Fetal dose estimation for Virtual Tangential-fields Arc Therapy whole breast irradiation by optically stimulated luminescence dosimeters.

Breast cancer is the most frequently diagnosed tumor in pregnant women and radiation therapy should carefully be weighted up because of the dose to the fetus. The aim of this study was to investigate fetal dose in a patient treated with Virtual Tangential-fields Arc Therapy (ViTAT), an innovative technique for whole breast irradiation. Optically stimulated luminescence detectors (OSLDs) were calibrated on a Varian TrueBeam linac, with both a 6X and 6XFFF beam quality, and used for out-of-field measurements. Fetal dose related with ViTAT technique was compared to the standard 3D conformal radiation therapy technique (3DCRT). The fetal dose delivered with a ViTAT technique planned with 6XFFF beam was also investigated. Measurements were taken on a phantom composed of Rando Alderson Phantom slices and solid water slabs. OSLDs were placed in a region identified by the height of the fundus from conception to the twentieth week using a custom made PMMA grid. Due to the higher number of monitor units, the peripheral dose of ViTAT delivered with 6X beams is higher than that of 3DCRT. However, nanoDots measurements prove that ViTAT can be used in place of 3DCRT while maintaining similar fetal dose levels if 6XFFF beams are used.

External photon radiation treatment for prostate cancer: Uncomplicated and cancer-free control probability assessment of 36 plans.

PURPOSE: To perform a systematic and thorough assessment, using the Uncomplicated and Cancer-Free Control Probability (UCFCP) function, of a broad range of photon prostate cancer RT treatments, on the same scenario (a unique pelvic CT set). UCFCP considers, together with the probabilities of local tumour control (TCP) and deterministic (late) sequelae (NTCP), the second primary cancer risk (SPCR) due to photon and neutron peripheral doses. METHODS AND MATERIALS: Thirty-six radiotherapy plans were produced for the same CT. 6, 10, 15 and 18 MV 3DCRT, IMRT and VMAT (77.4 Gy in 43 fractions) and 6 and 10 MV SBRT (36.25 Gy in 5 fractions with flattened and FFF beams) for Elekta, Siemens and Varian Linacs plans were included. DVH and peripheral organ dosimetry were used to compute TCP, NTCP, and SPCR (the competition and LNT models) for further plan ranking. RESULTS: Biological models (and parameters) used predicted an outcome which is in agreement with epidemiological findings. SBRT plans showed the lowest SPCR and a below average NTCPrectal. High energy plans did not rank worse than the low energy ones. Intensity modulated plans were ranked above the 3D conformal techniques. CONCLUSIONS: According to UCFCP, the best plans were the10 MV SBRTs. SPCR rates were low and did not show a substantial impact on plan ranking. High energy intensity-modulated plans did not increase in excess the average of SPCR. Even more, they ranked among the best, provided that MU were efficiently managed.

Delivering RapidArc®: A comprehensive study on accuracy and long term stability.

PURPOSE: To establish the reliability and accuracy of a UNIQUE Linac in delivering RapidArc treatments and assess its long term stability. MATERIALS AND METHODS: UNIQUE performance was monitored and analyzed for a period of nearly two years. 2280 Dynalog files, related to 179 clinical RapidArc treatments were collected. Different tumor sites and dose scheduling were included, covering the full range of our treatment plans. Statistical distributions of MLC motion error, gantry rotation error and MU delivery error were evaluated. The stochastic and systematic nature of each error was investigated together with their variation in time. RESULTS: All the delivery errors are found to be small and more stringent tolerances than those proposed by TG142 are suggested. Unlike MLC positional errors, where a linear relationship with leaf speed holds, other Volumetric Modulated Arc Therapy (VMAT) parameters reveal a random nature and, consequently, a reduced clinical relevance. MLC errors are linearly related only to leaf speed no matter the shape of the MLC apertures. Gantry rotation and MU delivery are as accurate as major competing Linacs. UNIQUE was found to be reliable and accurate throughout the investigation period, regardless of the specific tumor sites and fractionation schemes. CONCLUSIONS: The accuracy of RapidArc treatments delivered with UNIQUE has been established. The stochastic nature of delivery errors is proven. Long term statistics of the delivery parameter errors do not show significant variations, confirming the reliability of the VMAT delivery system.

Radiation dose in chest CT: Assessment of size-specific dose estimates based on water-equivalent correction.

PURPOSE: In the setting of thorax Computed Tomography (CT), the main purpose of this work is to quantify differences in Size-Specific Dose Estimate (SSDE), calculated on the basis of both effective diameter and water-equivalent diameter, accounting for patient size and tissue attenuation, respectively. MATERIALS AND METHODS: An in-house software was developed to measure water-equivalent diameter and effective diameter on each CT slice, for 133 CT examinations. SSDE was calculated according to the Report of American Association of Physicists in Medicine Task Group (AAPM TG) 204 and 220. The ratio between effective and water-equivalent diameters was studied as function of cross-sectional air-to-body area ratio, for the slice located in the middle of the scan range. RESULTS: When AAPM TG 220 prescriptions were applied, SSDE was mostly found larger than that obtained with AAPM TG 204 recommendations. On average, a difference of about 12% was observed, in spite of a considerable variability (from -18% to 53%). The ratio between effective and water-equivalent diameters ranged from 0.97 to 1.31, with a mean value of 1.15. Moreover, it was found considerably correlated with cross-sectional air-to-body ratio (Pearson's coefficient was 0.78 for women, 0.90 for men). CONCLUSION: The discrepancy obtained by calculating SSDE on the basis of AAPM TG 204, instead of AAPM TG 220 Report, may vary substantially. Fluctuations were justified by the variability observed in the relative amount of low attenuating tissues in body, which was proved to be considerably correlated to the ratio between effective and water-equivalent diameters.

