Dosimetric Effects Research Articles

Dosimetry Validation Study for Automated Head and Neck Cancer Organs at Risk Segmentation Using Stratified Learning and Neural Architecture Search

<h3>Purpose/Objective(s)</h3> Organs at risk (OARs) segmentation is an essential process in head and neck (H&N) cancer radiotherapy. We have reported high automated segmentation geometric accuracy of Stratified Learning and Neural Architecture Search method in terms of Dice Score (DSC) from our previous study. In this study, we would evaluate the dosimetric influence of our automated approach before integrating this method into the clinical workflow. <h3>Materials/Methods</h3> To measure the dosimetric effects brought by the OARs' variance, the intensity-modulated radiotherapy (IMRT) dose plans of 10 head and neck cancer patients were replanned using the original tumor target volumes and three substitute OAR contours permutations (deep learning generated stratified organs at risk segmentation (SOARS), SOARS revised by physician (SOARS-revised), and OAR delineated from scratch by physician (human reader)). We further examined the clinical dosimetric accuracy and the clinical reference OAR contours were overlaid on top of each replanned dose grid to evaluate the dosimetric differences. <h3>Results</h3> After replanning, SOARS and SOARS-revised contours have slightly smaller Diff (max dose) as compared to human reader contours (3.4%, 3.5% vs. 4.1%). For the Diff (mean dose), human reader, SOARS, SOARS-revised achieves similar results, i.e., 5.3%, 5.0%, and 5.0%, respectively. However, more OARs from the human reader have dose variations larger than 10% or 20% as compared to SOARS and SOARS-revised. Overall, our results indicate that using OAR contours from human reader, SOARS, and SOARS-revised lead to generally comparable dose accuracy in clinical practice. SOAR-related OAR contours have fewer OARs with dose error larger than 10% or 20%. <h3>Conclusion</h3> This study further validates the clinical applicability of a deep learning based automated H&N OAR segmentation method by comparing dosimetry of plans using OAR contours generated automatically and by a human reader to the gold standard contours. The dose variations calculated after planning on automated segmentation contours are less than 5%. Our proposed automated H&N OAR segmentation method not only achieves high geometric accuracy but also helps deliver treatment beams with little variances.

Dosimetric Effect of Respiratory Phase in MR-Guided SBRT for Locally Advanced Pancreatic Cancer

<h3>Purpose/Objective(s)</h3> Dose escalation of stereotactic body radiation therapy (SBRT) for locally advanced pancreatic cancer (LAPC) has traditionally been limited by organs-at-risk (OARs) tolerance; however, MR-guided SBRT (MRgSBRT) is currently being evaluated for dose escalation in multi-institutional trials. This novel treatment method incorporates soft tissue gating and on-table adaptive planning to achieve optimal dosimetry. Moreover, respiratory gating has previously been employed to minimize dose to OARs for tumors that sit close to the diaphragm. It can be inferred that the superior position of the diaphragm in end exhale breath hold (EE) allows for greater abdominal organ separation than in deep inspiration breath hold (DI). Therefore, we hypothesized that patients (pts) treated during EE for LAPC would demonstrate favorable treatment planning metrics including increased target coverage and decreased monitor unit delivery compared to pts treated in DI. <h3>Materials/Methods</h3> Five pts with LAPC previously treated with MRgSBRT were included in this analysis. For each pt, at the time of simulation, a CT scan was performed for heterogeneity corrections followed by two planning MRI scans: one in end exhale (MR_EE) and one in deep inspiration (MR_DI). All pts were ultimately treated in DI. For this study, both MR_EE and MR_DI scans were anonymized for contouring of targets and OARs. CTV was defined as the gross tumor (GTV) plus elective nodal coverage of the superior mesenteric (SM) and celiac axis (CA) nodes. PTV was defined as a 3 mm expansion on the CTV. The prescription dose was 50 Gy in 5 fractions every other day. Each plan was normalized by taking one of the luminal GI OARs (stomach, duodenum, small bowel, or large bowel) to the maximum tolerance of 33Gy to 0.5cc. Treatment plans were created using 18-21 beams per plan with a constant number of available beams per patient. A two-tailed t-test of equal variance was used to compare CTV coverage, PTVopt coverage, and monitor units per fraction (MU/fx) between the plans created on the MR_EE and MR_DI scans. <b>Results:</b> <h3>Conclusion</h3> Dose-escalated MRgSBRT for LAPC results in adequate target coverage while also respecting OAR tolerances. However, we were not able to demonstrate any statistically significant difference between the two breath hold techniques (EE vs DI) for CTV coverage, PTVopt coverage or monitor units per fraction for our cohort. Therefore, we recommend treating using the most comfortable and reproducible breath hold per pt. Further work is needed to characterize other factors such as patient compliance and gating efficiency.

