Accuracy and reliability of a commercial treatment planning system in nontarget regions in modern prostate radiotherapy.

BACKGROUND: The currently available treatment planning systems (TPSs) are neither designed nor intended for accurate dose calculations in nontarget regions. The aim of this work is to quantify the accuracy and reliability of nontarget doses calculated by a commercially available TPS. METHODS: Nontarget doses calculated by the collapsed cone (CC) (v5.2) algorithm implemented in the RayStation (v6) TPS were compared to measured values. Different scenarios were investigated, from simple static fields to intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) treatment plans. Deviations and confidence limits (CLs) were calculated between results of calculations and measurements-applying both local (δ) and global (Δ) normalization-for various points of interest (POIs). Results were based on a single-institution experience for one clinical test case (prostate) and evaluated against internationally accepted criteria. RESULTS: Overall, the TPS underestimated the nontarget dose by an average of -17.7% ± 25.3% for IMRT. Quantitatively similar results were obtained for VMAT (-17.6% ± 21.2%). POIs receiving < 5% of the prescription dose were significantly underestimated by the TPS (p-value < 0.05 for both IMRT and VMAT). Dose calculation accuracy was also determined by the contribution of secondary radiation, with measured doses for out-of-field POIs being significantly different from calculated values (p-value < 0.01 for both IMRT and VMAT). Although the CLδ in nontarget regions failed the proposed tolerance criteria (40%) for both IMRT (68.8%) and VMAT (52.6%), the CLΔ was within the tolerance limit (4%) for both treatment techniques (1.9% for IMRT and 1.3% for VMAT). No action levels (7%) were exceeded. CONCLUSIONS: Based on the currently available benchmarks our TPS is considered acceptable for clinical use, although the dose in some POIs was poorly predicted by the CC algorithm. Some areas were pointed out where TPSs and linear accelerator control systems can be improved.

A comparative analysis of Acuros XB and the analytical anisotropic algorithm for volumetric modulation arc therapy.

BACKGROUND: This study aimed to verify the dosimetric impact of Acuros XB (AXB) (AXB, Varian Medical Systems Palo Alto CA, USA), a two model-based algorithm, in comparison with Anisotropic Analytical Algorithm (AAA ) calculations for prostate, head and neck and lung cancer treatment by volumetric modulated arc therapy (VMAT ), without primary modification to AA. At present, the well-known and validated AA algorithm is clinically used in our department for VMAT treatments of different pathologies. AXB could replace it without extra measurements. The treatment result and accuracy of the dose delivered depend on the dose calculation algorithm. MATERIALS AND METHOD: Ninety-five complex VMAT plans for different pathologies were generated using the Eclipse version 15.0.4 treatment planning system (TPS). The dose distributions were calculated using AA and AXB (dose-to-water, AXBw and dose-to-medium, AXBm), with the same plan parameters for all VMAT plans. The dosimetric parameters were calculated for each planning target volume (PTV) and involved organs at risk (OA R). The patient specific quality assurance of all VMAT plans has been verified by Octavius®-4D phantom for different algorithms. RESULTS: The relative differences among AA, AXBw and AXBm, with respect to prostate, head and neck were less than 1% for PTV D95%. However, PTV D95% calculated by AA tended to be overestimated, with a relative dose difference of 3.23% in the case of lung treatment. The absolute mean values of the relative differences were 1.1 ± 1.2% and 2.0 ± 1.2%, when comparing between AXBw and AA, AXBm and AA, respectively. The gamma pass rate was observed to exceed 97.4% and 99.4% for the measured and calculated doses in most cases of the volumetric 3D analysis for AA and AXBm, respectively. CONCLUSION: This study suggests that the dose calculated to medium using AXBm algorithm is better than AAA and it could be used clinically. Switching the dose calculation algorithm from AA to AXB does not require extra measurements.

Peripheral blood lymphocytes differ in DNA damage response after exposure to X-rays with different physical properties.

