Intensity-modulated Radiotherapy Dose Distributions Research Articles

Prognostic Factors of Mandibular Osteoradionecrosis Including Accurate Colocalization of Avulsions and Dosimetric Dental Mapping Software, a Case-control Study

IntroductionOsteoradionecrosis of the mandible (ORN) remains a significant complication in the intensity modulated radiotherapy (IMRT) era. Dental dose cannot be predicted from heterogeneous IMRT dose distributions; mandibular dose metrics cannot guide dentist avulsion decisions in high risk ORN situations. Using a mapping tool to report dental root dose, avulsions and ORN sites, we reexamined ORN risk factors in a case-control study. MethodsFrom 2008 to 2019, 897 consecutive patients with oral cavity/oropharynx (OC/OR) or unknown primary cancer (CUP) and undergoing IMRT were analyzed to identify ORN cases. These were matched (1 ORN/2 controls) retrospectively for tumor, surgery, tobacco in a monocentric case-control study. Univariate and multivariate analyses integrated ORN factors and accurate dental dose data (grouped into 4 mandibular sectors). Generalizability was investigated in a simulated population database. Results171 patients were included. Median follow-up was 5.2 and 4.5 years in the ORN and control groups, respectively. Median time to ORN was 12 months. In univariate analysis, post-IMRT avulsions at the ORN site (Hazard ratio (HR) = 3.6; 95% confidence interval (CI) = 1.5-8.9; p=0.005), tumor laterality (HR =4.4; 95% CI = 1.4-14, p = 0.01), mean mandibular dose (HR = 1.1; 95% CI = 1.01-1.1; p = 0.018) and mean dose to the ORN site (HR = 1.1; 95% CI = 1.1-1.2; p < 0.001) correlated with higher ORN risk. In multivariate analysis, mean dose to the ORN site (HR = 1.1; 95% CI = 1.1-1.2; p < 0.001) and post-IMRT avulsions at the ORN site (HR = 4.6; 95% CI = 1.5-14.7; p = 0.009) were associated with ORN. For each increase in Gray in dental dose, the ORN risk increased by 12%. Simulations confirmed study observations. ConclusionDental dose and avulsions are associated with ORN with a 12% increase in risk with each additional Gray. Accurate dose information can help dentists in their decisions after IMRT.

Accounting for fractionation and heterogeneous dose distributions in the modelling of osteoradionecrosis in oropharyngeal carcinoma treatment

Background and purposeOsteoradionecrosis (ORN) of the mandible is a severe complication following radiotherapy (RT). With a renewed interest in hypofractionation for head and neck radiotherapy, more information concerning ORN development after high fraction doses is important. The aim of this explorative study was to develop a model for ORN risk prediction applicable across different fractionation schemes using Equivalent Uniform Doses (EUD). Material and methodsWe performed a retrospective cohort study in 334 oropharyngeal squamous cell carcinoma (OPSCC) patients treated with either a hypofractionated Stereotactic Body Radiation Therapy (HF-SBRT) boost or conventional Intensity Modulated Radiation Therapy (IMRT). ORN was scored with the CTCAE v5.0. HF-SBRT and IMRT dose distributions were converted into equivalent dose in 2 Gy fractions (α/β = 0.85 Gy) and analyzed using EUD. The parameter a that led to an EUD that best discriminated patients with and without grade ≥ 2 ORN was selected. Patient and treatment-related risk factors of ORN were analyzed with uni- and multivariable regression analysis. ResultsA total of 32 patients (9.6%) developed ORN grade ≥ 2. An EUD(a = 8) best discriminated between ORN and non-ORN (AUC = 0.71). In multivariable regression, pre-RT extractions (SHR = 2.34; p = 0.012), mandibular volume (SHR = 1.04; p = 0.003), and the EUD(a = 8) (SHR = 1.14; p < 0.001) were significantly associated with ORN. ConclusionRisk models for ORN based on conventional DVH parameters cannot be directly applied to HF-SBRT fractionation schemes and dose distributions. However, after correcting for fractionation and non-uniform dose distributions using EUD, a single model can distinguish between ORN and non-ORN after conventionally fractionated radiotherapy and hypofractionated boost treatments.

