Dosimetric comparison between VMAT with different dose calculation algorithms and protons for soft-tissue sarcoma radiotherapy.

BACKGROUND: To appraise the potential of volumetric modulated arc therapy (VMAT, RapidArc) and proton beams to simultaneously achieve target coverage and enhanced sparing of bone tissue in the treatment of soft-tissue sarcoma with adequate target coverage. MATERIAL AND METHODS: Ten patients presenting with soft-tissue sarcoma of the leg were collected for the study. Dose was prescribed to 66.5 Gy in 25 fractions to the planning target volume (PTV) while significant maximum dose to the bone was constrained to 50 Gy. Plans were optimised according to the RapidArc technique with 6 MV photon beams or for intensity modulated protons. RapidArc photon plans were computed with: 1) AAA; 2) Acuros XB as dose to medium; and 3) Acuros XB as dose to water. RESULTS: All plans acceptably met the criteria of target coverage (V95% >90-95%) and bone sparing (D(1 cm3) <50 Gy). Significantly higher PTV dose homogeneity was found for proton plans. Near-to-maximum dose to bone was similar for RapidArc and protons, while volume receiving medium/low dose levels was minimised with protons. Similar results were obtained for the remaining normal tissue. Dose distributions calculated with the dose to water option resulted ~5% higher than corresponding ones computed as dose to medium. CONCLUSION: High plan quality was demonstrated for both VMAT and proton techniques when applied to soft-tissue sarcoma.

Calcium and Iron Content of Cereal-Based Gluten-Free Products.

The impact of a gluten-free (GF) diet on the intake of calcium and iron is broadly unknown, as the micronutrient content of GF cereal-based products has scarcely been measured. The study aimed to measure the calcium and iron content of GF cereal-based products from the UK. Seventy-three GF products were analysed. A laboratory analysis of calcium and iron from GF food samples was performed by spectrophotometric and flame emission photometry, respectively. The values for wheat-based products were from a nutrient database. The calcium in GF white loaf samples varied greatly from 54 to 140 mg/100 g, with a lower average calcium content compared with wheat-based values (99 ± 29 mg/100 g n = 13 versus 177 mg/100 g; p < 0.01). Only 27% of the white loaves and rolls were fortified with calcium; this contrasts with 100% of white wheat-based loaves. The calcium in GF flour mixes ranged from 54 to 414 mg/100 g, with 66% fortified. GF white pasta had more calcium compared with wheat-based pasta (76 ± 27 mg/100 g n = 7 versus 24 mg/100 g; p = 0.002). The iron in GF bread loaves and pasta samples was similar to wheat-based comparators, whereas lower iron levels were observed in GF wraps (0.8 ± 0.2 n = 11 versus 1.6 mg/100 g). GF bread had a significantly higher fibre content, and the majority of GF bread had a lower protein content, compared with wheat-based bread products. These calcium and iron values provide a valuable addition towards enabling more accurate nutrient intake analysis for adults and children with coeliac disease.

On the impact of dose rate variation upon RapidArc implementation of volumetric modulated are therapy.

PURPOSE: A study was carried out to evaluate the robustness and mutual interplay of two variables concurring to generate modulation patterns of the RapidArc (RapidArc) implementation of volumetric modulated are therapy. Dose rate (DR) and gantry speed (GS) are free parameters optimized alongside field aperture shape by the RapidArc engine; however, they are limited by machine constraints and mutually compensate in order to deliver the proper MU/deg during the gantry rotation. METHODS: Four test cases (one geometrical and three clinical) were selected and RapidArc plans were optimized using maximum allowed dose rates from 100 to 600 MU/min. The maximum gantry speed was fixed at 4.8 deg/s. Qualitative analysis of DR and GS patterns from these cases was summarized together with quantitative assessment of delivery parameters. Pretreatment quality assurance measurements and scoring of plan quality aimed to determine whether preferable initial conditions might be identified or the optimization engine might be invariant to those variables and capable of providing adequate plans independently from the limits applied. RESULTS: The results of the study were: (i) High dynamic range in MU/deg is achievable across all dose rates by means of gantry speed modulation; (ii) there is a robust compensation mechanism between the two variables; (iii) from a machine delivery point-of-view, slightly improved accuracy is achieved when lower DRs are applied; however, this does not have practical consequences since measurements and plan evaluation showed a lack of clinically relevant deviation; and (iv) reduced total treatment time is a major advantage of high DR. CONCLUSIONS: A trend toward improved plan quality for clinical cases was observed with high DR but cannot be generalized, due to the limited amount of cases investigated and the consequent limited significance of the observed differences. As a minimum benefit, the reduced total treatment time should be considered as well.

Accuracy of Acuros XB and AAA dose calculation for small fields with reference to RapidArc(®) stereotactic treatments.

