FF Modes Research Articles

Radioprotective effects of melatonin against varying dose rates on radiotherapy-induced salivary gland damage scintigraphy findings

The aim of present study was to evaluate the acute radioprotective effect of melatonin against radiotherapy-induced rat salivary gland damage by applying salivary gland scintigraphy imaging technique in varying dose rates.Forty-eight Sprague Dawley rats were randomly distributed into six group: the control group, only melatonin (MEL) group, flattening filter (FF-RT) and flattening filter free (FFF-RT) only radiotherapy groups and radiotherapy plus melatonin groups, (FF-RT)+Mel and (FFF-RT)+Mel. The head and neck region of experimental rat was irradiated with in a single fraction of a 16 Gy in FF and FFF modes. Melatonin was administered at a single dose of 10 mg/kg through intraperitoneal injection, 15 min before radiation exposure. The assessment of salivary gland function was performed using gamma scintigraphy prodecure before and after radiotherapy.Statistically significant difference in target-to-background (T/B) ratio, between the only melatonin and control groups were not observed (p = 0.981). However, when acute effects of irradiation are considered after radiotherapy, salivary gland T/B ratios was a significant difference (p < 0.001). In the only radiotherapy groups applied considering in the FF and FFF options no statistically significant difference was observed in the salivary gland T/B ratio FF-RT group compared to the FFF-RT group (p = 0.999). The T/B ratio between the radiotherapy plus melatonin groups for FF-RT and FFF-RT beam was significantly higher in the compared to the only radiotherapy (p < 0.05).In conclusion, our findings suggest that melatonin is a radioprotective agent for treatment of low and high dose rate against acute salivary gland damage during radiotherapy of head and neck cancer.

The role of melatonin on acute thyroid damage induced by high dose rate X-ray in head and neck radiotherapy

The purpose of the present study was to investigate the acute period radioprotective effect of melatonin (Mel) against radiation-iömnduced tissue damage in thyroid gland of rats using thyroid gland scintigraphy imaging.Forty-eight Sprague Dawley female rats were divided into six groups; the control group, melatonin treatment group, low dose rate (LDR-RT), high dose rate (HDR-RT) only radiotherapy groups and radiotherapy plus melatonin groups, (LDR-RT) + Mel and (HDR-RT) + Mel. The head and neck region of each rat was irradiated with single dose of 16 Gy at 6 MV X-ray in FF and FFF beam mode. Melatonin was administered at a single dose of 10 mg/kg through intraperitoneal injection, 15 min prior to radiation exposure. Functional evaluation of the thyroid glands were made using scintigraphy imaging before and after radiotherapy.Statistically significant difference was not observed in thyroid gland to background ratio (T/B) for all experimental rat groups before exposure (p = 0.379). However, when acute effects of irradiation are considered after radiotherapy T/B ratio showed a significant difference (p < 0.001). In the radiotherapy only groups, for both FF and FFF modes, statistically significant difference was not observed in the T/B ratio of LDR-RT group compared to HDR-RT group (p = 0.946). In the radiotherapy plus melatonin groups, both FF and FFF modes, statistically significant difference was also observed in the T/B ratio of LDR-RT group compared to HDR-RT group (p < 0.05).We showed that melatonin administered before radiotherapy provided significant protective effect against acute rat thyroid gland dysfunction caused by single dose LDR-RT and HDR-RT.

Radioluminescence results from an Al2O3:C fiber prototype: 6 MV medical beam

The Investigations of this article focus on the response of an Al2O3:C radioluminescence (RL) prototype for medical dosimetry in a 6 MV photon beam. The prototype can be configured using two types of detectors coupled to fiber-optic cables - single crystal (1 × 1 × 2 mm3) and droplets (in two grain sizes, 38 and 4 μm, molded in r = 0.5 mm, l = 200 μm). By using the appropriate filters in addition to time gating it is possible to remove disturbance present during irradiation: the stem effect. Pre-irradiation of the dosimeters to a dose of 300 Gy made the memory effects in Al2O3:C negligible, so as to not impair the dosimetric properties of the system. The key findings are that the system is suitable for small field beam dosimetry, while giving overall good dose response in other features (i.e., beam profile, dose rate - FF and FFF modes). The results show that our prototype can be used for real time dose rate assessment in medical photon dosimetry without many correction factors. The 4 μm RL measurement results are in excellent agreement (i.e. below 1%) with the dose delivered according to standard beam data.

