2 film decreased by 11.5-19.6% in transmission mode and by 1.5-2.3% in reflection mode. Using an ellipsometer, variation of the refractive index of EBT2 film-the birefringence property-was found to be the largest between 45° (1.72 and 1.71) and 135° (1.8 and 1.77) for 300 and 800 cGy. Absorption spectra of EBT2 films measured with spectrometer were the function of film orientation. The readings in reflection scanning mode were more stable against room light than those in transmission scanning mode, although dose readings increased in both modes after the films were exposed to room light. CONCLUSIONS: The transmission scanning mode exhibited a strong dependence on film orientation during scanning and a change in optical density resulting from room light exposure, so a constant scanning orientation and minimal exposure to light can reduce uncertainty in the measured dose (23 ± 3%). The angular dependence was analyzed using Jones matrices and optical properties of EBT2 film were obtained using an ellipsometer and an UV/visible spectrometer. The reflection scanning mode has relatively good stability with respect to room light and film orientation on a scanner, although the large standard deviation of dose is a disadvantage in measurements of absolute dose. Reflection scanning mode can offer a potential advantage for film dosimetry in radiotherapy, although transmission scanning mode is still recommended for dosimetry as it provides better uncertainty results.

Effects of Radiation Dose on Liver After Free-breathing Volumetric Modulated Arc Therapy for Breast Cancer.

BACKGROUND/AIM: To evaluate the early effect of radiation dose on liver function in breast cancer patients undergoing free-breathing volumetric modulated arc therapy (FB-VMAT). PATIENTS AND METHODS: Medical records of 125 patients with breast cancer who underwent curative surgery followed by postoperative radiotherapy using FB-VMAT during 2018-2021 were reviewed. Results of the liver function test (LFT), performed within 1-week before and 6-months after radiotherapy, were collected and compared. The LFTs analyzed albumin, total and direct bilirubin, aspartate transaminase, alanine transferase, and alkaline phosphatase levels. The mean dose and relative liver volume receiving at least 10 Gy, 20 Gy, or 30 Gy were calculated. RESULTS: Median follow-up time was 21.4 months. One patient experienced locoregional and distant failures. The mean liver irradiation dose was 325.9 centigray (cGy) for all patients. The liver irradiation dose was higher in patients with right breast cancer than in those with left breast cancer (mean, 434.1 cGy vs. 260.6 cGy, p<0.001). Direct bilirubin and aspartate transaminase levels showed significant differences after FB-VMAT. LFT results outside normal limits were noted in 31 patients at follow-up, but nobody met the criteria of radiation-induced liver disease. Underlying liver disease, breast laterality, systemic treatment, or dose-volume histogram parameters were not associated with abnormal LFT results. CONCLUSION: FB-VMAT can deliver radiation doses safely without adversely affecting the liver. The mean dose ≤4 Gy could be a useful dose criterium of the liver for FB-VMAT plans.

Combined effect of dose gradient and rotational error on prescribed dose coverage for single isocenter multiple brain metastases in frameless stereotactic radiotherapy.

BACKGROUND: To evaluate the combined effect of rotational error and dose gradient on target dose coverage in frameless stereotactic radiotherapy. METHODS: Three spherical targets of different diameters (1, 1.5, and 2 cm) were drawn and placed equidistantly on the same axial brain computed tomography (CT) images. To test the different isocenter-target distances, 2.5- and 5-cm configurations were prepared. Volumetric modulated arc therapy plans were created for different dose gradients from the target, in which the dose gradients were modified using the maximum dose inside the target. To simulate the rotational error, CT images and targets were rotated in two ways by 0.5°, 1°, and 2°, in which one rotation was in the axial plane and the other was in three dimensions. The initial optimized plan parameters were copied to the rotated CT sets, and the doses were recalculated. The coverage degradation after rotation was analyzed according to the target dislocation and 12-Gy volume. RESULTS: A shallower dose gradient reduced the loss of target coverage under target dislocation, and the effect was clearer for small targets. For example, the coverage of the 1-cm target under 1-mm dislocation increased from 93 to 95% by increasing the Paddick gradient index from 5.0 to 7.9. At the same time, the widely accepted necrosis indicator, the 12-Gy volume, increased from 1.2 to 3.5 cm3, which remained in the tolerable range. From the differential dose volume histogram (DVH) analysis, the shallower dose gradient ensured that the dose-deficient under-covered target volume received a higher dose similar to that in the prescription. CONCLUSIONS: For frameless stereotactic brain radiotherapy, the gradient, alongside the margin addition, can be adjusted as an ancillary parameter for small targets to increase target coverage or at least limit coverage reduction in conditions with probable positioning error.

Evaluation of delivered monitor unit accuracy of gated step-and-shoot IMRT using a two-dimensional detector array.