Comparison between different Dosimetric Tools based on Intensity-modulated Radiotherapy (IMRT): A Phantom Study

This study examined the gamma passing rate (GPR) consistency during applying different kinds of gamma analyses and dosimeters to IMRT. Methods: Import treatment protocols for QA phantom irradiation have been recalculated. A gamma analysis was used for comparing the measured and calculated dose distribution of IMRT for different gamma criteria (2%/2mm, 3%/3mm, 4%/4mm, 3%/5mm, 3%/5mm). These criteria are evaluated when 5%, 10%, or 15% of the dose distribution is suppressed. Measured and calculated dose distribution was evaluated with gamma analysis to dose difference (DD) with DTA criteria (distance to agreement). IMRT QA plans to 25 patients from various sites were formed with the Varian Eclipse treatment planning system. Results: Results indicate different diverse hardware and software combinations show varied levels of agreement with expected analysis for the same pass-rate criterion. For a dosimetry audit of the IMRT technique, an EPID detector is superior to conventional methods comparable to Gafchromic EPT3 film and 2D array due to cost, time-consuming, and set up error to get result analysis. The gamma passing rate (GPR) average is increased by increasing the low-dose threshold for different dosimetric tools. For EPID, regardless of the gamma criterion employed, the %GP does not appear to be dependent on the low-dose threshold values (5%-15%) because it indicates that fulfilment the low-dose threshold to global normalization has little effect on patient-specific QA outcomes. Conclusions: It is concluded that GPRs differ depending on gamma, dosimetric tools, and the suppressing dose ratio. To get the best results of quality assurance, each institution should thus carefully develop its procedure for gamma analysis by defining the gamma index analysis and gamma criterion using its dosimetric tools.

A method of using deep learning to predict three-dimensional dose distributions for intensity-modulated radiotherapy of rectal cancer.

PurposeTo develop and test a three‐dimensional (3D) deep learning model for predicting 3D voxel‐wise dose distributions for intensity‐modulated radiotherapy (IMRT).MethodsA total of 122 postoperative rectal cancer cases treated by IMRT were considered in the study, of which 100 cases were randomly selected as the training–validating set and the remaining as the testing set. A 3D deep learning model named 3D U‐Res‐Net_B was constructed to predict 3D dose distributions. Eight types of 3D matrices from CT images, contoured structures, and beam configurations were fed into the independent input channel, respectively, and the 3D matrix of dose distributions was taken as the output to train the 3D model. The obtained 3D model was used to predict new 3D dose distributions. The predicted accuracy was evaluated in two aspects: (a) The dice similarity coefficients (DSCs) of different isodose volumes, the average dose difference of all voxels within the body, and 3%/5 mm global gamma passing rates of organs at risks (OARs) and planned target volume (PTV) were used to address the spatial correspondence between predicted and clinical delivered 3D dose distributions; (b) The dosimetric index (DI) including homogeneity index, conformity index, V50, V45 for PTV and OARs between predicted and clinical truth were statistically analyzed with the paired‐samples t test. The model was also compared with 3D U‐Net and the same architecture model without beam configurations input (named as 3D U‐Res‐Net_O).ResultsThe 3D U‐Res‐Net_B model predicted 3D dose distributions accurately. For the 22 testing cases, the average prediction bias ranged from −1.94% to 1.58%, and the overall mean absolute errors (MAEs) was 3.92 ± 4.16%; there was no statistically significant difference for nearly all DIs. The model had a DSCs value above 0.9 for most isodose volumes, and global 3D gamma passing rates varying from 0.81 to 0.90 for PTV and OARs, clearly outperforming 3D U‐Res‐Net_O and being slightly superior to 3D U‐Net.ConclusionsThis study developed a more general deep learning model by considering beam configurations input and achieved an accurate 3D voxel‐wise dose prediction for rectal cancer treated by IMRT, a potentially easier clinical implementation for more comprehensive automatic planning.

Application of Two-Dimensional Detector Arrays to the Delivery Quality Assurance in Breast IMRT

Most two-dimensional (2D) detector arrays used for the verification of the intensity-modulated radiotherapy (IMRT) dose distribution comprise diode detectors or ion chamber detectors. These detectors show different sensitivities depending on the angle of incidence of the IMRT beam on the detector. In this study, the effect of the radiation beam’s angle of incidence onto the detector was analyzed in order to find an optimal 2D detector array setup position for delivery quality assurance in breast IMRT. For breast IMRT plans, comprising mostly tangential fields, the dose distribution produced with true composite beams was measured by arranging 2D detector array devices at various angles. The change in dosimetric accuracy was then assessed as a function of the radiation beam’s angle of incidence. A total of 72 dose distributions were measured and analyzed for 12 breast IMRT plans. Both the diode detector array and the ion chamber array showed a higher gamma passing ratio in the sagittal dose distribution measurement, where the beam’s angle of incidence was relatively larger. This study indicates that using a 2D detector array placed at a relatively large angle of incidence offers a more accurate verification of the IMRT dose distribution.