Neutron dose distribution in the treatment room for an accelerator in the flattening filter‐free mode

AbstractObjectiveTo provide specific data and to make recommendations for radiation protection for clinical applications, the neutron cumulative dose, and exposure time in the treatment room around an accelerator operating at 10 MV in the conditions of both a conventional dose rate (with a flattening filter [FF] mode) and an ultra‐high dose rate (flattening filter‐free [FFF] mode) were measured and analyzed.MethodsIn the accelerator treatment room, the data were collected from four representative sites: the therapist's operating location, the distal end of the couch in the patient plane, the proximal maze, and the protective door. The field sizes were 5 × 5 cm2, 10 × 10 cm2, and 30 × 30 cm2; the output doses were 500 MU, 1000 MU, and 2000 MU; the solid water phantom simulating the patient's body was put on the isocenter; the dose rate in FF mode was a 10‐MV X‐ray at 600 MU/min; and the dose rate in FFF mode was 2400 MU/min. Under the same conditions, data were collected outside the selected area measuring three positions: the protective door, operating room, and center of the main protective wall position.ResultsThe neutron cumulative dose was negatively correlated with the field size; the closer to the target, the larger the neutron cumulative dose. The neutron cumulative dose was the largest at the distal end of the couch, and the neutron cumulative dose in FF mode was almost threefold as large as that in FFF mode. The neutron exposure time was positively correlated with the output dose. The FF mode takes slightly longer than the FFF mode. No neutron dose was read outside the accelerator room.ConclusionsOperating the accelerator in FFF mode will benefit both the patients and radiation therapists by reducing the neutron cumulative dose and exposure time. The current anti‐neutron accelerator room design is equally applicable to an ultra‐high‐dose rate (FFF) mode accelerator.

SU‐F‐T‐545: Dosimetric and Radiobiological Evaluation of Dose Calculation Algorithms On Prostate Stereotactic Body Radiotherapy Using Conventional Flattened and Flattening‐Filter‐Free Beam

Purpose:The purpose of this study is to evaluate the dosimetric and radiobiological impact of Acuros XB (AXB) and Anisotropic Analytic Algorithm (AAA) dose calculation algorithms on prostate stereotactic body radiation therapy plans with both conventional flattened (FF) and flattening‐filter free (FFF) modes.Methods:For thirteen patients with prostate cancer, SBRT planning was performed using 10‐MV photon beam with FF and FFF modes. The total dose prescribed to the PTV was 42.7 Gy in 7 fractions. All plans were initially calculated using AAA algorithm in Eclipse treatment planning system (11.0.34), and then were re‐calculated using AXB with the same MUs and MLC files. The four types of plans for different algorithms and beam energies were compared in terms of homogeneity and conformity. To evaluate the radiobiological impact, the tumor control probability (TCP) and normal tissue complication probability (NTCP) calculations were performed.Results:For PTV, both calculation algorithms and beam modes lead to comparable homogeneity and conformity. However, the averaged TCP values in AXB plans were always lower than in AAA plans with an average difference of 5.3% and 6.1% for 10‐MV FFF and FF beam, respectively. In addition, the averaged NTCP values for organs at risk (OARs) were comparable.Conclusion:This study showed that prostate SBRT plan were comparable dosimetric results with different dose calculation algorithms as well as delivery beam modes. For biological results, even though NTCP values for both calculation algorithms and beam modes were similar, AXB plans produced slightly lower TCP compared to the AAA plans.