Introduction: In radiology, low X-ray energies (<140 keV) are used to obtain an optimal image while in radiotherapy, higher X-ray energies (MeV) are used to eradicate tumor tissue. In radiation research, both these X-ray energies being used to extrapolate in vitro research to clinical practice. However, the energy deposition of X-rays depends on their energy spectrum, which might lead to changes in biological response. Therefore, this study compared the DNA damage response (DDR) in peripheral blood lymphocytes (PBLs) exposed to X-rays with varying beam quality, mean photon energy (MPE) and dose rate.Methods: The DDR was evaluated in peripheral blood lymphocytes (PBLs) by the É£-H2AX foci assay, the cytokinesis-block micronucleus assay and an SYTOX-based cell death assay, combined with specific cell death inhibitors. Cell cultures were irradiated with a 220 kV X-ray research cabinet (SARRP, X-Strahl) or a 6 MV X-ray linear accelerator (Elekta Synergy). Three main physical parameters were investigated: beam quality (V), MPE (eV) and dose rate (Gy/min). Additional copper (Cu) filtration caused variation in the MPE (78 keV, 94 keV, 118 keV) at SARRP; dose rates were varied by adjusting tube current for 220 kV X-rays (0.33-3 Gy/min) or water-phantom depth in the 6 MV set-up (3-6 Gy/min).Results: The induction of chromosomal damage and initial (30 min) DNA double-stranded breaks (DSBs) were significantly higher for 220 kV X-rays compared to 6 MV X-rays, while cell death induction was similar. Specific cell death inhibitors for apoptosis, necroptosis and ferroptosis were not capable of blocking cell death after irradiation using low or high-energy X-rays. Additional Cu filtration increased the MPE, which significantly decreased the amount of chromosomal damage and DSBs. Within the tested ranges no specific effects of dose rate variation were observed.Conclusion: The DDR in PBLs is influenced by the beam quality and MPE. This study reinforces the need for consideration and inclusion of all physical parameters in radiation-related studies.

Impact of proton therapy on the DNA damage induction and repair in hematopoietic stem and progenitor cells.

Proton therapy is of great interest to pediatric cancer patients because of its optimal depth dose distribution. In view of healthy tissue damage and the increased risk of secondary cancers, we investigated DNA damage induction and repair of radiosensitive hematopoietic stem and progenitor cells (HSPCs) exposed to therapeutic proton and photon irradiation due to their role in radiation-induced leukemia. Human CD34+ HSPCs were exposed to 6 MV X-rays, mid- and distal spread-out Bragg peak (SOBP) protons at doses ranging from 0.5 to 2 Gy. Persistent chromosomal damage was assessed with the micronucleus assay, while DNA damage induction and repair were analyzed with the γ-H2AX foci assay. No differences were found in induction and disappearance of γ-H2AX foci between 6 MV X-rays, mid- and distal SOBP protons at 1 Gy. A significantly higher number of micronuclei was found for distal SOBP protons compared to 6 MV X-rays and mid- SOBP protons at 0.5 and 1 Gy, while no significant differences in micronuclei were found at 2 Gy. In HSPCs, mid-SOBP protons are as damaging as conventional X-rays. Distal SOBP protons showed a higher number of micronuclei in HSPCs depending on the radiation dose, indicating possible changes of the in vivo biological response.

Microbiome and metabolome dynamics during radiotherapy for prostate cancer.

BACKGROUND: Prostate cancer patients treated with radiotherapy are susceptible to acute gastrointestinal (GI) toxicity due to substantial overlap of the intestines with the radiation volume. Due to their intimate relationship with GI toxicity, faecal microbiome and metabolome dynamics during radiotherapy were investigated. MATERIAL & METHODS: This prospective study included 50 prostate cancer patients treated with prostate (bed) only radiotherapy (PBRT) (n = 28) or whole pelvis radiotherapy (WPRT) (n = 22) (NCT04638049). Longitudinal sampling was performed prior to radiotherapy, after 10 fractions, near the end of radiotherapy and at follow-up. Patient symptoms were dichotomized into a single toxicity score. Microbiome and metabolome fingerprints were analyzed by 16S rRNA gene sequencing and ultra-high-performance liquid chromatography hybrid high-resolution mass spectrometry, respectively. RESULTS: The individual α-diversity did not significantly change over time. Microbiota composition (ß-diversity) changed significantly over treatment (PERMANOVA p-value = 0.03), but there was no significant difference in stability when comparing PBRT versus WPRT. Levels of various metabolites were significantly altered during radiotherapy. Baseline α-diversity was not associated with any toxicity outcome. Based on the metabolic fingerprint, no natural clustering according to toxicity profile could be achieved. CONCLUSIONS: Radiation dose and treatment volume demonstrated limited effects on microbiome and metabolome fingerprints. In addition, no distinctive signature for toxicity status could be established. There is an ongoing need for toxicity risk stratification tools for diagnostic and therapeutic purposes, but the current evidence implies that the translation of metabolic and microbial biomarkers into routine clinical practice remains challenging.