Impact of organ motion on volumetric and dosimetric parameters in stomach lymphomas treated with intensity‐modulated radiotherapy

PurposeInterplay effects may influence dose distributions to a moving target when using dynamic delivery techniques such as intensity‐modulated radiotherapy (IMRT). The aim of this study was to evaluate the impact of organ motion on volumetric and dosimetric parameters in stomach lymphomas treated with IMRT.MethodsTen patients who had been treated with IMRT for stomach lymphomas were enrolled. The clinical target volume (CTV) was contoured as the whole stomach. Considering interfractional uncertainty, the internal target volume (ITV) margin was uniformly 1.5 cm to the CTV and then modified based on the 4DCT images in case of the large respiratory motion. The planning target volume (PTV) was created by adding 5 mm to the ITV. The impact of organ motion on the volumetric and dosimetric parameters was evaluated retrospectively (4D simulation). The organ motion was reproduced by shifting the isocenter on the radiation treatment planning system. Several simulation plans were created to test the influence of the beam‐on timing in the respiration cycle on the dose distribution. The homogeneity index (HI), volume percentage of stomach covered by the prescribed dose (Vp), and D99 of the CTV were evaluated.ResultsThe organ motion was the largest in the superior‐inferior direction (10.1 ± 4.5 mm [average ± SD]). Stomach volume in each respiratory phase compared to the mean volume varied approximately within a ± 5% range in most of the patients. The PTV margin was sufficiently large to cover the CTV during the IMRT. There was a significant reduction in Vp and D99 but not in HI in the 4D simulation in free‐breathing and multiple fractions compared to the clinically‐used plan (P < 0.05) suggesting that interplay effects deteriorate the dose distribution. The absolute difference of D99 was less than 1% of the prescribed dose.ConclusionsThere were significant interplay effects affecting the dose distribution in stomach IMRT. The magnitude of the dose reduction was small when patients were treated on free‐breathing and multiple fractions.

A convolutional neural network approach for IMRT dose distribution prediction in prostate cancer patients.

The purpose of the study was to compare a 3D convolutional neural network (CNN) with the conventional machine learning method for predicting intensity-modulated radiation therapy (IMRT) dose distribution using only contours in prostate cancer. In this study, which included 95 IMRT-treated prostate cancer patients with available dose distributions and contours for planning target volume (PTVs) and organs at risk (OARs), a supervised-learning approach was used for training, where the dose for a voxel set in the dataset was defined as the label. The adaptive moment estimation algorithm was employed for optimizing a 3D U-net similar network. Eighty cases were used for the training and validation set in 5-fold cross-validation, and the remaining 15 cases were used as the test set. The predicted dose distributions were compared with the clinical dose distributions, and the model performance was evaluated by comparison with RapidPlan™. Dose–volume histogram (DVH) parameters were calculated for each contour as evaluation indexes. The mean absolute errors (MAE) with one standard deviation (1SD) between the clinical and CNN-predicted doses were 1.10% ± 0.64%, 2.50% ± 1.17%, 2.04% ± 1.40%, and 2.08% ± 1.99% for D2, D98 in PTV-1 and V65 in rectum and V65 in bladder, respectively, whereas the MAEs with 1SD between the clinical and the RapidPlan™-generated doses were 1.01% ± 0.66%, 2.15% ± 1.25%, 5.34% ± 2.13% and 3.04% ± 1.79%, respectively. Our CNN model could predict dose distributions that were superior or comparable with that generated by RapidPlan™, suggesting the potential of CNN in dose distribution prediction.

Modelling the effects of lung cancer motion due to respiration

Abstract Background and objectives: To justify the concept of validating conformal versus intensity-modulated approach in the treatment of non-small cell lung cancer (NSCLC). Materials and methods: For 10 patients representative of the spectrum of tumour sizes and locations, two plans were prepared: one with three-dimensional conformal radiation therapy (3DCRT) technique and the other with intensity-modulated radiation therapy (IMRT) technique. Preliminary measurements were performed in static conditions. For each of the field angles considered, the motion kernel was generated to simulate tumour motion trajectories, with the largest amplitude in the cranio-caudal direction of 4, 6, and 8 mm. The measurement results determined the agreement between the planned and measured doses. Results: No statistically significant differences were found between the motion patterns, with the smallest amplitudes for clinical target volume in 3DCRT. For IMRT, the significant differences between 0 mm vs. 6 mm and 0 mm vs. 8 mm amplitudes were found. The motion impact on delivered vs. planned doses had less effect on the oesophagus in 3DCRT compared to that in IMRT. The observed differences were comparable for the heart. Interpretation and conclusions: For maximal amplitudes below 4 mm, the disagreement between planned and delivered doses can be neglected. However, the amplitudes above 5 mm and 7 mm lead to significant changes in IMRT and 3DCRT dose distribution, respectively.

