Radiotherapy Standards Users Meeting, 1 Dec 2008 Abstracts. New calorimeters Simon Duane, NPL

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1 Radiotherapy Standards Users Meeting, 1 Dec 2008 Abstracts New calorimeters Simon Duane, NPL Calorimetry remains the most fundamental method for the measurement of absorbed dose and is the basis of almost all absorbed dose primary standards worldwide. Absorbed dose to water is realised either directly, by using water as the absorbing medium, or indirectly, by using another medium (usually graphite) and determining a factor to convert dose from that medium to water. NPL maintains separate primary standards of absorbed dose for high-energy electron and photon beams. These graphite calorimeters are the devices to which dosimetry for most external beam radiotherapy in the UK is traceable and, having been developed between 15 and 25 years ago, these devices are now due for replacement. Several projects in calorimetry at NPL are underway, including the construction of one new primary standard for measurements in both electron and photon beams and another for use in particle therapy beams. Two further calorimeters are in development, one for the measurement of absorbed dose in HDR brachytherapy another for use in small field/complex dose distributions such as IMRT. Their key features will be outlined in this presentation. Heat flows relatively quickly through graphite and the sensitive volume, or core, in a calorimeter is defined by the thermal barriers (preferably evacuated gaps) which surround it. These gaps perturb the dose distribution and so are kept as narrow as possible. The optimal geometry of the core is determined by the application. For measurements in a single external beam, whether electrons, photons or other particles, the point of interest is on the central axis at a defined depth from the front face of a phantom. The effect of dose gradients at the point of measurement is minimised by choosing a flat cylindrical core, of minimal thickness in the direction of the beam. For measurements in brachytherapy, the core is an annulus surrounding the source, 25 mm in radius, 2 mm thick and 5 mm high. For measurements in IMRT, the core is a squat cylinder, no more than 5 mm in diameter and 5 mm high. The temperature of the core changes in response to radiation, by ºC/Gy in graphite, but is also affected by heat transferred to/from its immediate environment. The correction due to heat transfer should be minimised, and is estimated by also measuring the temperature of the surrounding jacket. Self-heating within a brachytherapy source means that the source is always warmer than its environment: additional heat transfers due to this warm source present a significant perturbation to the radiation measurement. Likewise, the complex variation of dose rate with position and time significantly increases the effect of heat transfers in calorimetry for IMRT. In each case, it appears that isothermal calorimeter operation offers the possibility to largely eliminate these problems.

2 Volumetric modulated arc therapy (VMAT) James L Bedford, Karen E Rosser and Alan P Warrington Royal Marsden NHS Foundation Trust Introduction : Volumetric modulated arc therapy (VMAT) is a new technique where a dynamic multileaf collimator (DMLC) delivery with variable dose rate is accomplished during one or more gantry arcs. The concepts of VMAT are reviewed, and the commissioning and clinical introduction of the technique described. Methods of dose verification are discussed. Methods: An in-house inverse planning system, AutoBeam, was developed to handle the dynamic control points required for VMAT planning. Pinnacle 3 (Philips Radiation Oncology Systems, Madison, WI) was used for a final dose calculation. Quality assurance of treatment delivery was carried out. Beam flatness and symmetry were measured at the various dose rates used for VMAT delivery. Narrow dynamic apertures were measured using ionisation chamber and film to assess the DMLC delivery performance. Rotational accuracy of the arcing was evaluated using a prescription designed to give a rotationally symmetric dose distribution in a cylindrical phantom. Measurements of complete treatments were also carried out. In particular, a dosimetric study was performed, in which alanine dosimeters were used to calibrate several ionisation chambers in stylised VMAT-like beams. The calibrated ionisation chambers were then used to verify two treatment plans. A study of dose verification using Presage radiochromic dosimeters (Heuris Pharma, Lawrenceville, NJ) was also initiated. VMAT was clinically implemented, initially for lung treatment. Results : For relatively simple treatment sites such as prostate or lung, VMAT treatment plans show small improvements in dosimetry compared to conformal treatment plans. Delivery time is approximately half that of static delivery. More complex cases are still under evaluation. Beam symmetry is better than 3% at dose rates down to 35 MU/min. DMLC delivery shows dose matching of better than 4% at leaf matching planes, and overall agreement in dose of better than 4% compared to integration of a static beam profile. Rotational uniformity is better than 0.5%. Gamma (3% and 3 mm) for a complete treatment plan is typically better than 95%. Calibration of ionisation chambers in VMAT beams using alanine produces calibration factors less than 1% different to those produced using a well-established dosimetry protocol. The delivered dose measured using the calibrated ionisation chambers is less than 2% different to the dose calculated by Pinnacle 3. Clinical implementation has been well received. Conclusion : VMAT planning and delivery procedures have been developed and successfully implemented, with improved dosimetry and reduced treatment time, compared to conformal treatment. A new dosimetry protocol has been investigated for this new technique. We are grateful to Elekta Ltd and for their collaboration on this project. References : Bedford JL, Hansen VN, McNair HA, Aitken AH, Brock JEC, Warrington AP and Brada M. Treatment of lung cancer using volumetric modulated arc therapy and image guidance: A case study. Acta Oncol. 47: (2008). Bedford JL and Warrington AP. Commissioning of volumetric modulated arc therapy (VMAT). Int. J. Radiat. Oncol. Biol. Phys. in press (2009).

