Standard Linac Research Articles

A low Z linac and flat panel imager: comparison with the conventional imaging approach

Experimental and Monte Carlo simulations were conducted for an Elekta Ltd Precise Treatment System linac fitted with a low Z insert of sufficient thickness to remove all primary electrons. A variety of amorphous silicon based panels employing different scintillators were modelled to determine their response to a variety of x-ray spectra and produce an optimized portal imaging system. This study has shown that in a low Z configuration the vast majority of x-rays are produced in the nickel electron window, and with a combination of a carbon insert and caesium iodide based XVI-panel, significant improvement in the object contrast was achieved. For thin, head and neck-type geometries, contrast is 4.62 times greater for 1.6 cm bone in 5.8 cm water than the standard 6 MV/iViewGT system. For thicker, pelvis-type geometries contrast increases by a factor of 1.3 for 1.6 cm of bone in 25.8 cm water. To obtain images with the same signal-to-noise ratio as the 6 MV/iViewGT system, dose reductions of a factor of 15 and 4.2 are possible for 5.8 cm and 25.8 cm phantoms respectively. This design has the advantage of being easily implemented on a standard linac and provides a portal image directly from the therapy beam aperture.

SU‐DD‐A1‐05: Treatment Planning of Prone Position Accelerated Partial Breast Irradiation Customized for Magnetic Resonance Imaging Guidance

Purpose: It is important to deliver an optimized dose distribution to an accurately delineated target volume for a successful accelerated partial breast irradiation (APBI) treatment. Breast magnetic resonance imaging (MRI) could improve lumpectomy delineation. We investigated treatment plans for CyberKnife and conventional C‐arm LINAC delivery of MRI‐guided APBI in prone position. Method and Materials: We acquired a CT scan of a patient positioned on a mechanical assembly with the same geometry as the dedicated MR coil to simulate the position during breast MRI acquisition. Ellipsoidal virtual PTV's of 4 cm were defined in the anterior part and in the lower outer quadrant of the right breast. Treatment plans were generated by inverse planning techniques for each system. For CyberKnife, we minimized beam directions from posterior to anterior to protect critical normal tissues (heart, lung and contralateral breast) and normalized the maximum dose to 120% of the prescription dose (38.5 Gy). For the conventional LINAC, we excluded any beams directed towards the critical normal tissues and normalized the plan to achieve 99% of the prescription coverage for 99% of PTV. Results: LINAC‐based plans resulted in maximum doses of 41.6 Gy in PTV and ⩽ 0.5 Gy in the critical normal structures. CyberKnife‐based plans led to 90% of the prescription dose coverage for 95% of PTV. Maximum doses were ⩽ 0.5 Gy in contralateral breast, ⩽ 1.9 Gy in heart, and ⩽ 4.2 Gy in both lungs. The fraction of normal ipsilateral breast tissue receiving ⩾ 37 Gy was 1% and 5% for CyberKnife and LINAC plans respectively. Conclusion: CyberKnife and LINAC generate treatment plans for MRI‐guided prone position APBI with acceptable dose distributions. The standard LINAC requires careful beam configuration to secure clearances between equipments. CyberKnife results in better conformity and slightly higher doses in the critical normal tissues.

Extracranial stereotaxic body irradiation: preliminary results and the Orleans hospital experience

Extracranial stereotactic body irradiation (ESRT) provides an alternative treatment mode for patients with no other therapeutic solutions. It's based on previous intracranial stereotactic radiosurgery techniques. ESRT with dedicated systems including secure immobilization avoiding patient movement, accurate repositioning, and stereotactic registration of tumor targets and organs at risk (OAR) allows very high doses in few fractions on extra cranial locations. Standard linacs permit performing SBRT with specific immobilization devices including stereotactic location of the target. Different schemes of ESRT have been published. Comparison of these very diverse schemes is not easy since the prescription mode on target volumes and OAR are not always clearly defined. The Radiotherapy Department of Orléans Hospital chose to set up the ESRT on a conventional accelerator using an immobilization device including a Stereotactic Body Frame and an Active Breathing Control system (Elekta). From 120 patients treated with ESRT since 1999, 90 cases are presented; 28 lung metastases were treated with 40 Gy/5 fr/12 d and 17 liver metastases were treated with 30 Gy/5 fr/12 d prescribed on the 70 to 85 % isodose. The local control was obtained in 50 % of the cases. Twenty-eight patients were treated for T1-T2 N0 lung cancer with 50 Gy/10 fr/12 d on the 80 % isodose. Local control was obtained in 50 % of the cases. Seventeen liver cancers were treated with 30 Gy/10 fr/15 d prescribed on the 70 to 80 % isodose. Local control was obtained in 47 % of the cases. ECRT allows controlling localized tumors (primitives or metastases) and it is now a suitable part of our therapeutic tools as radiofrequency or cryotherapy techniques. Phase II studies are required to confirm its therapeutic potential.