SU-F-T-621: Impact of Vacuum and Treatment Couch On Surface Dose in Stereotactic Body Radiation Therapy With and Without a Flattening Filter

Purpose: When treating lung cancer patients with stereotactic body radiation therapy (SBRT), better immobilization is needed for accurate delivery of high-dose radiation. However, using a treatment couch (TrueBeamTM) and vacuum bag (BlueBAGTM) may increase the surface dose and skin toxicity. This study investigated the influence of couch and vacuum bag on the surface dose. Methods: The relative surface dose (D0/DMAX) was measured in an ion-chamber (Markus-type PTW, 0.05cm3) with a solid water phantom and SSD to 100 cm. A comprehensive comparison of different parameter settings, including the different energies (6MV-FFF, 10MV-FF, and 10MV-FFF), field sizes (3 X 3 cm2, 5 × 5 cm2, 8 × x cm2, 10 × 10 cm2, and 15 × 15 cm2), thickness of the vacuum bag (5mm, 15mm, 30mm, 39mm and 55mm), and couch (with and without), was performed. Results: The FFF increases the surface dose as compared to FF mode. In a similar setting with field of 10 × 10 cm2, FFF mode increases the surface dose from 26.0% to 32.8% for 6 MV, and 17.4% to 21.5% for 10 MV. When the beam passes through the couch, the surface dose increases to 3.6, 4.6, 2.9, and 3.7 times for 6 MV-FF, 10 MV-FF, 6 MV-FFF, and 10 MV-FFF, respectively. At the same energy, the surface dose increases to 3.93, 4.11, 4.23, 4.16 and 4.24 times at 5 mm, 15 mm, 30 mm, 39 mm and 55 mm thickness of the vacuum, respectively. Conclusion: Using a couch and vacuum significantly increases the surface dose. For SBRT with a superficial target close to the couch and immobilization vacuum, reduction of vacuum thickness and careful attention to skin dose in planning would be helpful in avoiding severe skin toxicity.

Small field detector correction factors: effects of the flattening filter for Elekta and Varian linear accelerators.

Flattening filter‐free (FFF) beams are becoming the preferred beam type for stereotactic radiosurgery (SRS) and stereotactic ablative radiation therapy (SABR), as they enable an increase in dose rate and a decrease in treatment time. This work assesses the effects of the flattening filter on small field output factors for 6 MV beams generated by both Elekta and Varian linear accelerators, and determines differences between detector response in flattened (FF) and FFF beams. Relative output factors were measured with a range of detectors (diodes, ionization chambers, radiochromic film, and microDiamond) and referenced to the relative output factors measured with an air core fiber optic dosimeter (FOD), a scintillation dosimeter developed at Chris O'Brien Lifehouse, Sydney. Small field correction factors were generated for both FF and FFF beams. Diode measured detector response was compared with a recently published mathematical relation to predict diode response corrections in small fields. The effect of flattening filter removal on detector response was quantified using a ratio of relative detector responses in FFF and FF fields for the same field size. The removal of the flattening filter was found to have a small but measurable effect on ionization chamber response with maximum deviations of less than ±0.9% across all field sizes measured. Solid‐state detectors showed an increased dependence on the flattening filter of up to ±1.6%. Measured diode response was within ±1.1% of the published mathematical relation for all fields up to 30 mm, independent of linac type and presence or absence of a flattening filter. For 6 MV beams, detector correction factors between FFF and FF beams are interchangeable for a linac between FF and FFF modes, providing that an additional uncertainty of up to ±1.6% is accepted.PACS number(s): 87.55.km, 87.56.bd, 87.56.Da

Flattening filter free beams from TrueBeam and Versa HD units: Evaluation of the parameters for quality assurance.