Measurement-based validation of a commercial Monte Carlo dose calculation algorithm for electron beams.

PURPOSE: This work presents the clinical validation of RayStation's electron Monte Carlo code by the use of diodes and plane parallel radiation detectors in homogenous and heterogeneous tissues. Results are evaluated against internationally accepted criteria. METHODS: The Monte Carlo-based electron beam dose calculation code was validated using diodes, air- and liquid-filled parallel radiation detectors on an Elekta linac with beam energies of 4, 6, 8, 10, and 12 MeV. Treatment setups with varying source-to-skin distances, different applicators, various cutouts, and oblique beam incidences were addressed, together with dose prediction behind lung-, air-, and bone-equivalent inserts. According to NCS (Netherlands Commission for Radiation Dosimetry) report 15 for nonstandard treatment setups, a dose agreement of 3% in the δ1 region (high-dose region around Zref ), a distance-to-agreement (DTA) of 3 mm or a dose agreement of 10% in the δ2 region (regions with high-dose gradients), and 4% in the δ4 region (photon tail/low-dose region) were applied. During validation, clinical routine settings of 2 × 2 × 2-mm3 dose voxels and a statistically dose uncertainty of 0.6% (250 000 histories/cm2 ) were used. RESULTS: RayStation's electron Monte Carlo code dose prediction was able to achieve the tolerances of NCS report 15. Output predictions as a function of the SSD improve with energy and applicator size. Cutout data revealed no field size or energy dependence on the accuracy of the dose prediction. Excellent agreement for the oblique incidence data was achieved and a maximum of one voxel difference was obtained for the DTA behind heterogeneous inserts. CONCLUSIONS: The accuracy of RayStation's Monte Carlo-based electron beam dose prediction for Elekta accelerators is confirmed for clinical treatment planning that is not only performed within an acceptable timeframe in terms of the number of histories but also addresses for homogenous and heterogeneous media.

Impact of radiotherapy parameters on the risk of lymphopenia in urological tumors: A systematic review of the literature.

BACKGROUND: We investigated how radiotherapy (RT) parameters may contribute to the risk of lymphopenia in urological tumors and we discussed how this may impact clinical outcomes. MATERIAL & METHODS: A systematic review was performed according to the preferred reporting items for systematic reviews and meta-analysis (PRISMA) guidelines. The PubMed, Embase and ISI Web Of Knowledge databases were searched. Study quality was assessed according to the Newcastle-Ottawa Scale. RESULTS: Overall, 8 articles reporting on a total of 549 urological cancer patients met the inclusion criteria. The pooled mean incidence of acute severe lymphopenia (absolute lymphocyte count < 500 cells/µL) was 17.1%. Extended radiation volumes may lead to an increased risk of developing lymphopenia. Medium-high doses (≥ 40 Gy) to the whole pelvic (odds ratio (OR) = 1.01; 95% confidence interval (CI) 1.00-1.01; p = 0.025) and iliac (OR = 1.04; 95% CI 1.01-1.08; p = 0.009) bone marrow (BM) were associated with acute grade 3 and late grade 2 lymphopenia, respectively. CONCLUSION: Multiple studies reported high and severe incidences of lymphopenia. Minimizing radiation volumes and unintentional irradiation of pelvic BM may reduce the incidence of lymphopenia, potentially improving clinical outcomes. More research is needed to further elucidate these findings and effectively implement recently developed new risk assessment tools.