The Dosimetric Effect of Variations in Bolus Placement Identified via Cherenkov Imaging

<h3>Purpose/Objective(s)</h3> The expanding use of Cherenkov imaging in radiation therapy (RT) clinics has presented the opportunity for a variety of applications. With the ability to visualize radiation dose on the surface of patients, Cherenkov imaging has emerged as a tool for detecting incidents in treatment delivery. With the use of transparent bolus, we can detect when a bolus has been misplaced during treatment setup. In this study, it is hypothesized that changes in treatment delivery due to bolus misplacement can be detected with Cherenkov imaging and verified with the treatment planning software (TPS). <h3>Materials/Methods</h3> Radiotherapy patients were imaged with a Cherenkov imaging system consisting of multiple time-gated, intensified CMOS cameras installed in the ceiling of two treatment bunkers at an academic center. All patients were treated according to their prescribed plan, and Cherenkov images were recorded for each fraction of their treatment. We investigated the dosimetric effect of misplaced bolus by mocking up the actual bolus positions for each case in the TPS. The dose distributions were then recalculated using an advanced dose calculation algorithm with 0.1 cm voxels, and compared to the original plan. <h3>Results</h3> Images from 622 patients were manually reviewed and a total of nine treatment incidents were detected. Two of these incidents involved variations in bolus placement from day to day during whole-breast RT, resulting in a lack of coverage for one or more beams during a single fraction of the patients' treatments. In Case 1, for a left breast treatment, the bolus did not cover the medial edge of the exit LPO field, resulting in a decrease in the minimum medial skin normalized dose in that area from 28.1% to 10.7% due to the lack of bolus backscatter into the skin. In Case 2, the lateral edge of a right breast RT treatment was left uncovered during the delivery of both the RPO and LAO fields, each contributing to a decrease in dose at the lateral edge. However, there was a slight increase in dose at the medial edge without the bolus due to a lack of beam attenuation at the lateral edge. Overall, the minimum normalized dose to the entire right breast decreased from 23.2% to 18.9%. <h3>Conclusion</h3> This first investigation into the effect of bolus misplacement detected via Cherenkov imaging demonstrated that these images can inform a mock-up of the actual treatment condition in the TPS for a dosimetric analysis. In comparison to the original plans with full bolus coverage, we confirmed that partial coverage results in a noticeable minimum dose reduction for relevant structures. This result validates that the lower Cherenkov emission seen from these fractions relative to the others can be at least partially attributed to the misaligned boluses, as Cherenkov intensity is proportional to dose. Ultimately, this method of treatment review is a valuable tool for radiation oncology quality assurance programs.

Feasibility and Dosimetric Benefits of Adaptive Planning in Prostate Cancer Radiotherapy Using a Novel Treatment Planning Machine with Integrated Dual kVCT/PET Imaging Systems

<h3>Purpose/Objective(s)</h3> Interfraction anatomical changes during intensity-modulated radiotherapy for prostate cancer may lead to suboptimal patients' treatment. This study assessed the dosimetric effects of anatomical changes on pretreatment kilovoltage computed tomography (kVCT) scans of a novel dual on-board kVCT/positiron emission tomography (PET) imaging radiotherapy system. <h3>Materials/Methods</h3> This retrospective study analyzed daily kVCT scans acquired on days 1, 5, 13, 17, and 22 of a post-prostatectomy patient treated with 44 Gy/22 fractions to the prostate bed and lymph nodes. The prostate bed CTV, pelvic lymph nodes, rectum and bladder were re-contoured on the daily kVCT, and treatment plans were subsequently created. The treatment doses were calculated using the HU density table of kVCT. Delineation and treatment plans created on the daily kVCT scans were compared with those created on the planning CT. Interfraction changes of bladder and rectum were analyzed based on median dose (Dmedian), mean dose (Dmean), maximum dose (Dmax), volume receiving 10 Gy (V10), volume receiving 30 Gy (V30), and overall volume. For bladder and rectum, the correlation coefficient between volume and Dmean was analyzed. <h3>Results</h3> For the kVCT days investigated in this study, mean bladder and rectum volume changes were 10.69 cc ± 10.08 cc and 1.31 cc ± 0.64 cc, respectively. Consequentially, the average changes in Dmedian, Dmean, Dmax for the bladder were 0.51 Gy ±0.32 Gy, 0.30 Gy ±0.22 Gy, and 0.75 Gy ±0.44 Gy, respectively. For the rectum, the average changes in Dmedian, Dmean, Dmax were 0.62 Gy ±0.36 Gy, 0.59 Gy ±0.49 Gy, and 0.76 Gy ±0.57 Gy, respectively. The average changes in V10, and V30 were 0.48 % ±0.56 %, and 1.1 %± 0.66 % for bladder and 1.48 % ±1.27 %, and 1.83 %± 1.34 % for rectum. The correlation coefficient (r2) for bladder-Dmean and Rectum-Dmean were 0.765 and 0.002, respectively. <h3>Conclusion</h3> This study showed that with adaptive planning, the interfraction dose variation was minimal because adaptive plans were optimized based on daily kVCT images (this is one primary benefit of adaptive planning). Even with adaptive planning, there is still a correlation between bladder dose and bladder volume. The high-quality kVCT images produced could potentially be used for target and OAR delineation and consequentially for treatment planning. Treatment delivery could further be improved by kVCT adaptive planning, which will account for possible interfraction variations.