PURPOSE: To assess the accuracy against measurements of two photon dose calculation algorithms (Acuros XB and the Anisotropic Analytical algorithm AAA) for small fields usable in stereotactic treatments with particular focus on RapidArc(®). METHODS: Acuros XB and AAA were configured for stereotactic use. Baseline accuracy was assessed on small jaw-collimated open fields for different values for the spot sizes parameter in the beam data: 0.0, 0.5, 1, and 2 mm. Data were calculated with a grid of 1 × 1 mm(2). Investigated fields were: 3 × 3, 2 × 2, 1 × 1, and 0.8 × 0.8 cm(2) with a 6 MV photon beam generated from a Clinac2100iX (Varian, Palo Alto, CA). Profiles, PDD, and output factors were measured in water with a PTW diamond detector (detector size: 4 mm(2), thickness 0.4 mm) and compared to calculations. Four RapidArc test plans were optimized, calculated and delivered with jaw settings J3 × 3, J2 × 2, and J1 × 1 cm(2), the last was optimized twice to generate high (H) and low (L) modulation patterns. Each plan consisted of one partial arc (gantry 110° to 250°), and collimator 45°. Dose to isocenter was measured in a PTW Octavius phantom and compared to calculations. 2D measurements were performed by means of portal dosimetry with the GLAaS method developed at authors' institute. Analysis was performed with gamma pass-fail test with 3% dose difference and 2 mm distance to agreement thresholds. RESULTS: Open square fields: penumbrae from open field profiles were in good agreement with diamond measurements for 1 mm spot size setting for Acuros XB, and between 0.5 and 1 mm for AAA. Maximum MU difference between calculations and measurements was 1.7% for Acuros XB (0.2% for fields greater than 1 × 1 cm(2)) with 0.5 or 1 mm spot size. Agreement for AAA was within 0.7% (2.8%) for 0.5 (1 mm) spot size. RapidArc plans: doses were evaluated in a 4 mm diameter structure at isocenter and computed values differed from measurements by 0.0, -0.2, 5.5, and -3.4% for Acuros XB calculations (1 mm spot size), and of -0.1, 0.3, 6.7, and -1.2% for AAA, respectively for J3 × 3, J2 × 2, J1 × 1H, J1 × 1L RapidArc plans. Gamma Agreement Index from 2D dose analysis was higher than 95% for J3 × 3 and J2 × 2 plans, being around 80% for J1 × 1 maps. Sensitivity with respect to the dosimetric leaf gap and transmission factor MLC parameters was evaluated in the four RapidArc plans, showing the need to properly set the dosimetric leaf gap for accurate calculations. CONCLUSIONS: Acuros XB and AAA showed acceptable characteristics for stereotactic small fields if adequate tuning of configuration parameters is performed. Dose calculated for RapidArc stereotactic plans showed an acceptable agreement against point and 2D measurements. Both algorithms can therefore be considered safely applicable to stereotactic treatments.

On the role of the optimization algorithm of RapidArc(®) volumetric modulated arc therapy on plan quality and efficiency.

PURPOSE: The RapidArc volumetric modulated arc therapy (VMAT) planning process is based on a core engine, the so-called progressive resolution optimizer (PRO). This is the optimization algorithm used to determine the combination of field shapes, segment weights (with dose rate and gantry speed variations), which best approximate the desired dose distribution in the inverse planning problem. A study was performed to assess the behavior of two versions of PRO. These two versions mostly differ in the way continuous variables describing the modulated arc are sampled into discrete control points, in the planning efficiency and in the presence of some new features. The analysis aimed to assess (i) plan quality, (ii) technical delivery aspects, (iii) agreement between delivery and calculations, and (iv) planning efficiency of the two versions. METHODS: RapidArc plans were generated for four groups of patients (five patients each): anal canal, advanced lung, head and neck, and multiple brain metastases and were designed to test different levels of planning complexity and anatomical features. Plans from optimization with PRO2 (first generation of RapidArc optimizer) were compared against PRO3 (second generation of the algorithm). Additional plans were optimized with PRO3 using new features: the jaw tracking, the intermediate dose and the air cavity correction options. RESULTS: Results showed that (i) plan quality was generally improved with PRO3 and, although not for all parameters, some of the scored indices showed a macroscopic improvement with PRO3. (ii) PRO3 optimization leads to simpler patterns of the dynamic parameters particularly for dose rate. (iii) No differences were observed between the two algorithms in terms of pretreatment quality assurance measurements and (iv) PRO3 optimization was generally faster, with a time reduction of a factor approximately 3.5 with respect to PRO2. CONCLUSIONS: These results indicate that PRO3 is either clinically beneficial or neutral in terms of dosimetric quality while it showed significant advantages in speed and technical aspects.