222. Optimization of pretreatment IMRT dose distribution verification: Bidimensional gaussian kernel implementation

Purpose Intensity modulated radiation therapy (IMRT) is a widely used technique delivering a highly conformal dose to the target together with a better sparing of organs at risk with respect to conventional irradiation techniques. Quality assurance (QA) of the generated IMRT treatment plan is of most importance to ensure that the patient receives the planned dose; a two-dimensional array of ionization chambers in a solid phantom can be used for the verification. Different methods exist for comparison of dose distributions and the gamma evaluation is the most commonly used. The aim of this work is to convolve the calculated dose distributions in order to optimize the comparison with the measured data obtained with the MatriXX ionization chamber detector arrays (IBA Dosimetry, Germany). Methods In this study a bidimensional gaussian kernel, which addresses the ionization chamber volume averaging effect, has been implemented to optimize the comparison of dose distributions for pretreatment patient-specific IMRT QA. It was determined by scanning a single chamber with a slit beam. A point spread function was reconstructed with a Gaussian function obtained with a MATLAB script. The kernel has been tested on head&neck and prostate treatment plans. Results It has been found that the convolution of the kernel with the treatment planning system calculated dose distribution can improve the comparison respect to the conventional method of IMRT verification. The gamma index passing rate values found with convolution method are higher in all considered clinical cases. Conclusions Intensity modulated radiation therapy (IMRT) is a widely used technique delivering a highly conformal dose to the target together with a better sparing of organs at risk with respect to conventional irradiation techniques. Quality assurance (QA) of the generated IMRT treatment plan is of most importance to ensure that the patient receives the planned dose; a two-dimensional array of ionization chambers in a solid phantom can be used for the verification. Different methods exist for comparison of dose distributions and the gamma evaluation is the most commonly used. The aim of this work is to convolve the calculated dose distributions in order to optimize the comparison with the measured data obtained with the MatriXX ionization chamber detector arrays (IBA Dosimetry, Germany). In this study a bidimensional gaussian kernel, which addresses the ionization chamber volume averaging effect, has been implemented to optimize the comparison of dose distributions for pretreatment patient-specific IMRT QA. It was determined by scanning a single chamber with a slit beam. A point spread function was reconstructed with a Gaussian function obtained with a MATLAB script. The kernel has been tested on head&neck and prostate treatment plans. It has been found that the convolution of the kernel with the treatment planning system calculated dose distribution can improve the comparison respect to the conventional method of IMRT verification. The gamma index passing rate values found with convolution method are higher in all considered clinical cases.

A Feasibility Study of IMRT of Lung Cancer Using Gafchromic EBT3 Film

Intensity modulated radiation therapy (IMRT) is an advanced method for delivery of three dimensional therapies, which provides optimal dose distribution with giving multiple nonuniform fluency to the patient. The complex dose distribution of IMRT should be checked to ensure that the accurate dose is delivered. Today, film dosimetry is a powerful tool for radiotherapy treatment Quality Assurance (QA) and a good method to verify dose distribution in phantoms. This study aimed to evaluate the accuracy of IMRT treatment planning system, Prowess Panther® software, with Gafchromic EBT3 films in a inhomogeneity phantom. The IMRT plan was generated by Prowess Panther® treatment planning system (TPS) version 5.2 on a inhomogeneity phantom, then it was irradiated by ONCOR linear accelerator (Linac) with 6 (MV) photon beam energy. The Gafchromic EBT3 film located between the phantom has measured the dose distribution. To compare between TPS calculated doses and film measured doses, Gamma criteria 3%/3 mm, 4%/4 mm, 5%/5 mm, 6%/6 mm and 7%/7 mm Dose Difference (DD) and Distance to Agreement (DTA), respectively were used. Gammas passing rates for PTV are obtained 67.5% for 3%/3mm, 78.8% for 4%/4mm, 86.3% for 5%/5mm, 91.2% for 6%/6mm and 94.3% for 7%/7mm and for organs at risk is 72.4% for 3%/3mm, 82.8% for 4%/4mm, 89.8% for 5%/5mm, 93.3% for 6%/6mm and 95.4% for 7%/7mm (respectively DD/DTA). By increasing the range of criteria the capability increased. The results show that the use of EBT3 film in a inhomogeneity phantoms allows us to evaluate the dose differences between the EBT3 measured dose distribution and TPS calculated dose distribution .Hence, a result Prowess Panther® TPS can be used for IMRT technique treatment.