Flattening filter free (FFF) beams generated by medical linear accelerators are today clinically used for stereotactical and non-stereotactical radiotherapy treatments. Such beams differ from the standard flattened beams (FF) in the high dose rate and the profile shape peaked on the beam central axis. Definition of new parameters as unflatness and slope for FFF beams has been proposed based on a renormalization factor for FFF profiles. The present study aims to assess the dosimetric differences between FFF beams generated by linear accelerators from different vendors, and to provide renormalization and parameter data of the two kinds of units. Dosimetric data from two Varian TrueBeam and two Elekta Versa HD linear accelerators, all with 6 and 10 MV nominal accelerating potentials, FF and FFF modes have been collected. Renormalization factors and related fit parameters according to Fogliata et al. ["Definition of parameters for quality assurance of flattening filter free (FFF) photon beams in radiation therapy," Med. Phys. 39, 6455-6464 (2012)] have been evaluated for FFF beams of both units and energies. Unflatness and slope parameters from profile curves were evaluated. Dosimetric differences in terms of beam penetration and near-the-surface dose were also assessed. FFF profile parameters have been updated; renormalization factors and unflatness from the Varian units are consistent with the published data. Elekta FFF beam qualities, different from the Varian generated beams, tend to express similar behaviour as the FF beam of the corresponding nominal energy. TPR20,10 for 6 and 10 MV FF and FFF TrueBeam beams are 0.665, 0.629 (6 MV) and 0.738, 0.703 (10 MV). The same figures for Versa HD units are 0.684, 0.678 (6 MV) and 0.734, 0.721 (10 MV). Renormalization factor and unflatness parameters evaluated from Varian and Elekta FFF beams are provided, in particular renormalization factors table and fit parameters.

SU‐E‐T‐195: Commissioning the Neutron Production of a Varian TrueBeam Linac

Purpose:The purpose of this work is the characterization of a new Varian TrueBeam™ facility in terms of neutron production, in order to estimate neutron equivalent dose in organs during radiotherapy treatments.Methods:The existing methodology [1] was used with the reference SRAMnd detector, calibrated in terms of thermal neutron fluence at the reference field operated by PTB (Physikalisch‐Technische‐Bundesanstalt) at the GeNF (Geesthacht‐Neutron‐Facility) with the GKSS reactor FRG‐1 [2]. Thermal neutron fluence for the 5 available possibilities was evaluated: 15 MV and 10&amp;6 MV with and without Flattening Filter (FF and FFF, respectively). Irradiation conditions are as described in [3]. In addition, three different collimator‐MLC configurations were studied for 15 MV: (a) collimator of 10×10 cm2 and MLC fully retracted (reference), (b) field sizes of 20×20 cm2 and 10×10 cm2 for collimator and MLC respectively, and (c) collimator and MLC aperture of 10×10 cm2.Results:Thermal fluence rate at the “reference point” [3], as a consequence of the neutron production, obtained for (a) conformation in 15 MV is (1.45±0.11) x10⁴ n•cm2/MU. Configurations (b) and (c) gave fluences of 96.6% and 97.8% of the reference (a). Neutron production decreases up to 8.6% and 5.7% for the 10 MV FF and FFF beams, respectively. Finally, it decreases up to 2.8% and 0.1% for the 6 MV FF and FFF modes, respectively.Conclusion:This work evaluates thermal neutron production of Varian TrueBeam™ system for organ equivalent dose estimation. The small difference in collimator‐MLC configuration shows the universality of the methodology [3]. A decrease in this production is shown when decreasing energy from 15 to 10 MV and an almost negligible production was found for 6 MV. Moreover, a lower neutron contribution is observed for the FFF modes.[1]Phys Med Biol,2012;57:6167–6191.[2]Radiat Meas,2010;45:1513–1517.[3]Med Phys,2015;42:276–281.

SU‐F‐BRE‐11: Neutron Measurements Around the Varian TrueBeam Linac

Purpose:With the emergence of flattening filter free (FFF) photon beams, several authors have noted many advantages to their use. One such advantage is the decrease in neutron production by photonuclear reactions in the linac head. In the present work we investigate the reduction in neutrons from a Varian TrueBeam linac using the Nested Neutron Spectrometer (NNS, Detec). The neutron spectrum, total fluence and source strength were measured and compared for 10 MV with and without flattening filter and the effect of moderation by the room and maze was studied for the 15 MV beam.Methods:The NNS, similar to traditional Bonner sphere detectors but operated in current mode, was used to measure the neutron fluence and spectrum. The NNS was validated for use in high dose rate environments using Monte Carlo simulations and calibrated at NIST and NRC Canada. Measurements were performed at several positions within the treatment room and maze with the linac jaws closed to maximize neutron production.Results:The measurements showed a total fluence reduction between 35‐40% in the room and maze when the flattening filter was removed. The neutron source strength Qn was calculated from in‐room fluence measurements and was found to be 0.042 × 102 n/Gy, 0.026 × 102 n/Gy and 0.59 × 1012 n/Gy for the 10 MV, the 10 MV FFF and 15 MV beams, respectively. We measured ambient equivalent doses of 11 mSv/hr, 7 mSv/hr and 218 mSv/hr for the 10 MV, 10 MV FFF and 15 MV by the head.Conclusion:Our measurements revealed a decrease in total fluence, neutron source strength and equivalent dose of approximately 35‐40% across the treatment room for the FFF compared to FF modes. This demonstrates, as expected, that the flattening filter is a major component of the neutron production for the TrueBeam.The authors greatly acknowledge support form the Canadian Nuclear Commission and the Natural Sciences and Engineering Research Council of Canada through the CREATE program. Co‐authors Dubeau and Witharana are employees of Detec (Gatineau, Quebec), manufacturer of the Nested Neutron Spectrometer.