The impact of treatment couch modelling on RapidArc.

A planning and dosimetric study was carried out on a cohort of six CT datasets from patients treated for prostate cancer to assess the impact of couch modelling on the accuracy of dose calculation for the volumetric modulated arc technique RapidArc. For each patient, RapidArc plans were optimized using the couch while final dose calculation was performed with different conditions (thin, medium, thick and no couch). Analysis was performed in terms of dose volume histograms, dose difference histograms and 3D-gamma tests. Pre-treatment verification measurements were performed using the PTW-729 array in conjunction with the Octavius phantom (PTW, Freiburg); similarly, HU characterization of couch was performed with the same phantom and ion chamber measurements comparing calculations and experimental data. A set of Hounsfield Units (HU) valid for low and high energy and the entire couch length was found as internal structure HU = -960, surface shell HU = -700. Analysis of dose plans showed that differences larger than 1.5 Gy for a 70 Gy prescription might be observed on significant fractions of PTVs. Smaller differences are visible in the medium low-dose regions. Pre-treatment verification on composite delivery confirmed these observations and, at the same time, showed good accuracy of dose calculations in the presence of couch modelling compared to delivery in the same conditions (GAI ranging from 95% to 100%). Results confirmed the reliability of the geometrical model build in the planning system Eclipse, and (i) there is no measurable effect if the wrong segment of the couch is used in the calculations; (ii) there are significant discrepancies of potential clinical impact at the level of the target volumes if calculations are performed without couch and delivery is performed with couch, and (iii) the effect is particularly relevant at low energy (6 MV in this case) that is the configuration clinically used by most of the centres adopting technologies based on intensity-modulated arcs.

Does deep inspiration breath hold reduce plan complexity? Multicentric experience of left breast cancer radiotherapy with volumetric modulated arc therapy.

PURPOSE: Volumetric modulated arc therapy (VMAT) for left breast treatments allows heart sparing without compromising PTV coverage. However, this technique may require highly complex plans. Deep Inspiration Breath Hold (DIBH) procedure increases the heart-to-breast distance, facilitating the dose sparing of the heart. The aim of the present work was to investigate if the cardiac-sparing benefits of the DIBH technique were achieved with lower plan modulation and complexity than Free Breathing (FB) treatments. METHODS AND MATERIALS: Ten left side breast cases were considered by two centers with different treatment planning systems (TPS) and Linacs. VMAT plans were elaborated in FB and DIBH according to the same protocol. Plan complexity was evaluated by scoring several complexity indices. A new global score index accounting for both plan quality and dosimetric parameters was defined. Pre-treatment QA was performed for all VMAT plans using EPID and Epiqa software. RESULTS: DIBH-VMAT plans were associated with significant PTV coverage improvement and mean heart dose reduction (p < 0.003), increasing the resulting global score index. All the evaluated complexity indices showed lower plan complexity for DIBH plans than FB ones, but only in few cases the results were statistically significant. All plans passed the gamma analysis with the selected criteria. CONCLUSIONS: The DIBH technique is superior to the FB technique when the heart needs further sparing, allowing a reduction of the doses to OARs with a slightly lower degree of plan complexity and without compromising plan deliverability. These benefits were achieved regardless of the technological scenarios adopted.

Intensity modulation with photons for benign intracranial tumours: a planning comparison of volumetric single arc, helical arc and fixed gantry techniques.

BACKGROUND AND PURPOSE: The potential benefits and limitations of the new RapidArc treatment concept compared to Helical Tomotherapy and fixed gantry intensity modulation techniques have been assessed at treatment planning level on 12 patients presenting with 'benign' brain tumours. MATERIALS AND METHODS: Plans for five acoustic neurinomas, five meningiomas and two pituitary adenomas were computed for an Helical Tomotherapy (HT) unit, for RapidArc delivery (RA) on a linac equipped with two types of MLC (RA_HD120 with the new High Definition MLC with 2.5mm leaf width at isocentre and RA_M120 with the standard Millennium with 5mm resolution) and for fixed beam IMRT with the High Definition MLC. Analysis was mostly performed on physical quantities derived from Dose-Volume Histograms (DVHs). RESULTS: Target coverage resulted basically equivalent among techniques. V(95%) (in %) was higher than 99% for all techniques, minimum significant dose (D(99%)) was 95.5+/-1.4 for IMRT, 96.2+/-1.4 and 97.0+/-1.2 for the RA_HD120 and RA_M120 approaches and 96.8+/-1.7 for HT, maximum significant dose (D(1%), in %) was 102.2+/-0.8, 102.7+/-0.5, 102.4+/-0.5 and 103.0+/-1.1, respectively, standard deviation (in %) was 1.4+/-0.4, 1.3+/-0.3, 1.1+/-0.2 and 0.8+/-0.3, respectively. Conformity Index (CI(95%)) was 0.47+/-0.12, 0.46+/-0.12, 0.43+/-0.11 and 0.38+/-0.11, respectively. For organs at risk all techniques respected planning objectives. Concerning the healthy tissue: V(10 Gy) (in %) was 9.4+/-5.5, 9.9+/-6.1, 9.2+/-6.1 and 12.1+/-8.8, respectively. Integral dose measured on the healthy tissue was 7.5+/-3.3, 9.7+/-3.4, 8.7+/-3.4, 10.4+/-4.2 10(3) Gy cm(3), respectively. CONCLUSIONS: For the class of tumours investigated in this report, HT and RA and IMRT proved to be adequate to properly treat patients. Further studies on more complex cases need to be investigated in order to assess the effectiveness of this new technique in a broader clinical perspective.