Dosimetric sensitivity with MU and MLC errors in IMRT versus VMAT plan for nasopharyngeal carcinoma

Objective To simulate the possible systematic delivery errors introduced by monitor units (MU) and multi-leaf collimator (MLC) in radiotherapy plans for nasopharyngeal carcinoma (NPC), and to analyze the dosimetric sensitivity of static intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) with these errors. Methods Five IMRT plans were replanned using VMAT modality with the same physical parameters, and then MU errors of 1.25%, 2.50%, and 5.00% were introduced into IMRT and VMAT plans. Meanwhile, to simulate leaf position errors during delivery, MLC position errors (0.25 mm, 0.50 mm, 1.00 mm, 1.50 mm, and 2.00 mm) were introduced by modifying the original plan documents. The types of MLC errors were as follows: (1) the MLC banks moved in the same direction; (2) the MLC banks moved in opposing directions (expand or contract the MLC gaps). The differences in dosimetric sensitivity introduced by MU and MLC errors between IMRT and VMAT plans for NPC were calculated by linear regression analysis. Results With the increase in MU errors, the doses to target and organs at risk (OARs) of IMRT and VMAT plans increased in a linear way, and met R2=0.992-1.000(P<0.05). For MLC errors, the average dosimetric sensitivity for target and OARs of IMRT and VMAT were -0.26%/mm and -0.65%/mm in case of offset errors, 4.87%/mm and 8.68%/mm in case of expansion errors, and -6.04%/mm and -9.88%/mm in case of indentation errors. In addition, the dosimetric sensitivity with the three types of MLC errors was greater for VMAT plan than for IMRT plan. Conclusions MU and MLC errors have a significant effect on the dose distribution of IMRT, and particularly VMAT, for NPC. It is important to execute routine quality assurance of MLC to ensure accurate radiotherapy. Key words: Nasopharyngeal neoplasms/intensity-modulated radiation therapy; Nasopharyngeal neoplasms/volumetric-modulated arc therapy; Dosimetry; Sensitivity

Repeatability of dose painting by numbers treatment planning in prostate cancer radiotherapy based on multiparametric magnetic resonance imaging

Dose painting by numbers (DPBN) refers to a voxel-wise prescription of radiation dose modelled from functional image characteristics, in contrast to dose painting by contours which requires delineations to define the target for dose escalation. The direct relation between functional imaging characteristics and DPBN implies that random variations in images may propagate into the dose distribution. The stability of MR-only prostate cancer treatment planning based on DPBN with respect to these variations is as yet unknown. We conducted a test–retest study to investigate the stability of DPBN for prostate cancer in a semi-automated MR-only treatment planning workflow. Twelve patients received a multiparametric MRI on two separate days prior to prostatectomy. The tumor probability (TP) within the prostate was derived from image features with a logistic regression model. Dose mapping functions were applied to acquire a DPBN prescription map that served to generate an intensity modulated radiation therapy (IMRT) treatment plan. Dose calculations were done on a pseudo-CT derived from the MRI. The TP and DPBN map and the IMRT dose distribution were compared between both MRI sessions, using the intraclass correlation coefficient (ICC) to quantify repeatability of the planning pipeline. The quality of each treatment plan was measured with a quality factor (QF). Median ICC values for the TP and DPBN map and the IMRT dose distribution were 0.82, 0.82 and 0.88, respectively, for linear dose mapping and 0.82, 0.84 and 0.94 for square root dose mapping. A median QF of 3.4% was found among all treatment plans. We demonstrated the stability of DPBN radiotherapy treatment planning in prostate cancer, with excellent overall repeatability and acceptable treatment plan quality. Using validated tumor probability modelling and simple dose mapping techniques it was shown that despite day-to-day variations in imaging data still consistent treatment plans were obtained.