Validation of a commercial TPS based on the VMC(++) Monte Carlo code for electron beams: commissioning and dosimetric comparison with EGSnrc in homogeneous and heterogeneous phantoms.

The aim of the present work was the validation of the VMC(++) Monte Carlo (MC) engine implemented in the Oncentra Masterplan (OMTPS) and used to calculate the dose distribution produced by the electron beams (energy 5-12 MeV) generated by the linear accelerator (linac) Primus (Siemens), shaped by a digital variable applicator (DEVA). The BEAMnrc/DOSXYZnrc (EGSnrc package) MC model of the linac head was used as a benchmark. Commissioning results for both MC codes were evaluated by means of 1D Gamma Analysis (2%, 2 mm), calculated with a home-made Matlab (The MathWorks) program, comparing the calculations with the measured profiles. The results of the commissioning of OMTPS were good [average gamma index (γ) > 97%]; some mismatches were found with large beams (size ≥ 15 cm). The optimization of the BEAMnrc model required to increase the beam exit window to match the calculated and measured profiles (final average γ > 98%). Then OMTPS dose distribution maps were compared with DOSXYZnrc with a 2D Gamma Analysis (3%, 3 mm), in 3 virtual water phantoms: (a) with an air step, (b) with an air insert, and (c) with a bone insert. The OMTPD and EGSnrc dose distributions with the air-water step phantom were in very high agreement (γ ∼ 99%), while for heterogeneous phantoms there were differences of about 9% in the air insert and of about 10-15% in the bone region. This is due to the Masterplan implementation of VMC(++) which reports the dose as "dose to water", instead of "dose to medium".

SU-E-T-466: TCP and NTCP: Is That All?

PURPOSE: Concerns about the secondary cancer risks associated to the peripheral neutron and photon contamination in photon modern radiotherapy (RT) techniques (e.g., Intensity Modulated RT -IMRT- or Intensity Modulated Arc Therapy -IMAT) have been widely raised. Benefits in terms of better tumor coverage have to be balanced against the drawbacks of poorer organ at risk sparing and secondary cancer risk in order to make the decision on the optimum treatment technique. The aim of this study was to develop a tool which estimates treatment success taking into consideration the neutron secondary cancer probability. METHODS: A methodology and benchmark dataset for radiotherapy real time assessment of patient neutron dose and application to a novel digital detector (DD) has been carried out (submitted to PMB, 2011). Our DD provides real time neutron equivalent dose distribution in relevant organs along the patient. This information, together with TCP and NTCP estimated from the DVH of target and organs at risks, respectively, have been built into a general biological model which allows us to evaluate the success of the treatments (Sánchez-Nieto et al., ESTRO meeting 2012). This model has been applied to make estimation of treatment success in a variety of treatment techniques (3DCRT, forward and inverse IMRT, RapidArc, Volumetric Modulated Arc Therapy and Helical Tomotherapy) to low and high energy. RESULTS: MU-demanding techniques at high energies were able to deliver treatment plans with the highest complicated-free tumour control. Nevertheless, neutron peripheral dose must be taken into consideration as the associated risk could be of the same order of magnitude than the usually considered NTCPs. CONCLUSIONS: The methodology developed to provide an online organ neutron peripheral dose can be successfully combined with biological models to make predictions on treatment success taking into consideration secondary cancer risks.

PET/CT and radiotherapy : data transfer, radiotherapy workflow and quality assurance.

The development of new technologies in radiation therapy has made it possible to introduce more sophisticated techniques that can deliver the prescribed dose with more conformation and accuracy and to apply dose escalation protocols without increasing the risk of healthy tissue damage. This has consented the simultaneous delivery of different dose levels to different parts of the target, making it possible to boost those tumour sub-volumes that are considered more radio resistant. The use of PET for radiotherapy planning purposes has become increasingly important in the last few years, because of its ability to provide valuable biologic and functional data. PET imaging can affect the treatment strategy definition and improve the target delineation and the assessment of therapy response. The most attractive aspect is the perspective to deliver differential doses inside target volumes for areas of different biologic behaviour based on functional imaging, moving closer to the goals of biologically conformal radiation therapy. Each single step of PET/CT-guided radiotherapy workflow, needs to be performed following high standard procedures, within a rigorous and appropriate quality assurance protocol to minimize the sources of errors and to maximize the efficacy of PET imaging in radiation therapy, ensuring safe and effective use of the technology. The present paper focuses on aspects concerning the use of PET/CT in radiation treatment process, with the aim to delineate different possible approaches to its clinical application and to highlight the critical aspects of the various subprocesses.