A time-of-flight detector for thermal neutrons from radiotherapy Linacs

Boron Neutron Capture Therapy (BNCT) is a therapeutic technique exploiting the release of dose inside the tumour cell after a fission of a 10B nucleus following the capture of a thermal neutron. BNCT could be the treatment for extended tumors (liver, stomach, lung), radio-resistant ones (melanoma) or tumours surrounded by vital organs (brain). The application of BNCT requires a high thermal neutron flux ( > 5 × 10 8 n cm - 2 s - 1 ) with the correct energy spectrum (neutron energy < 10 keV ), two requirements that for the moment are fulfilled only by nuclear reactors. The INFN PhoNeS (Photo Neutron Source) project is trying to produce such a neutron beam with standard radiotherapy Linacs, maximizing with a dedicated photo-neutron converter the neutrons produced by Giant Dipole Resonance by a high energy ( > 8 MeV ) photon beam. In this framework, we have developed a real-time detector to measure the thermal neutron time-of -flight to compute the flux and the energy spectrum. Given the pulsed nature of Linac beams, the detector is a single neutron counting system made of a scintillator detecting the photon emitted after the neutron capture by the hydrogen nuclei. The scintillator signal is sampled by a dedicated FPGA clock thus obtaining the exact arrival time of the neutron itself. The paper will present the detector and its electronics, the feasibility measurements with a Varian Clinac 1800/2100CD and comparison with a Monte Carlo simulation.

Monitortabellen für Elektronenstrahlung in der Strahlentherapie

The application of electron beams in radiotherapy is still based on tables of monitor units, although 3-D treatment planning systems for electron beams are available. This have several reasons: The need for 3-D treatment planning is not recognized; there is no confidence in the calculation algorithm; Monte-Carlo algorithms are too time-consuming; and the effort necessary to measure basic beam data for 3-D planning is considered disproportionate. However, the increasing clinical need for higher dosimetric precision and for more conformal electron beams leads to the requirement for more sophisticated tables of monitor units. The present paper summarizes and discusses the main aspects concerning the preparation of tables of monitor units for electron beams. The measurement equipment and procedures for measuring basic beam data needed for tables of monitor units for electron beams are described for a standard radiation therapy linac. The design of tables of monitor units for standard electron applicators is presented; this design can be extended for individual electron inserts, to variable applicator surface distances, to oblique beam incidence, and the use of bolus material. Typical data of an Elekta linac are presented in various tables.

SU-FF-T-382: Special Dosimetric/Measurement Considerations in Commissioning a Novel Integrated MiniMLC Linear Accelerator