From clinical observations of intensity-modulated radiotherapy to dedicated in vitro designs.

In this review, an overview of intensity-modulated radiotherapy (IMRT) and related high precision radiation techniques is presented. In addition, the related radiobiological issues are discussed. Hereby, we try to point to the potential differences in radiobiological effect between popular intensity-modulated radiotherapy and related techniques (IMRT+) and conventional or three-dimensional radiotherapy (3D-RT). Further, an overview of the existing in vitro and in vivo radiobiological models to investigate the effect of spatially and/or temporally fractionated dose distributions, as applied in IMRT+, on the biological outcome is given. More in detail, our radiobiological models will be presented. Additionally, we will discuss the (dis)advantages of the presented models, and give some consideration to improve the existing radiobiological models in terms of set-up and clinical relevance.

Intensity-modulated arc therapy with simultaneous integrated boost in the treatment of primary irresectable cervical cancer. Treatment planning, quality control, and clinical implementation.

PURPOSE: To report on the planning procedure, quality control, and clinical implementation of intensity-modulated arc therapy (IMAT) delivering a simultaneous integrated boost (SIB) in patients with primary irresectable cervix carcinoma. PATIENTS AND METHODS: Six patients underwent PET-CT (positron emission tomography-computed tomography) and MRI (magnetic resonance imaging) before treatment planning. Prescription (25 fractions) was (1) a median dose (D(50)) of 62, 58 and 56 Gy to the primary tumor (GTV_cervix), primary clinical target volume (CTV_cervix) and its planning target volume (PTV_cervix), respectively; (2) a D(50) of 60 Gy to the PET-positive lymph nodes (GTV_nodes); (3) a minimal dose (D(98)) of 45 Gy to the planning target volume of the elective lymph nodes (PTV_nodes). IMAT plans were generated using an anatomy-based exclusion tool with the aid of weight and leaf position optimization. The dosimetric delivery of IMAT was validated preclinically using radiochromic film dosimetry. RESULTS: Five to nine arcs were needed to create valid IMAT plans. Dose constraints on D(50) were not met in two patients (both GTV_cervix: 1 Gy and 3 Gy less). D(98) for PTV_nodes was not met in three patients (1 Gy each). Film dosimetry showed excellent gamma evaluation. There were no treatment interruptions. CONCLUSION: IMAT allows delivering an SIB to the macroscopic tumor without compromising the dose to the elective lymph nodes or the organs at risk. The clinical implementation is feasible.

Longitudinal radiomics of cone-beam CT images from non-small cell lung cancer patients: Evaluation of the added prognostic value for overall survival and locoregional recurrence.

BACKGROUND AND PURPOSE: The prognostic value of radiomics for non-small cell lung cancer (NSCLC) patients has been investigated for images acquired prior to treatment, but no prognostic model has been developed that includes the change of radiomic features during treatment. Therefore, the aim of this study was to investigate the potential added prognostic value of a longitudinal radiomics approach using cone-beam computed tomography (CBCT) for NSCLC patients. MATERIALS AND METHODS: This retrospective study includes a training dataset of 141 stage I-IV NSCLC patients and three external validation datasets of 94, 61 and 41 patients, all treated with curative intended (chemo)radiotherapy. The change of radiomic features extracted from CBCT images was summarized as the slope of a linear regression. The CBCT slope-features and CT-extracted features were used as input for a Cox proportional hazards model. Moreover, prognostic performance of clinical parameters was investigated for overall survival and locoregional recurrence. Model performances were assessed using the Kaplan-Meier curves and c-index. RESULTS: The radiomics model contained only CT-derived features and reached a c-index of 0.63 for overall survival and could be validated on the first validation dataset. No model for locoregional recurrence could be developed that validated on the validation datasets. The clinical parameters model could not be validated for either overall survival or locoregional recurrence. CONCLUSION: In this study we could not confirm our hypothesis that longitudinal CBCT-extracted radiomic features contribute to improved prognostic information. Moreover, performance of baseline radiomic features or clinical parameters was poor, probably affected by heterogeneity within and between datasets.