The Dosimetric Effects of Long On-Couch Time of Adaptive Ultra-Hypofractionated Radiotherapy for Prostate Cancer Patients on a 1.5 MR Linac

<h3>Purpose/Objective(s)</h3> To quantitatively characterize the dosimetric effects of long on-couch time of adaptive radiotherapy for prostate cancer patients on 1.5-T MR-Linac. <h3>Materials/Methods</h3> Seventeen patients consecutively treated with ultra-hypofractionated radiotherapy (UHF-RT) on 1.5-T MR-Linac were recruited from an on-going prospective phase II trial (NCT05183074). UHF-RT with dose of 36.25Gy in 5 fractions to whole prostate w/o seminal vesicles (SVs) (N=12) was delivered every other day, with a boost dose of 40Gy to whole prostate (CTV4000, N=16). Except the Pre-MR for registration and re-contouring and Position Verification-MR (PV-MR) for dose/position check which were conventionally used in Adapt-To-Shape (ATS) workflow, we also collected 3D-MR during beam-on phase (Bn-MR) and after RT (Post-MR). For each fraction, the contours of targets and organs-at-risk (OARs) on PV-MR, Bn-MR and Post-MR were projected from the Pre-MR by deformable registration, and manually adapted by the physician, followed by dose-recalculation of the ATS plan. The dose parameters of target volumes and OARs on each scan were generated in a commercially available software for final analysis. <h3>Results</h3> In total, 290 MR-scans were collected, including 85 Pre-MR, 85 PV-MR, 49 Bn-MR and 71 Post-MR scans, respectively. For each fraction, the volume differences of prostate and CTV on different scans were less than 3.0cc, indicating good alignment of target contouring. With median 49 (24-78) minutes of on-couch time, the mean PTV-V95% (34.4Gy) of plans on PV-MR, Bn-MR and Post-MR was 97.69±2.03%, 97.04±2.29%, and 96.82±2.59%, respectively, compared with 99.27±0.75% of ATS plan. Furthermore, the mean CTV-V100% (36.25Gy) of plans on PV-MR, Bn-MR and Post-MR was 99.32±1.20%, 98.59±1.84% and 98.65±1.87, respectively, compared with 99.93±0.30% of ATS plan. With excellent dose coverage of prostate-V100% (36.25Gy) (99.73±0.79% on PV-MR, 99.41±1.35% on Bn-MR and 99.38±1.35% on post-MR), the main reason of lower CTV-V100% was due to mild underdose of SVs (96.97±6.26% on PV-MR, 93.62±12.94% on Bn-MR and 94.69±9.22% on post-MR). With comparison to values of ATS plan, the actual dose of rectum wall during the whole workflow also varied, with mean △V38_Gy of 0.23±0.28cc on PV-MR, 0.41±0.51cc on Bn-MR and 0.39±0.52cc on post-MR, with mean △V36_Gy of 0.27±0.57cc on PV-MR, 0.39±0.71cc on Bn-MR and 0.30±0.68cc on post-MR, respectively. However, the V29_Gy of rectum wall increased by >15% was only observed in one scan. On the contrary, no obvious increase of bladder wall dose was seen due to gradual growth of bladder over the workflow. <h3>Conclusion</h3> This dosimetry analysis demonstrated reliable dose coverage of target volumes during beam-on period with adaptive ATS workflow on 1.5-T MR-Linac. The 3 mm CTV-PTV margin applied in our study have proven to be large enough for prostate irradiation, but might be insufficient for SVs incidentally. More attention should be paid to restrict the rectum wall high dose when optimizing ATS plan.

Patient-specific dose correction for prostate postimplant evaluation with flexible timing of postimplant imaging.

The dosimetric effect of edema on prostate implants have long been realized, but large uncertainties still exist in the estimation of dose actually received by the prostate. This study attempted to develop a new method to accurately estimate dose delivered to the prostate accounting for the variation of prostate volume and seed distribution, edema half-lives, and times for postimplant evaluation. A series of prostate seed implants for Cs-131, Pd-103, and I-125 with various prostate volumes were simulated in a water phantom using the TG-43 algorithm on the Varian Eclipse treatment planning system. Dose analysis was performed to derive a quantitative relationship between the prostate peripheral dose and the prostate radius with the variation of prostate volume and seed distribution. Using this relationship to calculate dynamically, the total dose accumulated in the prostate (DT ) accounting for the variation of prostate volume and seed distribution and edema half-lives. Moreover, the total dose can be estimated statically based on the prostate volume that can be determined in a computerized tomography (CT) image taken at a time after implantation. The statically estimated total dose (DCT ) was compared with DT to determine optimal imaging times as well as dose correction factors for other imaging times. An inverse power law was established between the prostate peripheral dose and prostate radius. The value of the power was 1.3 for Cs-131 and I-125, and 1.5 for Pd-103, respectively. DT was derived dynamically using the inverse power law. Given the edema half-lives, TE , of 4, 9.3, and 25 days and the volume expansion of 1.1 and 2.0 times of the prostate without edema, the optimal times for postimplant imaging were: 7, 9, and 16 days for TE = 4 days; 10, 14, and 28days for TE = 9.3 days; and 12, 19, 45 days TE = 25 days, for Cs-131, Pd-103, and I-125, respectively. DCT calculated using the prostate volume determined on the optimal days agreed with DT to 0.0%-1.8% and within 0.3% for most cases. For various prostate volumes, edema half-lives, and nonoptimal times, DCT was able to achieve a 1% accuracy. The postimplant dose calculation based on the proposed inverse power law for prostate seed implants with edema has improved the accuracy of postimplant dosimetry with accurate and patient-specific dose corrections accounting for prostate size, edema half-life, and postimplant imaging times. Optimal times for postimplant imaging have been accurately determined, and the high accuracy of postimplant dose calculation can be achieved for both optimal imaging times and nonoptimal imaging times.