Influence of sub-field area on dose distribution in intensity-modulated radiotherapy for cervical cancer

Objective To investigate the optimization of therapeutic regimen through the adjustment of the minimum sub-field area in intensity-modulated radiotherapy (IMRT) for cervical cancer, under the premise of no influence on the dose to target volume or organs at risk. Methods A total of 12 patients with pathologically confirmed cervical cancer were enrolled, and the prescribed dose to the planning target volume (PTV) was 50 Gy in 25 fractions. The Pinnacle 8.0m treatment planning system was used for all patients, and 16 IMRT plans were developed for each patient, with the application of 9 evenly distributed fixed incidence directions (0°, 40°, 80°, 120°, 160°, 200°, 240°, 280°, and 320°), a minimum sub-field number of 80, and a minimum sub-field hop count (MU) of 5 MU.The range of sub-field area was 2-81 cm2.Direct machine parameter optimization was used for inverse-planned optimization calculation, and all the plans met the requirements of the clinical prescribed dose. The dose-volume histogram was used to evaluate the dose distribution in target volume and organs at risk. Results With the sub-field area increasing from 2 cm2 to 81 cm2, the total hop count of IMRT plan was reduced from (1 405±170) MU to (490±47) MU (P=0.000), and when the sub-field area increased above 6 cm×6 cm, the total hop count was reduced significantly (P=0.000). In the IMRT plan with a minimum sub-field area of 2-49 cm2, there was no significant difference in dose between the target volume and the organs at risk (P>0.05). The dose to the rectum, the bladder, and both femoral heads showed no significant differences across the IMRT plans with different minimum sub-field areas (P>0.05). Conclusions When the Pinnacle 8.0m treatment planning system is used to develop IMRT plans for cervical cancer, the requirements for clinical dose can still be met with a minimum sub-field area reaching 7 cm×7 cm, and there are significant reductions in sub-field hop count and total hop count. Key words: Cervix neoplasms/intensity-modulated radiotherapy; Inverse planning optimization; Area of segment field

Failure in region of parotid gland after definitive intensity-modulated radiotherapy for Nasopharyngeal Carcinoma

To investigate the potential risk factors for parotid gland failure after intensity-modulated radiotherapy (IMRT) for nasopharyngeal carcinoma (NPC). Methods The clinical data of 1096 NPC patients who underwent IMRT in our hospital from January 2005 to December 2012 were analyzed retrospectively. Among these patients, 13 patients experienced parotid gland recurrence, and the recurrence in 12 patients was analyzed. A case-control study was performed with the side of the parotid gland with recurrence as the case group and the side of the parotid gland without recurrence as the control group. The association of parotid gland failure with the extent of tumor invasion, IMRT dose distribution, and local recurrence was analyzed. The differences between groups were analyzed with χ2 test or Fisher's the exact probability test. Results Among the 12 patients, 11 had stage III-IV primary NPC; after definitive IMRT, 9 had local tumor residues. The median time of parotid gland recurrence was 16(8-43) months. Of all the patients who experienced recurrence, 8 had recurrence in the superficial lobe of the parotid gland, 1 in the deep lobe, and 3 in both superficial and deep lobes. Recurrence was in the same side of primary tumor (P<0.001). Compared with the controls, the side of the parotid gland with recurrence had higher rate of cervical puncture/surgery (P=0.025). Parotid gland recurrence was often complicated by ipsilateral lymph node recurrence (67% vs. 8%, P=0.003), as well as the tendency of ipsilateral primary lesion recurrence (42% vs. 8%; P=0.059). Conclusions For NPC patients, the recurrence rate in the parotid gland is very low. Parotid gland recurrence may be related to locally advanced NPC, residues after treatment, the history of cervical puncture/surgery, and locoregional recurrence. The low radiotherapy dose in the parotid gland caused by IMRT may be an important reason for parotid gland recurrence. Key words: Nasopharyngeal neoplasms/intensity-modulated radiotherapy; Periparotid failure; Failure analysis

A simple optimization approach for improving target dose homogeneity in intensity-modulated radiotherapy for sinonasal cancer.

Homogeneous target dose distribution in intensity-modulated radiotherapy (IMRT) for sinonasal cancer (SNC) is challenging to achieve. To solve this problem, we established and evaluated a basal-dose-compensation (BDC) optimization approach, in which the treatment plan is further optimized based on the initial plans. Generally acceptable initial IMRT plans for thirteen patients were created and further optimized individually by (1) the BDC approach and (2) a local-dose-control (LDC) approach, in which the initial plan is further optimized by addressing hot and cold spots. We compared the plan qualities, total planning time and monitor units (MUs) among the initial, BDC, LDC IMRT plans and volumetric modulated arc therapy (VMAT) plans. The BDC approach provided significantly superior dose homogeneity/conformity by 23%–48%/6%–9% compared with both the initial and LDC IMRT plans, as well as reduced doses to the organs at risk (OARs) by up to 18%, with acceptable MU numbers. Compared with VMAT, BDC IMRT yielded superior homogeneity, inferior conformity and comparable overall OAR sparing. The planning of BDC, LDC IMRT and VMAT required 30, 59 and 58 minutes on average, respectively. Our results indicated that the BDC optimization approach can achieve significantly better dose distributions with shorter planning time in the IMRT for SNC.