A treatment planning study comparing volumetric arc modulation with RapidArc and fixed field IMRT for cervix uteri radiotherapy.

PURPOSE: A treatment planning study was performed to evaluate the performance of the novel volumetric modulated single arc radiotherapy on cervix uteri cancer patients. Conventional fixed field IMRT was used as benchmark. METHODS AND MATERIALS: CT datasets of eight patients were included in the study. Plans were optimised with the aim to assess organs at risk and healthy tissue sparing while enforcing highly conformal target coverage. Planning objectives for PTV were: maximum significant dose lower than 52.5 Gy and minimum significant dose higher than 47.5 Gy. For organs at risk, the median and maximum doses were constrained to be lower than 30 (rectum), 35 (bladder) and 25 Gy (small bowel) and 47.5 Gy; additional objectives were set on various volume thresholds. Plans were evaluated on parameters derived from dose volume histograms and on NTCP estimates. Peripheral doses at 5, 10 and 15 cm from the PTV surface were recorded to assess the low-level dose bath. The MU and delivery time were scored to measure expected treatment efficiency. RESULTS: Both RapidArc and IMRT resulted in equivalent target coverage but RapidArc had an improved homogeneity (D(5%)-D(95%) = 3.5 +/- 0.6 Gy for RapidArc and 4.3 +/- 0.8 Gy for IMRT) and conformity index (CI(90%) = 1.30 +/- 0.06 for RapidArc and 1.41 +/- 0.15 for IMRT). On rectum the mean dose was reduced by about 6 Gy (10 Gy for the rectum fraction not included in the PTV). Similar trends were observed for the various dose levels with reductions ranging from approximately 3 to 14.4 Gy. For the bladder, RapidArc allowed a reduction of mean dose ranging from approximately 4 to 6Gy and a reduction from approximately 3 to 9 Gy w.r.t. IMRT. Similar trends but with smaller absolute differences were observed for the small bowel and left and right femur. NTCP calculations on bladder and rectum confirmed the DVH data with a potential relative reduction ranging from 30 to 70% from IMRT to RapidArc. The healthy tissue was significantly less irradiated in the medium to high dose regions (from 20 to 30 Gy) and the integral dose reduction with RapidArc was about 12% compared to IMRT. Concerning peripheral dose, the relative difference between IMRT and RapidArc was of 9 +/- 2%, 43 +/- 11% and 36 +/- 5% at 5, 10 and 15 cm from the PTV surface, respectively. The MU/Gy from RapidArc was 245 +/- 17 corresponding to an expected average beam on time of 73 +/- 10 s per fractions of 2 Gy. IMRT plans presented higher values with an average of MU/Gy = 479 +/- 63. CONCLUSION: RapidArc was investigated for cervix uteri cancer showing significant improvements in organs at risk and healthy tissue sparing with uncompromised target coverage leading to better conformal avoidance of treatments w.r.t. conventional IMRT. This, in combination with the confirmed short delivery time, can lead to clinically significant advances in the management of this highly aggressive cancer type. Clinical protocols are now advised to evaluate prospectively the potential benefit observed at the planning level.

Testing the GlaaS algorithm for dose measurements on low- and high-energy photon beams using an amorphous silicon portal imager.