Multi-dimensional dosimetric verification of stereotactic radiotherapy for uveal melanoma using radiochromic EBT film.

Since 1997, linac based stereotactic radiotherapy (SRT) of uveal melanoma has been continuously developed at the Department of Radiotherapy, Medical University Vienna. The aim of the present study was (i) to test a new type of radiochromic film (Gafchromic EBT) for dosimetric verification of class solutions for these treatments and (ii) to verify treatment plan acceptance criteria, which are based on gamma values statisitcs. An EPSON Expression 1680 Pro flat bed scanner was utilized for film reading. To establish a calibration curve, films were cut in squares of 2 x 2 cm2, positioned at 5 cm depth in a solid water phantom and were irradiated with different dose levels (0.5 and 5 Gy) in a 5 x 5 cm2 field at 6 MV. A previously developed solid phantom (polystyrene) was used with overall dimensions corresponding to an average human head. EBT films were placed at four different depths (10, 20, 25 and 30 mm) and all films were irradiated simultaneously. Four different treatment plans were verified that resemble typical clinical situations. These plans differed in irradiation technique (conformal mMLC or circular arc SRT) and in tumour size (PTV of 1 or 2.5 cm3). In-house developed software was applied to calculate gamma (gamma) index values and to perform several statistical operations (e.g. gamma-area histograms). At depths of 10 mm gamma1%, (gamma-value where 1% of the points have an equal or higher value in the region of interest) were between 1-3 and maximum gamma > 1 (% of gamma-values > 1 in the region of interest) areas were almost 30%. At larger depths, i.e. more close to the isocenter, gamma 1% was < 1 and gamma > 1 areas were mostly < 5%. Average gamma values were about 0.5. Besides the compromised accuracy in the buildup region, previously defined IMRT acceptance criteria [Stock et al., Phys. Med Biol. 50 (2005) 399-411] could be applied as well to SRT. Radiochromic EBT films, in combination with a flat-bed scanner, were found to be an ideal multidimensional dosimetric tool for treatment plan quality assurance. EBT films are a suitable and reliable dosimetric tool that could replace traditionally used radiographic films. The presented acceptance criteria for SRT treatment plans might be used as a benchmarking data-set for other stereotactic applications and/or other equipment (planning system and delivery hardware) combinations.

Automated Instead of Manual Treatment Planning? A Plan Comparison Based on Dose-Volume Statistics and Clinical Preference.

PURPOSE: Automated planning aims to speed up treatment planning and improve plan quality. We compared manual planning with automated planning for lung stereotactic body radiation therapy based on dose-volume histogram statistics and clinical preference. METHODS AND MATERIALS: Manual and automated intensity modulated radiation therapy plans were generated for 56 patients by use of software developed in-house and Pinnacle 9.10 Auto-Planning, respectively. Optimization times were measured in 10 patients, and the impact of the automated plan (AP) on the total treatment cost was estimated. For the remaining 46 patients, each plan was checked against our clinical objectives, and a pair-wise dose-volume histogram comparison was performed. Three experienced radiation oncologists evaluated each plan and indicated their preference. RESULTS: APs reduced the average optimization time by 77.3% but only affected the total treatment cost by 3.6%. Three APs and 0 manual plans failed our clinical objectives, and 13 APs and 9 manual plans showed a minor deviation. APs significantly reduced D2% (2% of the volume receives a dose of at least D2%) for the spinal cord, esophagus, heart, aorta, and main stem bronchus (P < .05) while preserving target coverage. The radiation oncologists found >75% of the APs clinically acceptable without any further fine-tuning. CONCLUSIONS: APs may help to create satisfactory treatment plans quickly and effectively. Because critical appraisal by qualified professionals remains necessary, there is no such thing as "fully automated" planning yet.

Late radiotherapy-induced lower intestinal toxicity (RILIT) of intensity-modulated radiotherapy for prostate cancer: the need for adapting toxicity scales and the appearance of the sigmoid colon as co-responsible organ for lower intestinal toxicity.