SU‐C‐211‐04: Evaluation of IMRT and VMAT Treatment Plan Quality Delivered with and Without Flattening Filter Using Pareto Optimal Fronts

Purpose: To evaluate treatment plan quality of IMRT and VMAT plans for high energy photon beams delivered with (FF) and without (FFF) flattening filter. Methods: 9‐field IMRT and 360° single arc VMAT plans, based on 10 MV beams with and without flattening filter, were created using Monaco (Elekta/CMS, v.2.04, USA) for three different patients (prescribed PTV dose 78 Gy). This treatment planning systems was commissioned for FF and FFF beams provided by an Elekta linac. The Pareto optimal fronts were created by calculating different treatment plans with varying rectum constraints and evaluating the PTV volume receiving less than 95% of the prescribed dose (V &lt;95%) and the volume of the rectum receiving 70Gy or more (V 70Gy). Treatment plan efficiency was evaluated by recording number of monitor units (MUs) and by measuring the delivery time (T) using an Elekta Precise linear accelerator in FF and FFF mode. Results: The POFs of the rectum for both IMRT and VMAT in FFF mode were similar or even superior to the FF‐ modalities. For two of the three patients the POFs of IMRT and VMAT revealed a systematic difference of about 2% in target coverage. The POFs of the remaining patient showed virtually no difference for all four modalities. The delivery time of IMRT FFF decreased by about 23% compared to IMRT FF. In contrast to the IMRT techniques, delivery time increased by 20% when using VMAT FFF compared to VMAT FF. Conclusions: The evaluation of the POFs confirms that a flattening filter is not necessary for static and rotational IMRT treatments. Similar studies are needed for other IMRT indications, like head‐and‐neck cancer, to draw final conclusions. Sequencing algorithms for FFF‐beams need to be optimized and further improvement of the mechanical MLC properties are desirable for VMAT in FFF mode. Corporate sponsored research by Elekta

Reduced Neutron Production Through Use of a Flattening-Filter–Free Accelerator

To measure and compare neutron fluence around an accelerator operating at 18 MV, both with the flattening filter present (FF mode) and absent (flattening-filter-free [FFF] mode). The neutron fluence was measured at several locations in the patient plane using gold foil activation in neutron moderators. Differences in neutron fluence between the FF and FFF mode were assessed in three frameworks: (1) measured per monitor unit of machine-on time, (2) determined per dose on the central axis, and (3) determined for a complete course of prostate intensity-modulated radiotherapy. Neutron fluence per monitor unit was approximately 20% lower when the accelerator was operated in the FFF mode than when it was in FF mode. The total amount of neutron fluence that would be obtained during the entire course of prostate intensity-modulate radiotherapy was 69% lower when the accelerator was operated in the FFF mode than when it was in the FF mode. This reduction in neutron fluence would correspond to a drastic reduction in the neutron dose equivalent received by the patient as a byproduct of high-energy radiotherapy. It would also correspond to a reduction in activation within the treatment vault and subsequent exposure to radiation therapists. When feasible, operating the accelerator without a FF will benefit both patients and radiation therapists by reducing the number of unwanted neutrons and resultant exposure. This reduces the risk of negative effects from such exposure (e.g., second cancers).