The GLAaS algorithm for pretreatment intensity modulation radiation therapy absolute dose verification based on the use of amorphous silicon detectors, as described in Nicolini et al. [G. Nicolini, A. Fogliata, E. Vanetti, A. Clivio, and L. Cozzi, Med. Phys. 33, 2839-2851 (2006)], was tested under a variety of experimental conditions to investigate its robustness, the possibility of using it in different clinics and its performance. GLAaS was therefore tested on a low-energy Varian Clinac (6 MV) equipped with an amorphous silicon Portal Vision PV-aS500 with electronic readout IAS2 and on a high-energy Clinac (6 and 15 MV) equipped with a PV-aS1000 and IAS3 electronics. Tests were performed for three calibration conditions: A: adding buildup on the top of the cassette such that SDD-SSD = d(max) and comparing measurements with corresponding doses computed at d(max), B: without adding any buildup on the top of the cassette and considering only the intrinsic water-equivalent thickness of the electronic portal imaging devices device (0.8 cm), and C: without adding any buildup on the top of the cassette but comparing measurements against doses computed at d(max). This procedure is similar to that usually applied when in vivo dosimetry is performed with solid state diodes without sufficient buildup material. Quantitatively, the gamma index (gamma), as described by Low et al. [D. A. Low, W. B. Harms, S. Mutic, and J. A. Purdy, Med. Phys. 25, 656-660 (1998)], was assessed. The gamma index was computed for a distance to agreement (DTA) of 3 mm. The dose difference deltaD was considered as 2%, 3%, and 4%. As a measure of the quality of results, the fraction of field area with gamma larger than 1 (%FA) was scored. Results over a set of 50 test samples (including fields from head and neck, breast, prostate, anal canal, and brain cases) and from the long-term routine usage, demonstrated the robustness and stability of GLAaS. In general, the mean values of %FA remain below 3% for deltaD equal or larger than 3%, while they are slightly larger for deltaD = 2% with %FA in the range from 3% to 8%. Since its introduction in routine practice, 1453 fields have been verified with GLAaS at the authors' institute (6 MV beam). Using a DTA of 3 mm and a deltaD of 4% the authors obtained %FA = 0.9 +/- 1.1 for the entire data set while, stratifying according to the dose calculation algorithm, they observed: %FA = 0.7 +/- 0.9 for fields computed with the analytical anisotropic algorithm and %FA = 2.4 +/- 1.3 for pencil-beam based fields with a statistically significant difference between the two groups. If data are stratified according to field splitting, they observed %FA = 0.8 +/- 1.0 for split fields and 1.0 +/- 1.2 for nonsplit fields without any significant difference.

The impact of photon dose calculation algorithms on expected dose distributions in lungs under different respiratory phases.

A planning study was carried out on a cohort of CT datasets from breast patients scanned during different respiratory phases. The aim of the study was to investigate the influence of different air filling in lungs on the calculation accuracy of photon dose algorithms and to identify potential patterns of failure with clinical implications. Selected respiratory phases were free breathing (FB), representative of typical end expiration, and deep inspiration breath hold (DIBH), a typical condition for clinical treatment with respiratory gating. Algorithms investigated were the pencil beam (PBC), the anisotropic analytical algorithm (AAA) and the collapsed cone (CC) from the Varian Eclipse or Philips Pinnacle planning system. Reference benchmark calculations were performed with the Voxel Monte Carlo (VMC++). An analysis was performed in terms of physical quantities inspecting either dose-volume or dose-mass histograms and in terms of an extension to three dimensions of the gamma index of Low. Results were stratified according to a breathing phase and algorithm. Collectives acquired in FB or DIBH showed well-separated average lung density distributions with mean densities of 0.27 +/- 0.04 and 0.16 +/- 0.02 g cm(-3), respectively, and average peak densities of 0.17 +/- 0.03 and 0.09 +/- 0.02 g cm(-3). Analysis of volume-dose or mass-dose histograms proved the expected deviations on PBC results due to the missing lateral transport of electrons with underestimations in the low dose region and overestimations in the high dose region. From the gamma analysis, it resulted that PBC is systematically defective compared to VMC++ over the entire range of lung densities and dose levels with severe violations in both respiratory phases. The fraction of lung voxels with gamma > 1 for PBC reached 25% in DIBH and about 15% in FB. CC and AAA performed, in contrast, similarly and with fractions of lung voxels with gamma > 1 in average inferior to 2% in FB and 4-5% (AAA) or 6-8% (CC) in DIBH. In summary, PBC proved to be severely defective in calculations involving lungs and particularly for cases where specific respiratory phases (e.g. DIBH) are assumed for treatment. In contrast, CC and AAA manifested a high degree of consistency against the Monte Carlo method and provided stable results over the entire range of clinically relevant densities.

Basic dosimetric verification in water of the anisotropic analytical algorithm for Varian, Elekta and Siemens linacs.

Since early 2007 a new version of the Anisotropic Analytical Algorithm (AAA) for photon dose calculations was released by Varian Medical Systems for clinical usage on Elekta linacs and also, with some restrictions, for Siemens linacs. Basic validation studies were performed and reported for three beams. 4,6 and 15 MV for an Elekta Synergy, 6 and 15 MV for a Siemens Primus and, as a reference, for 6 and 15 MV from a Varian Clinac 2100C/D. Generally AAA calculations reproduced well measured data and small deviations were observed for open and wedged fields. PDD curves showed in average differences between calculation and measurement smaller than 1% or 1.2 mm for Elekta beams, 1% or 1.8 mm for Siemens beams and 1% or 1 mm for Varian beams. Profiles in the flattened region matched measurements with deviations smaller than 1% for Elekta and Varian beams, 2% for Siemens. Percentage differences in Output Factors were observed as small as 1% in average.