PURPOSE: To report on: 1. Late radiotherapy-induced lower intestinal toxicity (RILIT) after intensity-modulated radiotherapy (IMRT) for prostate cancer. 2. The correlation between late RILIT and volume parameters of the rectum, sigmoid colon and small bowel. MATERIALS AND METHODS: We included 241 patients with a follow-up of >or=18 months for this analysis. Late RILIT consisted of 8 different symptoms, comprising the 5 symptoms from the RTOG toxicity score supplemented with urgency, fecal incontinence and anal pain. Late RILIT and late RTOG toxicity were scored prospectively and correlated with: 1. Different rectum, sigmoid colon and small bowel volume parameters. 2. Patient-related morbidity. We calculated the median, quartile and percentiles for the different volume parameters and correlated them with grade 1-3 late RILIT. RESULTS: Median follow-up was 42 months. Three patients developed grade 3 red blood loss. We registered grade 2 RILIT and RTOG toxicity in 13% and 10%, respectively, the most frequent grade 1 symptom being fecal urgency. The intermediate rectal volume parameters were significantly correlated with late RILIT. We were able to calculate cut-off dose-volume histograms (DVHs) that predict for grade 0-2 RILIT. CONCLUSIONS: After IMRT for prostate cancer, the overall incidence of grade >or=2 RILIT is low. Cut-off DVHs can be used for patient counseling.

Comparison of 6 MV and 18 MV photons for IMRT treatment of lung cancer.

BACKGROUND AND PURPOSE: To compare 6 MV and 18 MV photon intensity modulated radiotherapy (IMRT) for non-small cell lung cancer. MATERIALS AND METHODS: Doses for a cohort of 10 patients, typical for our department, were computed with a commercially available convolution/superposition (CS) algorithm. Final dose computation was also performed with a dedicated IMRT Monte Carlo dose engine (MCDE). RESULTS: CS plans showed higher D(95%) (Gy) for the GTV (68.13 vs 67.36, p=0.004) and CTV (67.23 vs 66.87, p=0.028) with 18 than with 6 MV photons. MCDE computations demonstrated higher doses with 6 MV than 18 MV in D(95%) for the PTV (64.62 vs 63.64, p=0.009), PTV(optim) (65.48 vs 64.83, p=0.014) and CTV (66.22 vs 65.64, p=0.027). Dose inhomogeneity was lower with 18 than with 6 MV photons for GTV (0.08 vs 0.09, p=0.007) and CTV (0.10 vs 0.11, p=0.045) in CS but not MCDE plans. 6 MV photons significantly (D(33%); p=0.045) spared the esophagus in MCDE plans. Observed dose differences between lower and higher energy IMRT plans were dependent on the individual patient. CONCLUSIONS: Selection of photon energy depends on priority ranking of endpoints and individual patients. In the absence of highly accurate dose computation algorithms such as CS and MCDE, 6 MV photons may be the prudent choice.

Dosimetric characterization of GafChromic EBT film and its implication on film dosimetry quality assurance.

The suitability of radiochromic EBT film was studied for high-precision clinical quality assurance (QA) by identifying the dose response for a wide range of irradiation parameters typically modified in highly-conformal treatment techniques. In addition, uncertainties associated with varying irradiation conditions were determined. EBT can be used for dose assessment of absorbed dose levels as well as relative dosimetry when compared to absolute absorbed dose calibrated using ionization chamber results. For comparison, a silver halide film (Kodak EDR-2) representing the current standard in film dosimetry was included. As an initial step a measurement protocol yielding accurate and precise results was established for a flatbed transparency scanner (Epson Expression 1680 Pro) that was utilized as a film reading instrument. The light transmission measured by the scanner was found to depend on the position of the film on the scanner plate. For three film pieces irradiated with doses of 0 Gy, approximately 1 Gy and approximately 7 Gy, the pixel values measured in portrait or landscape mode differed by 4.7%, 6.2% and 10.0%, respectively. A study of 200 film pieces revealed an excellent sheet-to-sheet uniformity. On a long time scale, the optical development of irradiated EBT film consisted of a slow but steady increase of absorbance which was not observed to cease during 4 months. Sensitometric curves of EBT films obtained under reference conditions (SSD = 95 cm, FS = 5 x 5 cm(2), d = 5 cm) for 6, 10 and 25 MV photon beams did not show any energy dependence. The average separation between all curves was only 0.7%. The variation of the depth d (range 2-25 cm) in the phantom did not affect the dose response of EBT film. Also the influence of the radiation field size (range 3 x 3-40 x 40 cm(2)) on the sensitometric curve was not significant. For EDR-2 films maximum differences between the calibration curves reached 7-8% for X6MV and X25MV. Radiochromic EBT film, in combination with a flatbed scanner, presents a versatile system for high-precision dosimetry in two dimensions, provided that the intrinsic behaviour of the film reading device is taken into account. EBT film itself presents substantial improvements on formerly available models of radiographic and a radiochromic film and its dosimetric characteristics allow us to measure absorbed dose levels in a large variety of situations with a single calibration curve.