Dosimetric comparison of AcurosBV with AAPM TG43 dose calculation formalism in cervical intraductal high‐dose‐rate brachytherapy using three different applicators

AbstractPurposeTo compare the dosimetric effects of American Association of Physicists in Medicine (AAPM) TG43 dose formalism and AcurosBV (grid‐based Boltzmann solver, GBBS) formalism on high‐dose‐rate (HDR) brachytherapy planning for cervical cancer patients irradiated using three different applicators.MethodsA TG43 plan and a AcurosBV plan were generated for each of the 30 patients. Twenty patients who had undergone whole pelvic radiotherapy followed by cervical HDR brachytherapy and the remaining 10 patients who underwent total hysterectomy only gave HDR brachytherapy also were enrolled in this study. The patients were divided into three groups according to the types of applicators used: tandem and ovoid (T&amp;O), tandem and ring (T&amp;R), and cylinder. To compare the dosimetric parameters, the cumulative dose‐volume histograms (DVHs) were measured. We also compared the doses at 90% of the volume (D90%), the volume receiving 100% and 150% of the prescribed dose (V100% and V150%) for the clinical target volume (CTV‐HR), and the doses of point A, the dose receiving 0.1 and 2 cc of the volume (D0.1cc and D2cc) for the organs‐at‐risk (OARs).ResultsCompared with the AcurosBV plans, TG43 plans predicted higher D90%, V100%, and V150% of CTV‐HR, dose of point A, and D0.1cc and D2cc of OARs in three types of applicators. Except D2cc of sigmoid in T&amp;R and cylinder applicators, the D90%, V100%, and V150% of CTV‐HR; the dose of point A and the D0.1cc and D2cc of bladder, rectum, and small bowel exhibited significant discrepancies (all p &lt; 0.05). The effects of the three types of applicators on the dose distribution were quite different due to the differences in the materials: The dose difference of CTV‐HR and OARs was greatest (around 10%) for T&amp;O applicators but only 1–5% for T&amp;R and cylinder applicators.ConclusionsAcurosBV was more accurate in calculating the doses in the air cavity and high‐density substance than TG43. In the clinical setting, AcurosBV exhibited different dosimetric distributions in the cervix plans for HDR brachytherapy, especially in treatment planning when using T&amp;O applicators. The AcurosBV algorithm should be considered when using T&amp;O applicators or other materials with a much higher or lower density (metal or air) than soft tissue. However, If the density is close to that of soft tissue, considering AcurosBV algorithm requires more calculation time, TG43 could still be selected when using applicators in clinical.

Clinical validation of deep learning algorithms for radiotherapy targeting of non-small-cell lung cancer: an observational study

Artificial intelligence (AI) and deep learning have shown great potential in streamlining clinical tasks. However, most studies remain confined to in silico validation in small internal cohorts, without external validation or data on real-world clinical utility. We developed a strategy for the clinical validation of deep learning models for segmenting primary non-small-cell lung cancer (NSCLC) tumours and involved lymph nodes in CT images, which is a time-intensive step in radiation treatment planning, with large variability among experts. In this observational study, CT images and segmentations were collected from eight internal and external sources from the USA, the Netherlands, Canada, and China, with patients from the Maastro and Harvard-RT1 datasets used for model discovery (segmented by a single expert). Validation consisted of interobserver and intraobserver benchmarking, primary validation, functional validation, and end-user testing on the following datasets: multi-delineation, Harvard-RT1, Harvard-RT2, RTOG-0617, NSCLC-radiogenomics, Lung-PET-CT-Dx, RIDER, and thorax phantom. Primary validation consisted of stepwise testing on increasingly external datasets using measures of overlap including volumetric dice (VD) and surface dice (SD). Functional validation explored dosimetric effect, model failure modes, test-retest stability, and accuracy. End-user testing with eight experts assessed automated segmentations in a simulated clinical setting. We included 2208 patients imaged between 2001 and 2015, with 787 patients used for model discovery and 1421 for model validation, including 28 patients for end-user testing. Models showed an improvement over the interobserver benchmark (multi-delineation dataset; VD 0·91 [IQR 0·83-0·92], p=0·0062; SD 0·86 [0·71-0·91], p=0·0005), and were within the intraobserver benchmark. For primary validation, AI performance on internal Harvard-RT1 data (segmented by the same expert who segmented the discovery data) was VD 0·83 (IQR 0·76-0·88) and SD 0·79 (0·68-0·88), within the interobserver benchmark. Performance on internal Harvard-RT2 data segmented by other experts was VD 0·70 (0·56-0·80) and SD 0·50 (0·34-0·71). Performance on RTOG-0617 clinical trial data was VD 0·71 (0·60-0·81) and SD 0·47 (0·35-0·59), with similar results on diagnostic radiology datasets NSCLC-radiogenomics and Lung-PET-CT-Dx. Despite these geometric overlap results, models yielded target volumes with equivalent radiation dose coverage to those of experts. We also found non-significant differences between de novo expert and AI-assisted segmentations. AI assistance led to a 65% reduction in segmentation time (5·4 min; p<0·0001) and a 32% reduction in interobserver variability (SD; p=0·013). We present a clinical validation strategy for AI models. We found that in silico geometric segmentation metrics might not correlate with clinical utility of the models. Experts' segmentation style and preference might affect model performance. US National Institutes of Health and EU European Research Council.