Accuracy of dose calculation algorithms for virtual heterogeneous phantoms and intensity-modulated radiation therapy in the head and neck.

This study verified the dose calculation accuracy of the analytical anisotropic algorithm (AAA), Acuros XB version 10 (AXB10), and version 11 (AXB11) installed in an Eclipse treatment planning system, by comparing with Monte Carlo (MC) simulations. First, the algorithms were compared in terms of dose distributions using four types of virtual heterogeneous multi-layer phantom for 6 and 15 MV photons. Next, the clinical head and neck intensity-modulated radiation therapy (IMRT) dose distributions for 6 MV photons were evaluated using dose volume histograms (DVHs) and three-dimensional gamma analysis. In percentage depth doses (PDDs) for virtual heterogeneous phantoms, AAA overestimated absorbed doses in the air cavity, bone, and aluminum in comparison with MC, AXB10, and AXB11. The PDDs of AXB10 almost agreed with those of MC and AXB11, except for the air cavity. The dose in the air cavity was higher for AXB10 than for AXB11, because their electron cutoff energies are set at 500 and 200 keV, respectively. For head and neck IMRT dose distributions, the D95 in the clinical target volume (CTV) for AAA was almost the same as that for AXB10 and was approximately 7 % larger than that for MC. Comparing each approach with MC using a criterion of 3 %/3 mm, the pass rates for AXB10, AXB11, and AAA were 92.4, 94.7, and 90.4 % in the CTV, respectively. In conclusion, AAA produces dose errors in heterogeneous regions, while AXB11 provides calculation accuracy comparable to MC. AXB10 overestimates the dose in regions that include an air cavity.

SU‐E‐T‐427: Feasibility Study for Evaluation of IMRT Dose Distribution Using Geant4‐Based Automated Algorithms

Purpose:For intensity‐modulated radiation therapy (IMRT) treatment planning validation using Monte Carlo (MC) simulations, a precise and automated procedure is necessary to evaluate the patient dose distribution. The aim of this study is to develop an automated algorithm for IMRT simulations using DICOM files and to evaluate the patient dose based on 4D simulation using the Geant4 MC toolkit.Methods:The head of a clinical linac (Varian Clinac 2300 IX) was modeled in Geant4 along with particular components such as the flattening filter and the multi‐leaf collimator (MLC). Patient information and the position of the MLC were imported from the DICOM‐RT interface. For each position of the MLC, a step‐ and‐shoot technique was adopted. PDDs and lateral profiles were simulated in a water phantom (50×50×40 cm3) and compared to measurement data. We used a lung phantom and MC‐dose calculations were compared to the clinical treatment planning used at the Seoul National University Hospital.Results:In order to reproduce the measurement data, we tuned three free parameters: mean and standard deviation of the primary electron beam energy and the beam spot size. These parameters for 6 MV were found to be 5.6 MeV, 0.2378 MeV and 1 mm FWHM respectively. The average dose difference between measurements and simulations was less than 2% for PDDs and radial profiles. The lung phantom study showed fairly good agreement between MC and planning dose despite some unavoidable statistical fluctuation.Conclusion:The current feasibility study using the lung phantom shows the potential for IMRT dose validation using 4D MC simulations using Geant4 tool kits.This research was supported by Korea Institute of Nuclear safety and Development of Measurement Standards for Medical Radiation funded by Korea research Institute of Standards and Science. (KRISS‐2015‐15011032)

SU‐E‐T‐313: The Accuracy of the Acuros XB Advanced Dose Calculation Algorithm for IMRT Dose Distributions in Head and Neck