PURPOSE: To overcome the problem of organ motion in intensity-modulated radiation therapy (IMRT), gated IMRT is often used for the treatment of lung cancer. In this study, the authors investigated the accuracy of the delivered monitor units (MUs) from each segment during gated IMRT using a two-dimensional detector array for user-specific verification purpose. METHODS: The authors planned a 6 MV photon, seven-port step-and-shoot lung IMRT delivery. The respiration signals for gated IMRT delivery were obtained from the one-dimensional moving phantom using the real-time position management (RPM) system (Varian Medical Systems, Palo Alto, CA). The beams were delivered using a Clinac iX (Varian Medical Systems, Palo Alto, CA) with the Millennium 120 MLC. The MatriXX (IBA Dosimetry GmbH, Germany) was validated through consistency and reproducibility tests as well as comparison with measurements from a Farmer-type ion chamber. The authors delivered beams with varying dose rates and duty cycles and analyzed the MatriXX data to evaluate MU delivery accuracy. RESULTS: There was quite good agreement between the planned segment MUs and the MUs computed from the MatriXX within +/- 2% error. The beam-on times computed from the MatriXX data were almost identical for all cases, and they matched well with the RPM beam-on and beam-off signals. A slight difference was observed between them, but it was less than 40 ms. The gated IMRT delivery demonstrated an MU delivery accuracy that was equivalent to ungated IMRT, and the delivered MUs with a gating signal agreed with the planned MUs within +/- 0.5 MU regardless of dose rate and duty cycle. CONCLUSIONS: The authors can conclude that gated IMRT is able to deliver an accurate dose to a patient during a procedure. The authors believe that the methodology and results can be transferred to other vendors' devices, particularly those that do not provide MLC log data for a verification purpose.

Dosimetric characteristics of linear accelerator photon beams with small monitor unit settings.

Several studies on the effect of tumor cell killing by dose rate variation have implied that the use of a shorter treatment time is more favorable for intensity modulated radiation therapy (IMRT). Aiming at step-and-shoot IMRT with higher dose rates, the stabilities of beam output and profiles with small monitor unit (MU) settings were investigated for various dose rates. With the use of a Varian 21EX (Varian Medical Systems Inc., Palo Alto, CA), static and step-and-shoot IMRT beam output along with profiles were measured by use of an ion chamber and a two-dimensional diode array detector as a function of monitor units and dose rates. For a static case, as the MU approached 1, the beam output increased up to 2% for 300 MU/min and 4.5% for 600 MU/min, showing a larger overdose as the dose rate increased. Deterioration of the beam symmetry and flatness were also observed as the MU decreased to 1 monitor unit. For the step-and-shoot IMRT case, a large dosimetric error of more than 10% was also detected with the use of a small MU segment. However, no definite correlation with the dose rate was observed due to the combined beam start-up effects by the grid pulse and finite communication time between the machine console and multileaf collimator (MLC) controller. For step-and-shoot IMRT with higher dose rates, beam output and beam profile stability with small MU needs to be checked, and adequate MU limitation where segments are not allowed need to be reflected in the step-and-shoot IMRT planning.

Electron dose distributions caused by the contact-type metallic eye shield: Studies using Monte Carlo and pencil beam algorithms.

A metallic contact eye shield has sometimes been used for eyelid treatment, but dose distribution has never been reported for a patient case. This study aimed to show the shield-incorporated CT-based dose distribution using the Pinnacle system and Monte Carlo (MC) calculation for 3 patient cases. For the artifact-free CT scan, an acrylic shield machined as the same size as that of the tungsten shield was used. For the MC calculation, BEAMnrc and DOSXYZnrc were used for the 6-MeV electron beam of the Varian 21EX, in which information for the tungsten, stainless steel, and aluminum material for the eye shield was used. The same plan was generated on the Pinnacle system and both were compared. The use of the acrylic shield produced clear CT images, enabling delineation of the regions of interest, and yielded CT-based dose calculation for the metallic shield. Both the MC and the Pinnacle systems showed a similar dose distribution downstream of the eye shield, reflecting the blocking effect of the metallic eye shield. The major difference between the MC and the Pinnacle results was the target eyelid dose upstream of the shield such that the Pinnacle system underestimated the dose by 19 to 28% and 11 to 18% for the maximum and the mean doses, respectively. The pattern of dose difference between the MC and the Pinnacle systems was similar to that in the previous phantom study. In conclusion, the metallic eye shield was successfully incorporated into the CT-based planning, and the accurate dose calculation requires MC simulation.

Monte Carlo-based treatment planning for a spoiler system with experimental validation using plane-parallel ionization chambers.

A beam spoiler is often used to increase the build-up dose near the surface for treatment of superficial treatment areas. Photon-beam spoilers produce a large amount of contaminant electrons, conditions for which standard, commercial treatment-planning system dose-calculation algorithms are inadequate for producing accurate dose calculations. In this study, we implemented a Monte Carlo (MC) dose-calculation algorithm for this spoiler system. With and without a spoiler of 1 cm Lucite, depth doses and transverse profiles in the build-up region were measured for field sizes of 5 x 5 cm2 and 10 x 10 cm2 at the spoiler-to-surface distances (STSDs) of 6, 10 and 15 cm. An Attix chamber and a Markus chamber were used for depth doses, whereas a diode detector was used for transverse profiles. An MC simulation using BEAM/DOSXYZ was used to compare the calculated and the measured data. The MC calculations agreed with the Attix chamber measurements within 2% for all STSDs and field sizes, whereas the Markus data--even with corrections made-showed a discrepancy of about 3.5% with a maximum difference of 7.3% for a field size of 10 x 10 cm2 at an STSD of 6 cm. The MC treatment-planning system was successfully applied to a head-and-neck case using 6 MV photon beams with a beam spoiler.