Imaging investigation of small motion target for lung stereotactic body radiotherapy (SBRT)

Purpose: Motion management is important in lung tumor SBRT. Small target four-dimensional computed tomography (4DCT) imaging could give significant target motion information. In this study, Quality of 4DCT for small target was investigated to provide information for treatment planning in SBRT of small target, which is in favor of magnifying the target margin characteristics during 4DCT scan. Method and Materials: A Real Time Position Management (RPM) phantom from Varian Medical System and 6 CT-spot pellets from Beekley Medical were employed for this study. Each pellet was equivalent to a 4mm diameter ball. The CT spot pellets were attached to the RPM phantom. Two pellet set planes formed 8cm distance and generated two speeds which were 0.2cm and 0.4mm per second given a period with 5.0 seconds. With a GE LightSpeedTM RT CT scanner, the phantom was executed with helical scan with 1.25mm slice thickness with and without motion. Then a 4D CT scan in axial cine mode was carried on at the same phantom. The image exams were exported the GE Advantage Workstation for process. The exams were reviewed and sorted into 10 phases retrospectively. The 0% phase image set, 50% phase image set, maximum intensity projection (MIP) image set, and average intensity projection (Ave-IP) image set were exported to the Eclipse treatment planning system for analysis. In image sets, multi-dimensional variables such as center coordinate of pellets, distance between pellets, and volume of the pellets were used for analysis. Results: At 350 HU window width and 40HU window level, image sets from helical scan for the static phantom is used as reference, comparing to helical scan from moving phantom, retrospective MIP, average intensity projection, 0% phase and 50% phase, for pellets moving at 0.4 cm per second, the average coordinate deviation are 0.49cm, 0.22cm, 0.19cm, 0.59cm and 0.06cm with standard deviation 0.83cm, 0.29cm, 0.32cm, 0.69cm, and 0.07cm, the percentage distance differences are 10.2%, 3.3%, 7.3%, 0.1%, and 0.1% with standard deviation of 13.2%, 4.4%, 9.6%, 0.1%, and 0.1%. and the percentage volumes changes are 17.4%, 171.5%, 18.4%, 14.2% and 7.8% with standard deviation at 25.4%, 201.8%, 23.7%, 19.85% and 10%. Conclusions: A simple and sensitive 4DCT quality assurance method was developed by combining Varian RPM phantom and CT-spot pellets from Beekley Medical and it shows that the 50% phase image set is better for SBRT planning. Further study will include dosimetric effect on the small target moving in different directions.

Evaluation of dosimetric effects of metallic artifact reduction and tissue assignment on Monte Carlo dose calculations for 125 I prostate implants.

Monte Carlo (MC) simulation studies, aimed at evaluating the magnitude of tissue heterogeneity in 125 I prostate permanent seed implant brachytherapy (BT), customarily use clinical post-implant CT images to generate a virtual representation of a realistic patient model (virtual patient model). Metallic artifact reduction (MAR) techniques and tissue assignment schemes (TAS) are implemented on the post-implant CT images to mollify metallic artifacts due to BT seeds and to assign tissue types to the voxels corresponding to the bright seed spots and streaking artifacts, respectively. The objective of this study is to assess the combined influence of MAR and TAS on MC absorbed dose calculations in post-implant CT-based phantoms. The virtual patient models used for 125 I prostate implant MC absorbed dose calculations in this study are derived from the CT images of an external radiotherapy prostate patient without BT seeds and prostatic calcifications, thus averting the need to implement MAR and TAS. The geometry of the IsoSeed I25.S17plus source is validated by comparing the MC calculated results of the TG-43 parameters for the line source approximation with the TG-43U1S2 consensus data. Four MC absorbed dose calculations are performed in two virtual patient models using the egs_brachy MC code: (1) TG-43-based Dw,w-TG 43 , (2) Dw,w-MBDC that accounts for interseed scattering and attenuation (ISA), (3) Dm,m that examines ISA and tissue heterogeneity by scoring absorbed dose in tissue, and (4) Dw,m that unlike Dm,m scores absorbed dose in water. The MC absorbed doses (1) and (2) are simulated in a TG-43 patient phantom derived by assigning the densities of every voxel to 1.00gcm-3 (water), whereas MC absorbed doses (3) and (4) are scored in the TG-186 patient phantom generated by mapping the mass density of each voxel to tissue according to a CT calibration curve. The MC absorbed doses calculated in this study are compared with VariSeed v8.0 calculated absorbed doses. To evaluate the dosimetric effect of MAR and TAS, the MC absorbed doses of this work (independent of MAR and TAS) are compared to the MC absorbed doses of different 125 I source models from previous studies that were calculated with different MC codes using post-implant CT-based phantoms generated by implementing MAR and TAS on post-implant CT images. The very good agreement of TG-43 parameters of this study and the published consensus data within 3% validates the geometry of the IsoSeed I25.S17plus source. For the clinical studies, the TG-43-based calculations show a D90 overestimation of more than 4% compared to the more realistic MC methods due to ISA and tissue composition. The results of this work generally show few discrepancies with the post-implant CT-based dosimetry studies with respect to the D90 absorbed dose metric parameter. These discrepancies are mainly Type B uncertainties due to the different 125 I source models and MC codes. The implementation of MAR and TAS on post-implant CT images have no dosimetric effect on the 125 I prostate MC absorbed dose calculation in post-implant CT-based phantoms.