Three dimensional radiation dosimetry in lung-equivalent regions by use of a radiation sensitive gel foam: proof of principle.

A polymer hydrogel foam is proposed as a potential three dimensional experimental dosimeter for radiation treatment verification in low-density tissue such as the lung. A gel foam is created by beating a radiation sensitive polymer gel mixture in an anoxic atmosphere. The mass density of the gel foam is in the order of 0.25-0.35 kg/dm3. Both nuclear magnetic resonance (NMR) spin-spin relaxation rate (R2) and magnetization transfer ratio (MTR) have been used to map the dose distribution from the gel dosimeter. It is found that MTR has significant advantages compared to R2 for mapping the dose distribution in the polymer gel foam dosimeters. The magnetization transfer ratio is found to be less dependent on the density and microstructure of the gel foam dosimeter while spin-spin relaxation dispersion has been observed making the spin-spin relaxation rate dependent on the interecho time interval. Optical microscopy reveals a microstructure that shows great similarity with human lung tissue. It is also shown how NMR hydrogen proton density measurements can be used to map the density distributions in gel dosimeters.

Accuracy of patient dose calculation for lung IMRT: A comparison of Monte Carlo, convolution/superposition, and pencil beam computations.

The accuracy of dose computation within the lungs depends strongly on the performance of the calculation algorithm in regions of electronic disequilibrium that arise near tissue inhomogeneities with large density variations. There is a lack of data evaluating the performance of highly developed analytical dose calculation algorithms compared to Monte Carlo computations in a clinical setting. We compared full Monte Carlo calculations (performed by our Monte Carlo dose engine MCDE) with two different commercial convolution/superposition (CS) implementations (Pinnacle-CS and Helax-TMS's collapsed cone model Helax-CC) and one pencil beam algorithm (Helax-TMS's pencil beam model Helax-PB) for 10 intensity modulated radiation therapy (IMRT) lung cancer patients. Treatment plans were created for two photon beam qualities (6 and 18 MV). For each dose calculation algorithm, patient, and beam quality, the following set of clinically relevant dose-volume values was reported: (i) minimal, median, and maximal dose (Dmin, D50, and Dmax) for the gross tumor and planning target volumes (GTV and PTV); (ii) the volume of the lungs (excluding the GTV) receiving at least 20 and 30 Gy (V20 and V30) and the mean lung dose; (iii) the 33rd percentile dose (D33) and Dmax delivered to the heart and the expanded esophagus; and (iv) Dmax for the expanded spinal cord. Statistical analysis was performed by means of one-way analysis of variance for repeated measurements and Tukey pairwise comparison of means. Pinnacle-CS showed an excellent agreement with MCDE within the target structures, whereas the best correspondence for the organs at risk (OARs) was found between Helax-CC and MCDE. Results from Helax-PB were unsatisfying for both targets and OARs. Additionally, individual patient results were analyzed. Within the target structures, deviations above 5% were found in one patient for the comparison of MCDE and Helax-CC, while all differences between MCDE and Pinnacle-CS were below 5%. For both Pinnacle-CS and Helax-CC, deviations from MCDE above 5% were found within the OARs: within the lungs for two (6 MV) and six (18 MV) patients for Pinnacle-CS, and within other OARs for two patients for Helax-CC (for Dmax of the heart and D33 of the expanded esophagus) but only for 6 MV. For one patient, all four algorithms were used to recompute the dose after replacing all computed tomography voxels within the patient's skin contour by water. This made all differences above 5% between MCDE and the other dose calculation algorithms disappear. Thus, the observed deviations mainly arose from differences in particle transport modeling within the lungs, and the commissioning of the algorithms was adequately performed (or the commissioning was less important for this type of treatment). In conclusion, not one pair of the dose calculation algorithms we investigated could provide results that were consistent within 5% for all 10 patients for the set of clinically relevant dose-volume indices studied. As the results from both CS algorithms differed significantly, care should be taken when evaluating treatment plans as the choice of dose calculation algorithm may influence clinical results. Full Monte Carlo provides a great benchmarking tool for evaluating the performance of other algorithms for patient dose computations.