Purpose:To investigate the accuracy of the Acuros XB version 11 (AXB11) advanced dose calculation algorithm by comparing with Monte Caro (MC) calculations. The comparisons were performed with dose distributions for a virtual inhomogeneity phantom and intensity‐modulated radiotherapy (IMRT) in head and neck.Methods:Recently, AXB based on Linear Boltzmann Transport Equation has been installed in the Eclipse treatment planning system (Varian Medical Oncology System, USA). The dose calculation accuracy of AXB11 was tested by the EGSnrc‐MC calculations. In additions, AXB version 10 (AXB10) and Analytical Anisotropic Algorithm (AAA) were also used. First the accuracy of an inhomogeneity correction for AXB and AAA algorithms was evaluated by comparing with MC‐calculated dose distributions for a virtual inhomogeneity phantom that includes water, bone, air, adipose, muscle, and aluminum. Next the IMRT dose distributions for head and neck were compared with the AXB and AAA algorithms and MC by means of dose volume histograms and three dimensional gamma analysis for each structure (CTV, OAR, etc.).Results:For dose distributions with the virtual inhomogeneity phantom, AXB was in good agreement with those of MC, except the dose in air region. The dose in air region decreased in order of MC&lt;AXB11&lt;AXB10. This may be caused by the difference of the electron cut‐off energy for algorithms, ie: 0.700 MeV for MC, 0.711 MeV for AXB11, and 1.011 MeV for AXB 10. Since the AAA algorithm is based on the dose kernel of water, the doses in regions for air, bone, and aluminum considerably became higher than those of AXB and MC. The pass rates of the gamma analysis for IMRT dose distributions in head and neck were similar to those of MC in order of AXB11&lt;AXB10&lt;AAA.Conclusion:The dose calculation accuracy of AXB11 was almost equivalent to the MC dose calculation.

Three-dimensional gamma analysis of dose distributions in individual structures for IMRT dose verification.

Our purpose in this study was to implement three-dimensional (3D) gamma analysis for structures of interest such as the planning target volume (PTV) or clinical target volume (CTV), and organs at risk (OARs) for intensity-modulated radiation therapy (IMRT) dose verification. IMRT dose distributions for prostate and head and neck (HN) cancer patients were calculated with an analytical anisotropic algorithm in an Eclipse (Varian Medical Systems) treatment planning system (TPS) and by Monte Carlo (MC) simulation. The MC dose distributions were calculated with EGSnrc/BEAMnrc and DOSXYZnrc user codes under conditions identical to those for the TPS. The prescribed doses were 76 Gy/38 fractions with five-field IMRT for the prostate and 33 Gy/17 fractions with seven-field IMRT for the HN. TPS dose distributions were verified by the gamma passing rates for the whole calculated volume, PTV or CTV, and OARs by use of 3D gamma analysis with reference to MC dose distributions. The acceptance criteria for the 3D gamma analysis were 3/3 and 2 %/2 mm for a dose difference and a distance to agreement. The gamma passing rates in PTV and OARs for the prostate IMRT plan were close to 100 %. For the HN IMRT plan, the passing rates of 2 %/2 mm in CTV and OARs were substantially lower because inhomogeneous tissues such as bone and air in the HN are included in the calculation area. 3D gamma analysis for individual structures is useful for IMRT dose verification.

Dose verification of a clinical intensity-modulated radiation therapy eye case by the magnetic resonance imaging of N-isopropylacrylamide gel dosimeters

Abstract In this study, N-isopropylacrylamide (NIPAM) polymer gel, together with magnetic resonance imaging (MRI), was used to measure the relative three-dimensional (3D) dose distribution of an intensity-modulated radiation therapy (IMRT) eye case. The gels were enclosed in cylindrical acrylic vessels with 10 cm outer diameter and 10 cm length. The gels were subsequently irradiated by delivering 5 Gy of a prescribed dose with a 6 MV linear accelerator using five fields. The 3D maps of the proton relaxation rate R2 were obtained using a 1.5 T MRI system correlated with the dose. The treatment planning system (TPS) data and NIPAM gel dosimeter data were compared with the experimental results in the form of relative dose distributions, including isodose curves, dose profiles, and gamma index maps. Results indicated that the linear relationship of the R2—dose for NIPAM gel dosimeters reached 0.999 within the dose range of 0 Gy to 12 Gy. Comparison of planar dose distributions among the gel dosimeters and TPS showed that the isodose lines corresponded to selected planes in the axial plane. For the 50% to 110% dose analysis, the maximum dose differences varied from 4.04% to 13.53%. Gamma evaluation of the planar dose profile resulted in pass rates of 96.84%, 83.16%, and 53.42% when the acceptance criteria of 3%/3 mm, 2%/2 mm, and 1%/1 mm, respectively, were used in the axial plane. Overall, the results showed that NIPAM polymer gel dosimeters can serve as a high-resolution, accurate, 3D tool for IMRT dose distribution verification.