Dosimetric effects of oral contrast in the planning of conventional radiotherapy and IMRT, for rectal cancer treatment

Abstract Introduction: Contrast media are frequently used during radiation therapy simulation. However, there are concerns about dosimetric variations when dose calculation is done on contrast-enhanced computed tomography (CT). This study evaluates the dosimetric effect of oral contrast during three-dimensional conformal radiotherapy (3D-CRT) and volumetric modulated arc radiotherapy (VMAT) planning. Methods: Rectal cancer patients were consecutively enrolled. For each patient, one unenhanced CT and one contrast-enhanced CT were taken using oral and intravenous contrast. Then, a 3D-CRT plan and an Intensity-modulated radiation therapy (IMRT)/VMAT plan were generated in the enhanced CT, and the dose distribution was recalculated in the respective unenhanced CT. The beam intensities were kept the same as for the enhanced CT plans. Finally, the unenhanced and enhanced plans were compared by calculating the gamma index. Results: For 3D-CRT plans, there were statistically significant differences in second phase planning target volume (PTV) D2% (Mean difference (MD) between unenhanced and enhanced CT 0·01 Gy, 95% CI [0·003 to 0·02 Gy]) and in maximum doses to the bladder (MD 0·26 Gy, 95% CI [0·05 to 0·47 Gy]). For IMRT/VMAT plans, there were statistically significant differences in small intestine V45 Gy (MD 3·1 cc, 95% CI [0·81 to 5·4 cc]), bladder V45 Gy (MD 2·9%, 95% CI [1·4 to 4·3%]) and maximum dose to the bladder (MD 0·65 Gy, 95% CI [0·46 Gy to 0·85 Gy]). In addition, for PTV D98% the MD between unenhanced and enhanced CT was 0·22 Gy 95% CI [0·05 to 0·39]. Conclusions: For most of the dose metrics, the differences were not clinically meaningful. The greatest differences were found in VMAT plans, especially in V45 Gy of the small intestine. This difference could lead to an underestimation of dose–volume metrics when the plan is based on an enhanced CT. The use of small bowel oral contrast does not significantly influence dose calculations and may not affect the acceptability of plans when adhering to constraints.

Dosimetric effects of the ipsilateral shoulder position variations in the sitting-positioned boron neutron capture therapy for lower neck tumor

We aimed to evaluate dosimetric effects of ipsilateral shoulder position variations (ISPVs) in sitting-positioned boron neutron capture therapy (BNCT) for lower neck tumor. The ISPVs were simulated using deformed shoulder images that can simulate arbitrary shape. The dose–volume parameters for the tumor in the rotated shoulder plans considerably varied compared with that for the mucosa. Even in a small number of cases, these differences were clearly observed among patients. The ISPVs in lower neck BNCT have great dosimetric effects.

Dosimetric effects of respiratory motion during stereotactic body radiation therapy of lung tumors

Background Respiratory-induced lung tumor motion may affect the delivered dose in stereotactic body radiation therapy (SBRT). Previous studies are often based on phantom studies for one specific treatment technique. In this study, the dosimetric effect of tumor motion was quantified in real patient geometries for different modulated treatments and tumor motion amplitudes for lung-SBRT. Material and Methods A simulation method using deformable image registrations and 4-dimensional computed tomographies (4DCT) was developed to assess the dosimetric effects of tumor motion. The method was evaluated with ionization chamber and Gafchromic film measurements in a thorax phantom and used to simulate the effect for 15 patients with lung tumors moving 7.3–27.4 mm. Four treatment plans with different complexities were created for each patient and the motion-induced dosimetric effect to the gross tumor volume (GTV) was simulated. The difference between the planned dose to the static tumor and the simulated delivered dose to the moving tumor was quantified for the near minimum (D98%), near maximum (D2%) and mean dose (Dmean) to the GTV as well as the largest observed local difference within the GTV (Maxdiff). Results No correlation was found between the dose differences and the tumor motion amplitude or plan complexity. However, the largest deviations were observed for tumors moving >15.0 mm. The simulated delivered dose was within 2.5% from the planned dose for D98% (tumors moving <15 mm) and within 3.3% (tumors moving >15 mm). The corresponding values were 1.7% vs. 6.4% (D2%); 1.7% vs. 2.4% (Dmean) and 8.9% vs. 35.2% (Maxdiff). Using less complex treatment techniques minimized Maxdiff for tumors moving >15.0 mm. Conclusion The dosimetric effects of respiratory-induced motion during lung SBRT are patient and plan specific. The magnitude of the dosimetric effect cannot be assessed solely based upon tumor motion amplitude or plan complexity.