Purpose: To present an overview of commissioning a novel integrated miniMLC linear accelerator system, including the complete steps in final clinical implementation. Special precautions due to the collimator design are identified. Reference datasets for electrons are included. Method and Materials: Elekta BeamModulator has 40 pairs of 4mm leaves. There are no movable backup diaphragms and the field is defined only by the open leaves. Smaller field sizes and more precise leaf positioning (<1mm) are specified. The film scanning system was verified to within 0.2mm accuracy. Microchambers were used for in‐water scanning. Brass and graphite miniphantoms were constructed for head‐scatter factor measurements of fields from 0.8cm×0.8cm to 16cm×16cm. A complete set of scan and point measurement data was collected for photons and electrons. Beams were modeled in XIO and PrecisePlan and an independent calculation program (RADCALC). Before actual clinical implementation, periodic QA baselines were established, and site specific IMRT plans and QA measurements were performed on phantoms. Results and Conclusion: An extensive and comprehensive program was employed in commissioning. Beam data collection and calibration were internally verified by at least two independent measurements and checked against standard datasets. Treatment planning system modeling followed the guidelines of TG53. When compromises had to be made, the best fits were chosen for situations mimicking IMRT segments (4cm×4cm and 4.8cm×4.8cm). QA measurements of 3D conformal plans and IMRT plans achieved the following agreement statistics: 3mm DTA, 3% difference, produced pass rate of 97.8% average (2.6%STD). Dose point measurements with chambers agreed to plan values within 3.6%. After comparisons between 3D dose, independent monitor unit(MU) and manual calculations; Radcalc and XIO independent MU calculation programs were deemed unusable (the discrepancy reaching 5.4%), due to incorrect modeling of head‐scatter factors for this collimator. Additionally, measured electron cone factors varied up to 13% from standard Elekta linacs.

AN ANALYSIS OF THE BEAT-INDUCING PROPERTIES OF CONDUCTORS' GESTURES

Although the number of proton therapy centres is growing worldwide, proton therapy is still a limited resource. The primary reasons are gantry size and cost. Therefore, we investigate the potential of a new design for proton therapy, which may facilitate proton treatments in conventional bunkers and allow the widespread use of protons.The treatment room consists of a standard Linac for IMRT, a motorized couch for treatments in lying position, and a horizontal proton beamline equipped with pencil beam scanning. As proton beams are limited to a coronal plane, treatment plans may be suboptimal for many tumour sites. However, high-quality plans may be realized by combining protons and photons. Treatment planning is performed by simultaneously optimizing IMRT and IMPT plans based on their cumulative physical dose. We demonstrate this concept for three head&neck cancer cases.Optimal combinations use photons to improve dose conformity while protons reduce the integral dose to normal tissues. In fact, combined treatments improve on single-modality IMRT and fixed beamline IMPT plans for quality-of-life-limiting OARs and retain most of the integral dose reduction in the healthy tissues of the pure IMPT plans. The lower doses that can be obtained with multi-modality treatments reduce the risk for side effects compared to single-modality IMRT plans.Combined proton–photon treatments may play a role in developing a new solution for proton therapy without a gantry. Optimal combinations improve on IMRT plans and reduce the risk of side effects while making protons available to more patients.

SU-FF-T-223: Evaluation of Dosimetric Properties of 6 &amp; 10 MV Photon Beams From a Liner Accelerator with No Flattening Filter

Purpose: To study the dosimetric properties of an IMRT treatment beam after removing the flattening filter. Our goal is to show that more efficient IMRT treatments can be performed on a standard IMRT-capable linac after the flattening filter has been removed. Method and Materials: Measurements and Monte Carlo simulations were performed of 6 and 10 MV photon beams from an Elekta SL-25. Comparisons of the properties of these beams were made, with and without the flattening filter. %dd curves, beam profiles and photon fluence spectra were the primary method of comparing the beams. A study using the LATCH option in BEAMnrc was also performed to determine the original of scattered photons that contribute to the dose outside of the treatment field. Results: Dose outside the treatment field was found to be reduced by up to 5.8%, depending on the field size. Photon fluence on the central axis of the 6 and 10 MV beams increased by a factor of 2.35 and 4.18. Conclusion: Our Monte Carlo study showed that by removing the flattening filter the amount of laterally scattered photons from the jaws and MLC's was reduced leading to lower photon fluencies outside of the treatment field. It is hypothesized that inverse treatments plans created using these filter-free beams would lead to a higher degree of conformation of the dose to the target. The increase in photon fluence that is induced by removal of the flattening filter is predicted to lead to shorter treatment times.