Applying Lean-Six-Sigma Methodology in radiotherapy: Lessons learned by the breast daily repositioning case.

BACKGROUND & PURPOSE: Lean Six Sigma Methodology (LSSM) was introduced in industry to provide near-perfect services to large processes, by reducing improbable occurrence. LSSM has been applied to redesign the 2D-2D breast repositioning process (Lean) by the retrospective analysis of the database (Six Sigma). MATERIALS & METHODS: Breast patients with daily 2D-2D matching before RT were considered. The five DMAIC (define, measure, analyze, improve, and control) LSSM steps were applied. The process was retrospectively measured over 30 months (7/2014-12/2016) by querying the RT Record&Verify database. Two Lean instruments (Poka-Yoke and Visual Management) were considered for advancing the process. The new procedure was checked over 6 months (1-6/2017). RESULTS: 14,931 consecutive shifts from 1342 patients were analyzed. Only 0.8% of patients presented median shifts >1 cm. The major observed discrepancy was the monthly percentage of fractions with almost zero shifts (AZS = 13.2% ±â€¯6.1%). Ishikawa fishbone diagram helped in defining the main discrepancy con-causes. Procedure harmonization involving a multidisciplinary team to increase confidence in matching procedure was defined. AZS was reduced to 4.8% ±â€¯0.6%. Furthermore, distribution symmetry improvement (Skewness moved from 1.4 to 1.1) and outlier reduction, verified by Kurtosis diminution, demonstrated a better "normalization" of the procedure after the LSSM application. CONCLUSIONS: LSSM was implemented in a RT department, allowing to redesign the breast repositioning matching procedure.

A treatment planning study using non-coplanar static fields and coplanar arcs for whole breast radiotherapy of patients with concave geometry.

PURPOSE: A treatment planning study was performed to evaluate the performance of new radiotherapy techniques based on non-coplanar multiple fields or on dynamic conformal arcs for early stage breast treatments. METHODS AND MATERIALS: CT datasets of 7 different patients that were deemed unsuitable for tangential beam treatment due to a large volume of lung in the treatment fields were used as input for the study. Standard tangential field plans and inversely modulated IMRT plans were used as benchmark to evaluate performances of conformal plans with 3 non-coplanar fields (3F-NC), with 2 short dynamic conformal arcs (2-Arc) or hybrid plans with one static conformal field and one dynamic conformal arc (P-Arc). All plans were designed to achieve the higher target coverage and minimum ipsilateral lung involvement depending on the planning technique with a key objective to avoid involvement of the contralateral breast. The following planning objectives were selected. For PTV: D(1%) (maximum significant dose) lower than 110% and D(99%) (minimum significant dose) higher than 90%. For the ipsilateral lung a mean dose lower than 15 Gy and/or a volume receiving more than 20 Gy lower than 22%. For contralateral breast, all techniques but IMRT were set to have no beam impinging this organ at risk, while for IMRT plans were further designed to keep the mean dose lower than 5 Gy and to minimise the volume receiving a dose higher than 70% of the prescribed dose. RESULTS: P-Arc resulted to be on average a better technique, as it provides a PTV dose distribution highly conformal (Conformity index 1.45), homogeneous (D(5%)-D(95%)=15.6%), with adequate coverage (V(90%)=96.4%) and a limited involvement of the ipsilateral lung (MLD approximately 9 Gy, V(5 Gy) approximately 36%, NTCP<2%) when compared to four other treatment techniques. 3F-NC presented similar but slightly worse performances on target: Conformity index 1.57, D(5%)-D(95%)=18.1%, V(90%)=95.7%). 3F-NC on ipsilateral lung resulted as the P-Arc. The tangential approach, the 2-Arc or the IMRT techniques, resulted to be inferior to the previous in either conformality (tangentials), ipsilateral lung sparing (tangentials, 2-Arc and IMRT) and in contralateral or healthy tissue involvement (IMRT). CONCLUSION: For early stage breast cancer when high sparing of lung tissues is required and no involvement of contralateral breast is allowed, the P-Arc or the 3F-NC techniques might be recommended in terms of dosimetric expectations.

On the dosimetric behaviour of photon dose calculation algorithms in the presence of simple geometric heterogeneities: comparison with Monte Carlo calculations.