Dosimetric effect of modelling non-homogeneous LINAC couch using cone-beam computed tomography on quality assurance (QA) results

Abstract Aim: To evaluate the dosimetric effect of modelling a non-homogeneous couch on patients’ quality assurance (QA) gamma pass rates for intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) techniques. Materials and Methods: A non-homogeneous treatment couch (TxT 550 TTM, CIVCO, USA) was imaged using the LINAC mounted cone-beam computer tomography (CBCT) system. Modelling this couch in different situations, including incomplete (homogeneous model), correct model and not defined situations in the treatment planning system (TPS), was performed based on the geometrical and material densities data extracted from the CBCT images. Calculated gamma pass rates between TPS dose calculations and the measurements in a phantom for different couch models were obtained and compared at two gamma criteria (2%-2 mm and 3%-3 mm). Results: Comparing TPS calculations for the correct modelled couch and the measurements showed high gamma pass rates for both the IMRT and VMAT techniques (96·5 ± 0·9%, 99·2 ± 0·5% for IMRT in 2%-2 mm and 3%-3 mm criteria; 97·5 ± 0·8%, 99·4 ± 0·5% for VMAT). The overall gamma pass rate of the IMRT plan QAs was reduced by about 2% and 3% on average for incomplete and no couch modelling, respectively. These reductions for VMAT techniques were 2·5% and 4·3%, respectively. Conclusions: Non-homogeneous couches have different parts with different attenuations, which can be correctly defined using LINAC CBCT. Modelling of treatment couch has a significant effect on patient QA results for VMAT and IMRT plans, especially in radiation fields/subfield transmitting from the couch. We suggest using LINAC CBCTs as an appropriate device for couch modelling in modulated radiotherapy techniques.

Comparison of Dose Distribution around Central Vaginal Applicator Brachytherapy Treatment Calculated with Water Based TG43 &amp; Model Based TG186

Aim: The focus of this research was to look at the dosimetric effect and evaluate the dose difference between TG-186 and TG-43 plans for cervical carcinoma using central vaginal applicators, in order to help clinicians to choose right algorithm. Introduction: TG-43 protocol is for calculation of dose-rate distributions around photon-emitting brachytherapy sources. The radiation is assumed to be carried out through an infinite homogeneous water phantom in the TG-43 dose calculation approach. Thus any heterogeneities within or outside the patient are not taken into account. As a result the accuracy of dose calculations in places near to air or bone may be impacted. The recently published AAPM Radiation Therapy Committee Task Group 186 (TG-186) report has provided new guidelines for dose calculation and reporting in brachytherapy to address the shortcomings of the current TG-43U1 dose calculation methodology. Heterogeneity correction algorithms have only recently been made available to the BT community in contrary to external beam radiotherapy. Materials and Methods: A retrospective study including 15 patients with cervical cancer who had undergone postoperative brachytherapy was performed. External Beam Radiotherapy (EBRT) was performed with a total dose of 50 Gy in 25 fractions as 2 Gy/per fraction using conventional fractionation schedule.Based on TG-43 and TG-186 algorithms plans were done for 15 cervical patients on the Nucletron- oncentra planning system and treated with Ir-192 brachytherapy source. The plans were not re optimized and therefore the dwell positions and dwell times were identical between the two plans. Retrospectively the plans were recalculated using the TG-186 algorithms. For each patient DVH is used to calculate dose in 0.1cc CGy, 1cc CGy, 2cc CGy and 5cc CGy of OAR doses as well as prescription point doses. Results: D w,m, found small changes in Prescription doses and in D0.1 cc, D1 cc, D2 cc, D5 cc for rectum and bladder with all dose parameters for individual patients differing from TG-43 values by &lt; 1%. Conclusion: The dose difference between TG43 and TG186 algorithms is for most clinical cases not significant for target volumes and OARs. Cylinder applicators although TG43 algorithm overestimated the tissue dose the difference of dose distribution caused by the two algorithms was almost negligible because the difference of dose distribution was not much (less than 1%) and both were located around the applicator.

The geometric and dosimetric effect of algorithm choice on propagated contours from CT to cone beam CTs

PurposeAdaptive radiotherapy relies of rapid re-contouring, online more so than offline. Intra-patient contour propagation via non-rigid registration offers a solution but can be of limited accuracy. However, the dosimetric significance of the inaccuracies is unknown. Here we evaluate the dosimetric reliability of contours generated by different commercially-available software packages. MethodPlanning CT contours for ten head and neck cancer patients were propagated via five commercial packages to five CBCT scans acquired throughout treatment. The treatment plan was recalculated on each of the CBCTs for each set of propagated contours, and DVH parameters extracted for the spinal cord, brainstem, parotids and larynx. The propagated contours were compared to two gold standard contours: contours manually outlined and a consensus STAPLE contours generated from the propagated contours. Geometrical similarity was evaluated using mean distance to agreement (mDTA), Hausdorff distance, centroid agreement and Dice similarity coefficient. Dosimetric reliability was assessed against clinical constraints and comparing via the intraclass correlation coefficient (ICC). ResultsAll propagated contours were similar to the STAPLE (mDTA < 1.0 mm) whilst larger differences were seen for the manual contours (mDTA < 3.0 mm). The dosimetric comparison showed that the propagated contours gave excellent dose estimates for most organs. The spinal cord reliability was moderate (ICC > 0.66). ConclusionsLarge differences in geometric metrics rarely had a statistically significant impact on DVH parameters for the OARs studied. For that reason, propagated contours on treatment CBCT images are suitable for estimating dose to the OARs.

Dosimetric effects of embolization material artefacts in arteriovenous malformations stereotactic radiosurgery on treatment planning calculation.