SU‐FF‐J‐07: A Low‐Z Target with No Flattener and Reduced Energy for Improved Contrast in Megavoltage Cone‐Beam CT

Purpose: To improve contrast in MV cone‐beam CT using a low‐Z target (LZT). Method and Materials: A high‐Z tungsten target (HZT) at 6 MV and low‐Z carbon targets of different thicknesses were used with no flattener. The maximum energy was used with LZT while eliminating primary electron leakage into the monitor chamber. Output and dose distributions were measured. Images were acquired at 7 frames/sec with a 1024×1024 400‐micrometer pixel flat panel detector with 1mm Cu build‐up and a Lanex fast scintillator. Phantoms were QC‐3V, a contrast/spatial resolution phantom, a sheep head and a cadaver. Results: Beam energy was 3.5±0.5 MV and 4.5±0.5 MV for the 1.016 cm and 1.35 cm carbon targets, respectively. The higher energy was more stable with higher output: 0.299 cGy/sec compared to 0.084 cGy/sec. Surface dose was 80%. Field diameter at isocenter was limited to 36 cm by the electron monitor, when present. Computed contrast‐to‐noise ratio (CNR) for the contrast‐spatial resolution phantom for LZT was 3.5 times that of HZT at 10 cGy. Computed CNR for HZT for the 2D QC‐3V phantom images is 24 and for LZT is 100 with dose per projection of 0.035cGy. The f50 of LZT is 0.41 lp/mm. Adaptive noise filtering with a kernel of 8×8 increased the CNR by a factor of 2.4 without degrading resolution. Preliminary cone beam images with the low Z target show remarkable bone contrast and indicate improved soft tissue contrast. Conclusion: A stable 4.5 MV beam was produced on a standard treatment linac equipped with a carbon target, showing improved CNR over images taken with the treatment target and sufficient output for cone‐beam CT. A direct comparison to images taken with the carbon and tungsten targets on the same cadaver, expected to show an improvement in soft tissue contrast, will be available at the meeting.

Phase-II study on stereotactic radiotherapy of locally advanced pancreatic carcinoma

Background and purposeThe majority of patients with pancreatic cancer have advanced disease at the time of diagnosis and are not amenable for surgery. Stereotactic radiotherapy (SRT) may be an alternative treatment for patients with locally advanced disease. The effect of SRT was investigated in the present phase-II trial. Patients and methodsTwenty-two patients with locally advanced and surgically non-resectable, histological proven pancreatic carcinoma were included into the trial. The patients were immobilized by the Elekta stereotactic body frame (SBF) or a custom made body frame. SRT was given on standard LINAC with standard multi-leaf collimator. Central dose was 15Gy×3 within 5–10 days. ResultsEvaluation of response was found to be very difficult due to radiation and tumour related tissue reaction. Only two patients (9%) were found to have a partial response (PR), the remaining had no change (NC) or progression (PD) after treatment. Six patients had local tumour progression, but only one patient had an isolated local failure without simultaneous distant metastasis. Median time to local or distant progression was 4.8 months. Median survival time was 5.7 months and only 5% were alive 1 year after treatment. Acute toxicity reported 14 days after treatment was pronounced. There was a significant deterioration of performance status (P=0.008), more nausea (P=0.001) and more pain (P=0.008) after 14 days compared with base-line. However, 8 of 12 patients (66%) improved in performance status, scored less nausea, pain, or needed less analgesic drugs at 3 months after treatment. Four patients suffered from severe mucositis or ulceration of the stomach or duodenum and one of the patients had a non-fatal ulcer perforation of the stomach. ConclusionsSRT was associated with poor outcome, unacceptable toxicity and questionable palliative effect and cannot be recommended for patients with advanced pancreatic carcinoma.

SU-FF-T-238: Dosimetry of Small Beams Used in Radiosurgery: A Comparison Between Different Detectors and Monte Carlo Simulation