Absorbed dose measurements in the build-up region of flattened versus unflattened megavoltage photon beams.

This study evaluated absorbed dose measurements in the build-up region of conventional (FF) versus flattening filter-free (FFF) photon beams. The absorbed dose in the build-up region of static 6 and 10MV FF and FFF beams was measured using radiochromic film and extrapolation chamber dosimetry for single beams with a variety of field sizes, shapes and positions relative to the central axis. Removing the flattening filter generally resulted in slightly higher relative build-up doses. No considerable impact on the depth of maximum dose was found.

An inter-centre quality assurance network for IMRT verification: results of the ESTRO QUASIMODO project.

BACKGROUND AND PURPOSE: IMRT necessitates extension of existing inter-centre quality assurance programs due to its increased complexity. We assessed the feasibility of an inter-centre verification method for different IMRT techniques. MATERIALS AND METHODS: Eight European radiotherapy institutions of the QUASIMODO network, have designed an IMRT plan for a horseshoe-shaped PTV surrounding a cylindrical OAR in a simplified pelvic phantom. All centres applied common plan objectives but used their own equipment for planning and delivery. They verified the delivery of this plan according to a common protocol with radiographic film and ionisation chamber measurements. The irradiated films, the results of the ionisation chamber measurements and the computed dose distributions were sent to one analysis centre that compared the measured and computed dose distributions with the gamma method and composite dose-area histograms. RESULTS: 4% (relative to the prescribed dose) and 3mm (distance-to-agreement) were decided feasible gamma criteria. The composite dose-area histograms showed a maximum local deviation of 3.5% in the mean dose of the PTV and 5% in the OAR. Systematic differences could be identified, and in some cases explained. CONCLUSIONS: This multi-centre dosimetric verification study demonstrated both the feasibility of a multi-centre quality assurance network to evaluate any IMRT planning and delivery system combination, as well as the validity of the methodology involved.

IMRT treatment planning:- a comparative inter-system and inter-centre planning exercise of the ESTRO QUASIMODO group.

BACKGROUND AND PURPOSE: The purpose of this work was a comparison of realistic IMRT plans based on the same CT-image data set and a common predefined set of dose objectives for the planning target volume and the organs at risk. This work was part of the larger European QUASIMODO IMRT verification project. MATERIALS AND METHODS: Eleven IMRT plans were produced by nine different European groups, each applying a representative set of clinically used IMRT treatment planning systems. The plans produced were to be deliverable in a clinically acceptable treatment time with the local technical equipment. All plans were characterized using a set of different quality measures such as dose-volume histograms, number of monitor units and treatment time. RESULTS: Only one plan was able to fulfil all dose objectives strictly; six plans failed some of the objectives but were still considered to be clinically acceptable; four plans were not able to reach the objectives. Additional quality scores such as the number of monitor units and treatment time showed large variations, which mainly depend on the delivery technique. CONCLUSION: The presented planning study showed that with nearly all presently available IMRT planning and delivery systems comparable dose distributions could be achieved if the planning goals are clearly defined in advance.