Background and PurposeStereotactic Radiosurgery (SRS) is a specialized radiotherapy treatment technique for Arteriovenous Malformations (AVM) in which Computed Tomography (CT) images are used for dose calculations. The purpose of this study was to investigate CT image distortions caused by embolic agents and quantify the influence of these distortions on dose calculations.MethodsEight AVM patients administered embolic agents prior to SRS were included. Original plans were compared to new recalculated plans using two sets of images. The first set was created by masking the embolic material and artefacts, the second was the diagnostic CT images. In addition, treatment plans were created for an anthropomorphic phantom with water inserts, then with known volumes of embolic materials to study the dosimetric effect of each material.ResultsRelative to patients’ original plans, maximum Monitor Unit (MU) difference was −4.4% with whole brain masking, −1.3% with artefact masking, −4.1% with embolic masking, and −4.5% with artefact-free diagnostic images. Calculated dose differences were within ± 3.5% for all plans. In phantom, Gamma pass rate was 96% for both embolic agents with conformal fields and 99.9% with dynamic arcs. Dose and MU differences in phantom plans were negligible.ConclusionRelative dose differences between the original plans and the corrected ones were not clinically remarkable. We recommend evaluating the effect of embolic materials on individual patients’ plans. The whole brain corrected planning CT images or diagnostic CT images could be utilized to calculate the magnitude of dose reduction caused by embolic materials and correct it if necessary.

Dosimetric Effect of Injection Ports in Tissue Expanders on Post-Mastectomy Volumetric Modulated Arc Therapy (VMAT) Planning for Left-Sided Breast Cancer

This study aimed to compare the dosimetric effect of traditional metallic ports and radio frequency identification (RFID) ports (Motiva Flora®) on post-mastectomy volumetric modulated arc therapy (VMAT) planning for left-sided breast cancer. Computed tomography (CT) simulation was performed on an anthropomorphic torso phantom by attaching two types of tissue expander on the left chest wall. For the comparison of CT artifacts, five points of interest were selected and compared: point A = central chest wall, B = medial chest wall, point C = lateral chest wall, point D = axilla, and point E = left anterior descending artery. VMAT planning using two partial arcs with a single isocenter was generated, and dosimetric parameters were investigated. Compared to metallic ports, RFID ports tremendously decreased distortion on CT images, with the exception of point D. The V5Gy, V10Gy, V15Gy, V20Gy, V30Gy, and Dmean values of the heart in RFID ports were lower than those in metallic ports. The V5Gy, V15Gy, V20Gy, V30Gy, and Dmean values of the ipsilateral lung in RFID ports were also lower than those in metallic ports. RFID ports showed superior dosimetric results for doses to the heart and lungs as compared to that in metallic ports.

Mean Brain Dose Remains Uninfluenced by the Lesion Number for Gamma Knife Stereotactic Radiosurgery for 10+ Metastases

Gamma Knife (GK) stereotactic radiosurgery (SRS) is increasingly used as an initial treatment for patients with 10 or more brain metastases. However, the clinical and dosimetric consequences of this practice are not well established. We performed a single-institution, retrospective analysis of 30 patients who received Gamma Knife SRS for 10 or more brain metastases in 1 session. We utilized MIM Software to contour the whole brain and accumulated the doses from all treated lesions to determine the mean dose delivered to the whole brain. Patient outcomes were determined from chart review. Our cohort had a median number of 13 treated lesions (range 10-26 lesions) for a total of 427 treated lesions. The mean dose to the whole brain was determined to be 1.8±0.91Gy (range 0.70-3.8Gy). The mean dose to the whole brain did not correlate with the number of treated lesions (Pearson r= 0.23, P= 0.21), but was closely associated with tumor volume (Pearson r= 0.95, P < 0.0001). There were no significant correlations between overall survival and number of lesions or aggregate tumor volume. Fourteen patients (47%) underwent additional SRS sessions and 6 patients (20%) underwent whole-brain radiotherapy with a median of 6.6months (range 3.0-50months) after SRS. Two patients (6.6%) developed grade 2 radionecrosis following SRS beyond earlier whole-brain radiotherapy. The mean dose to the whole brain in patients treated with Gamma Knife SRS for 10 or more brain metastases remained low with an acceptable rate of radionecrosis. This strategy allowed the majority of patients to avoid subsequent whole-brain radiotherapy.

Influence dosimetric study of different couches in radiotherapy treatments

Radiotherapy is a recommended procedure, in average, for 52% of cancer cases as one of the treatment forms, therefore, it is important for the clinical practice to investigate the affecting factors in dose distribution received by the patients, and for example, devices and treatment couch. With the introduction of treatments with modulated intensity techniques like IMRT and VMAT, the number of incidence fields used for patient treatment increased, which makes the couch’s dosimetric effect more significant in these modalities. The attenuation data acquisition referring to the treatment couches, as well as the TPS data evaluation, show as important parameters for the clinical practice because they show what happens with the dose delivery during the treatment, ensuring a better quality and safety to the treatments. This research presents experimental results evaluating the couch’s impact in the treatments by a study of perturbation in the distribution of surface dose, dose attenuation according to the gantry’s angle for the couches BrainLABTM, ExactTM and iBEAMTM. Then we propose better density values for the couches BrainLABTM and ExactTM for their inclusion in EclipseTM TPS. Lastly, we compare the dose difference considering the presence or not of couch in the planning. In conclusion, the beam’s attenuation increase by the couches and the doses alterations on the skin must be taken in consideration in the treatment planning process. It is of great relevance that each treatment center perform internal tests to determinate the best density values for available TPS.