Purpose: To compare experimental results from different detectors to Monte Carlo simulation in the dosimetry of small fields employed in the Cyberknife radiosurgery system. The Monte Carlo code has also been used to decide whether protocols that require a 10×10 cm2 field can be used for dosimetry under reference conditions. Method and Materials: Total scatter factors (sc,p) have been measured by means of the PTW PinPoint, Exradin A16 and T14P microchambers, Thomson and Nielsen micromosfet, PTW diamond and diode detectors, and MD 55-2 radiochromic films. A Monte Carlo calculation (BEAM code) was used to simulate the Cyberknife linac and to calculate sc,p and correction factors to be applied to raw data for various detectors. Two Farmer-type chambers were also simulated in order to calculate the ratio between kQ factors for the Cyberknife and of for a standard 6MV linac. Results: Monte Carlo simulations gave results in agreement within ±1% with experimental results for sc,p, PDD and profiles for collimators above 10 mm. Results for sc,p of smaller collimators (5 to 10 mm) agreed with radiochromic film measurements within ± 2%, while results with ionization chambers and diamond suffer from a systematic underestimation, resolved after application of the Monte Carlo correction factor. Micro-mosfet detectors showed good results as compared to Monte Carlo and radiochromic films, but suffered from poor reproducibility. The PTW diode detector seemed to systematically overestimate sc,p values; for the 5 mm collimator, a value of 0.746 has been obtained versus values in the range 0.67–0.71 obtained with Monte Carlo, radiochromic films, ionization chambers and diamond. The ratio between kQ factors at different beam qualities is 1.002 ± 0.5%. Conclusion: Monte Carlo can be used in the dosimetry of small beams to help solve experimental problems due to the finite dimension and to non-water equivalence of the detectors.

A low-energy-spread rf accelerator for a far-infrared free electron laser

A high electron current and a small energy spread are essential for the operation of a free electron laser (FEL). In this paper we discuss the design and performance of the accelerator for FELIX, the free electron laser for infrared experiments. The system consists of a thermionic gun, a prebuncher, a buncher and two standard commercial linac sections. The gun is operated with a pulse duration of 280 ps and a bunch charge of 200 pC. After compression to 35 ps by the prebuncher, the bunches are accelerated to 4 MeV in the buncher and simultaneously compressed to 6 ps. The principle of the method is that the order of the electrons is conserved in the buncher, so that the resulting more or less linear energy-phase relationship along each bunch can be compensated effectively against space charge forces and the accelerating field gradient in the linacs, via an appropriate choice of the phase of the rf wave. Behind the linacs an rms energy spread of 0.30% has been measured.

Heavy-ion accelerators for inertial confinement fusion

Two concepts have been applied to the classical problem of accelerators for the ignition of indirectly driven inertial fusion. The first is the use of non-Liouvillian stacking based on photoionisation of a singly charged ion beam. A special FEL appears the most suited device to generate the appropriate light beam intensity at the required wavelength. The second is based on the use of a large number of (&gt;1000) beamlets–or “beam straws”–all focussed by an appropriate magnetic structure and concentrated on the same spot on the pellet. The use of a large number of beams–each with a relatively low-current density–elegantly circumvents the problems of space charge, making use of the non-Liouvillian nature of the stopping power of the material of the pellet. The present conceptual design is based on a low-current (〈i〉 ≈ 50 mA) heavy-ion beam accelerated with a standard LINAC structure and accumulated in a stack of rings with the help of photoionisation. Beams are then extracted simultaneously from all the rings and further subdivided with the help of a switchyard of alternate paths separating and synchronising the many bunches from each ring before they hit the pellet. Single beam straws carry a reasonable number of ions: Beams and technology are directly relatable to the ones presently employed, for instance, at the CERN-PS. Space-charge-dominated conditions arise only during the last few turns before extraction and in the beam transport channel to the reaction chamber. In a practical example, we aim at a peak power of 500 TW delivered to the pellet for a duration of 10–15 ns. High-energy (10 GeV) beam straws of Ba doubly ionised ions are concentrated on several (four) focal spots of a radius of about 1 mm. The power density deposited on these tiny cylindrical absorbers inside a hermetic “hohlraum” is about 2.5 × 1016 w/g. These conditions are believed to be optimal for X-ray conversion, i.e., with an estimated conversion efficiency of about 90%.

Role of Tertiary Collimation for Linac‐Based Radiosurgery

AbstractFor the stereotactic irradiation of intracranial lesions with a linear accelerator, the use of a tertiary collimating system can be expected to confer several benefits in comparison to the use of the standard linac collimating jaw system. Moving the collimator closer to the patient should lead to reduced penumbra and reduced susceptibility to positioning error. In addition, the use of a circular rather than a square aperture should yield a more advantageous dose distribution for irradiation techniques which follow spherical geometry. An investigation using film dosimetry with a tissue‐equivalent head phantom demonstrated that the tertiary system was superior to the standard jaw system as expected. © 1993 Wiley‐Liss, Inc.