A comparative study was performed to reveal differences and relative figures of merit of seven different calculation algorithms for photon beams when applied to inhomogeneous media. The following algorithms were investigated: Varian Eclipse: the anisotropic analytical algorithm, and the pencil beam with modified Batho correction; Nucletron Helax-TMS: the collapsed cone and the pencil beam with equivalent path length correction; CMS XiO: the multigrid superposition and the fast Fourier transform convolution; Philips Pinnacle: the collapsed cone. Monte Carlo simulations (MC) performed with the EGSnrc codes BEAMnrc and DOSxyznrc from NRCC in Ottawa were used as a benchmark. The study was carried out in simple geometrical water phantoms (rho = 1.00 g cm(-3)) with inserts of different densities simulating light lung tissue (rho = 0.035 g cm(-3)), normal lung (rho = 0.20 g cm(-3)) and cortical bone tissue (rho = 1.80 g cm(-3)). Experiments were performed for low- and high-energy photon beams (6 and 15 MV) and for square (13 x 13 cm2) and elongated rectangular (2.8 x 13 cm2) fields. Analysis was carried out on the basis of depth dose curves and transverse profiles at several depths. Assuming the MC data as reference, gamma index analysis was carried out distinguishing between regions inside the non-water inserts or inside the uniform water. For this study, a distance to agreement was set to 3 mm while the dose difference varied from 2% to 10%. In general all algorithms based on pencil-beam convolutions showed a systematic deficiency in managing the presence of heterogeneous media. In contrast, complicated patterns were observed for the advanced algorithms with significant discrepancies observed between algorithms in the lighter materials (rho = 0.035 g cm(-3)), enhanced for the most energetic beam. For denser, and more clinical, densities a better agreement among the sophisticated algorithms with respect to MC was observed.

GLAaS: an absolute dose calibration algorithm for an amorphous silicon portal imager. Applications to IMRT verifications.

A new calibration algorithm (GLAaS) to derive absolute dose maps from images acquired with the Varian PV-aS500 electronic portal imager (based on amorphous silicon detectors) has been developed incorporating the dependence of detector response on primary and transmitted radiation and on field size. Detector calibration and algorithm validation were performed at different depths (10.0, 3.8, 1.5, and 0.8 cm) in solid water to investigate various application possibilities. Calibration data were obtained against ion chamber measurements. Validation experiments were performed on intensity-modulated fields and comparison was carried out against calculated dose maps as well as against film measurements. Split fields were acquired independently and PV-aS500 images were summed offline with the new algorithm allowing complex fields to be verified in conditions most closely resembling clinical conditions. Excellent results were obtained for the 3.8, 1.5, and 0.8 depths on a set of 34 modulated fields including both split and nonsplit fields. Applying the gamma index analysis (with distance to agreement and dose thresholds set to 3 mm and 4%, respectively), only 2.3% of the field area showed gamma > 1 at 1.5 cm depth (8.1%, 3.1%, 2.7% at 10.0, 3.8, and 0.8 and 2.5% with films at 10 cm depth). Tests were also performed to verify GLAaS at gantry angles different from 0 degrees. No statistical differences were obtained for the comparison between split and nonsplit fields and between different gantry angles. Highly significant statistical differences were obtained when comparing independent samples of 240 fields verified either with GLAaS or with film. Fields verified with GLAaS presented a mean area with gamma > 1 of 2.1 +/-1.3% while for film this value was 3.9 +/- 3.4% (p<0.001). Absolute dosimetry proved to be reliable with the PV-aS500 detector with the GLAaS algorithm. The minimal settings at depths of 1.5 or 3.8 cm would allow the use of the detector at any gantry angle without the need for any special fixation tool.

Dosimetric validation of the anisotropic analytical algorithm for photon dose calculation: fundamental characterization in water.

In July 2005 a new algorithm was released by Varian Medical Systems for the Eclipse planning system and installed in our institute. It is the anisotropic analytical algorithm (AAA) for photon dose calculations, a convolution/superposition model for the first time implemented in a Varian planning system. It was therefore necessary to perform validation studies at different levels with a wide investigation approach. To validate the basic performances of the AAA, a detailed analysis of data computed by the AAA configuration algorithm was carried out and data were compared against measurements. To better appraise the performance of AAA and the capability of its configuration to tailor machine-specific characteristics, data obtained from the pencil beam convolution (PBC) algorithm implemented in Eclipse were also added in the comparison. Since the purpose of the paper is to address the basic performances of the AAA and of its configuration procedures, only data relative to measurements in water will be reported. Validation was carried out for three beams: 6 MV and 15 MV from a Clinac 2100C/D and 6 MV from a Clinac 6EX. Generally AAA calculations reproduced very well measured data, and small deviations were observed, on average, for all the quantities investigated for open and wedged fields. In particular, percentage depth-dose curves showed on average differences between calculation and measurement smaller than 1% or 1 mm, and computed profiles in the flattened region matched measurements with deviations smaller than 1% for all beams, field sizes, depths and wedges. Percentage differences in output factors were observed as small as 1% on average (with a range smaller than +/-2%) for all conditions. Additional tests were carried out for enhanced dynamic wedges with results comparable to previous results. The basic dosimetric validation of the AAA was therefore considered satisfactory.