Analysis of predictive factors for lung injury after forward-planned intensity-modulated radiotherapy in whole breast irradiation.

PURPOSE: This study was performed to assess frequency, timings of occurrence, and predictors of radiologic lung damage (RLD) after forward-planned intensity-modulated radiotherapy (FIMRT) for whole breast irradiation. METHODS: We retrospectively reviewed medical records of 157 breast cancer patients and each of their serial chest computed tomography (CT) taken 4, 10, 16, and 22 months after completion of breast radiotherapy (RT). FIMRT was administered to whole breast only (n=152), or whole breast and supraclavicular regions (n=5). Dosimetric parameters, such as mean lung dose and lung volume receiving more than 10 to 50 Gy (V10-V50), and clinical parameters were analyzed in relation to radiologic lung damage. RESULTS: In total, 104 patients (66.2%) developed RLD after whole breast FIMRT. Among the cases of RLD, 84.7% were detected at 4 months, and 15.3% at 10 months after completion of RT. More patients of 47 or younger were found to have RLD at 10 months after RT than patients older than the age (11.7% vs. 2.9%, p=0.01). In univariate and multivariate analyses, age >47 and V40 >7.2% were significant predictors for higher risk of RLD. CONCLUSION: RLD were not infrequently detected in follow-up CT after whole breast FIMRT. More detected cases of RLD among younger patients are believed to have developed at later points after RT than those of older patients. Age and V40 were significant predictors for RLD after whole breast intensity-modulated radiotherapy.

Application of a dummy eye shield for electron treatment planning.

Metallic eye shields have been widely used for near-eye treatments to protect critical regions, but have never been incorporated into treatment plans because of the unwanted appearance of the metal artifacts on CT images. The purpose of this work was to test the use of an acrylic dummy eye shield as a substitute for a metallic eye shield during CT scans. An acrylic dummy shield of the same size as the tungsten eye shield was machined and CT scanned. The BEAMnrc and the DOSXYZnrc were used for the Monte Carlo (MC) simulation, with the appropriate material information and density for the aluminum cover, steel knob and tungsten body of the eye shield. The Pinnacle adopting the Hogstrom electron pencil-beam algorithm was used for the one-port 6-MeV beam plan after delineation and density override of the metallic parts. The results were confirmed with the metal oxide semiconductor field effect transistor (MOSFET) detectors and the Gafchromic EBT2 film measurements. For both the maximum eyelid dose over the shield and the maximum dose under the shield, the MC results agreed with the EBT2 measurements within 1.7%. For the Pinnacle plan, the maximum dose under the shield agreed with the MC within 0.3%; however, the eyelid dose differed by -19.3%. The adoption of the acrylic dummy eye shield was successful for the treatment plan. However, the Pinnacle pencil-beam algorithm was not sufficient to predict the eyelid dose on the tungsten shield, and more accurate algorithms like MC should be considered for a treatment plan.

Clinical and experimental investigation of pseudoaneurysm in the anterior communicating artery area on 3-dimensional time-of-flight cerebral magnetic resonance angiography.

OBJECTIVE: To investigate the hemodynamic mechanism of pseudoaneurysm in the anterior communicating artery (AcoA) area in magnetic resonance (MR) angiography. METHODS: For the clinical study, a total of 62 patients who undertook digital subtraction angiography (DSA) because of the rupture of an aneurysm originating from a location other than the AcoA area were examined with MR angiography. The relation between signal defect at the AcoA in MR angiography and anatomic variation of the anterior cerebral artery (ACA) was evaluated. For the experimental study, MR angiography and DSA were performed on elastic silicon vascular phantoms with 2 different bifurcation angles (70 degrees and 140 degrees). Hemodynamic factors producing signal defects were evaluated, and the results were compared by computational fluid dynamics (CFD). RESULTS: In a clinical study, 21 of 62 patients had a hypogenetic A1 segment on either side of the ACA. Their MR angiography showed signal defects in the axilla area of the bifurcated AcoA complex in 14 patients, 7 of which could make the residual normal vessel seem to be an aneurysm. All the cases with an intact AcoA complex showed no signal defect. In an experimental study, MR angiography of vascular phantoms with broad-angle bifurcation (140 degrees) showed signal defects at the axilla areas of bifurcation, and these were shown as turbulent flow in DSA and CFD. Phantoms with narrow-angle bifurcation (70 degrees) did not show a significant signal defect, however. CONCLUSIONS: A hypoplastic A1 segment accompanying a broad bifurcation angle of the contralateral A1 segment may cause a pseudoaneurysm in MR angiography because of signal defect in the AcoA area.