Critical appraisal of a non-coplanar technique for radiotherapy of breast minimising lung involvement.

BACKGROUND AND PURPOSE: To appraise the potential benefit of a conformal technique with non-coplanar fields to minimise lung irradiation in the radiation treatment of breast. PATIENTS AND METHODS: A comparative study was carried out at planning level for six patients selected for their inadequate sparing of healthy lung tissue with the reference tangential technique. Plans were designed for the conventional tangential technique, for an alternative conformal approach with three beams and for the newly proposed technique with two non-coplanar beams. RESULTS: In average for the new technique compared to the reference, mean lung dose dropped from approximately 16 to 10.5 Gy, V(20 Gy) from 29.5 to 18.2% and the dose delivered to 1/3 (1/4) of the lung volume dropped from 28.5% (67.3%) to 8.7% (13.4%). For PTV, the volume receiving at least 90% of the prescribed dose resulted 97.4% for the new and 97.3% for the reference. Conformity index improved significantly from 2.58 for the reference to 1.84 for the new technique. CONCLUSIONS: For a subgroup population of breast cancer patients, where conventional techniques failed to achieve high conformal avoidance, a treatment modality with non-coplanar beams was developed and clinically tested for six patients. It resulted dosimetrically adequate, particularly when the risk of toxicity is relevant.

IMRT with the sliding window: comparison of the static and dynamic methods. Dosimetric and spectral analysis.

BACKGROUND AND PURPOSE: Aim of the study is the critical appraisal of the two delivery techniques for intensity-modulated treatments commonly known as dynamic and static step and shoot, in the framework of a sliding window multileaf sequencing method. METHODS: The study was performed using the solution commercialised by Varian with the Eclipse treatment planning system (TPS) and the Clinac accelerator. For a set of intensity modulated fluences, the calculated dose maps and the dose delivered to films were compared for the static and dynamic modes to verify the capability of the TPS to accurately model both the delivery modes. For these investigations, the gamma concept of Low et al. [Low D, Harms W, Mutic S, Purdy J. A technique for the quantitative evaluation of dose distributions. Med Phys 1998;25: 656-60] was applied demonstrating, in general, optimal modeling for both static and dynamic tests. Optimal and actual fluences, were analysed to ascertain the degree of the TPS accuracy in converting 'ideal' maps into realistic leaf motions. RESULTS: Among the methods used, the Webb's Modulation Index [Webb S. Use of a quantitative index of beam modulation to characterize dose conformality: illustration by a comparison of full beamlet IMRT, few-segment IMRT and conformal unmodulated radiotherapy. Phys Med Biol. 2003;48: 2051-2062] proved to be useful. Also, dose volume histogram analysis was applied on deliverable plans using more clinical tools. CONCLUSION: The dynamic delivery method seems to offer a more reliable agreement with the optimal calculations and, clinically, a slightly superior performance in terms of target coverage.

Experimental method to obtain scattering contribution in portal dose images.

A method for evaluating scattered dose contribution in portal images acquired under clinical conditions (phantom-device distance of 30 cm) is presented. This method is based on radiographic film and ionisation chamber measurements and is valid for homogenecus polystyrene phantoms and square fields of different size. The portal imaging device consisted of a radiographic film placed between slabs of polystyrene under full build-up conditions (1.5 cm for 6 MV beam and 3 cm for 18 MV and 1 cm of polystyrene backscatter material. First the primary dose image in the portal plane P(i,j) is obtained using a projection algorithm, then the scattered dose image S(i,j) is found by subtracting the primary dose image in the portal plane P(i,j) from the total dose image acqu red in the portal plane T(i,j). The ratio S(i,j)/T between the scattered dose distribution and the dose value measured on the bearn axis in the portal plane was found to be uniform within the radiation field for all the geometrical configuration of phantoms and fields studied. Under these conditions the mean value of the scatter fraction S/T evaluated within a ROI centred on the bearn axis accurately describes the scatter fraction distribution S(i,j)/T within the whole radiation field. S/T ranges from 7.4% to 31.1% in the 6 MV beam and from 8.9% to 30.8% in the 18 MV beam. Finally an analytical method to evaluate the ratio S/T has been developed from the experimental results. It comprises phantom, accelerator head and portal imaging device contributions and depends on field size and phantom thickness.