3 Duane S, Nicholas D, Palmans H, Schaeken B, Sephton J, Sharpe P, Thomas R, Tomsej M, Tournel K, Verellen D and Vynckier S. Dosimetry audit for tomotherapy using alanine/epr. Med. Phys. 33: (2006). Acoustics and Ionising Radiation, Formulation and Strategy Alan DuSautoy The organisation of the Acoustics and Ionising Radiation Team and how it fits into with government will be described. The objectives and aims set for us by the National Measurement System Policy Unit will be reviewed. I will show 1. How programmes will be formulated in future 2. Introduce the concept of Rolling Formulation and 3. Describe how Acoustics and Ionising Radiation will implement these changes. Lastly I will give the Overview for Acoustics and Ionising Radiation and introduce the Theme Roadmap for radiotherapy. This will form the core of the strategy in this area to be published shortly. New services at NPL to support radiotherapy Vere Smyth With the installation of the new Elekta linac, NPL now has the opportunity to develop new expertise and services to support the introduction and delivery of intensity modulated radiation therapy (IMRT) and image-guided radiation therapy (IGRT). We have just completed a consultation process with radiotherapy departments to find out where the main dosimetry problems are, and how NPL could offer metrology support. It is clear that the present NPL services, which concentrate on dose at a reference point, need to be extended to address the increasingly complex dose distributions delivered to patients. Following the feedback received from departments, a new project is proposed as part of the National Measurement System programme. This will address the topics of dosimetry audit for IMRT, quality assurance and dose verification practice for IMRT, and the possible implementation of the electronic portal imaging device (EPID) for dosimetry. Possible new services following from this will be discussed. Looking further ahead, it seems that medical imaging modalities are becoming more important to the calculation and verification of 3D radiation dose. NPL could potentially take a lead in producing performance and evaluation standards that will be needed to help direct the development and safe clinical introduction of the new technology.

4 6-dimensional radiosurgery - Cyberknife at the Harley Street Clinic Dr Ian Cowley Harley Street Clinic Accuray's Cyberknife is the world's first robotic stereotactic radiosurgery treatment machine, delivering precision-targeted beams of radiation to disease sites throughout the body without the need for invasive frames or uncomfortable immobilisation. A 6MV linac mounted on a 6-jointed robotic arm can target X-ray beams as small as 5 mm diameter at almost any angle into the patient's body, giving unparalleled dose-sculpting opportunities for targets throughout the patient. The heart of the system is the industrial-grade robotic arm. Similar to those used in the Citroen Picasso TV adverts, the arm is designed to provide positional accuracy of the linac of 0.12 mm. The 6 joint allow the floor-mounted robot to reach around the patient and achieve almost any angular position. The radiation is delivered through a series of fixed conical applicators ranging from 5 mm to 60 mm in diameter, or a new IRIS motorised collimator which can simulate each of the 12 fixed cones. The system is able to achieve an end-to-end accuracy of 0.95 mm by utilising image guidance throughout the treatment. Kilovoltage imaging is used to check the patient position at regular intervals and correct beam geometry via the robotic arm. Initial patient alignment is achieved using the same imagers and a 6D robotic couch. The system is even advanced enough to be able to track moving targets in the lungs as the patient breathes. Accuray have installed 140 Cyberknife systems to date, and The Harley Street Clinic is excited to be the first Cyberknife centre in the UK. The centre will treat its first patient in February D dosimetry using the IQSCAN system Julia Pearce As radiotherapy treatments become increasingly complex so the need for 3D dosimeters increases. Recent progression in radiotherapy with treatments such as intensity modulated radiation therapy (IMRT) has placed demands on existing methods of dosimetry. Radiation can be delivered using many beams from different directions and with different shapes to establish a dose distribution that conforms tightly to the planned target volume and that limits radiation to normal tissues and critical organs. Current dosimeters, such as ionisation chambers and films are only 1 or 2 dimensional and so are limited in their ability to integrate dose over a 3 dimensional volume. As a result, dosimeters capable of measuring a dose distribution in 3 dimensions are being developed. 3D dosimeters tend to fall into two categories: gel dosimeters or dyed plastics. Gel dosimeters are based on chemical reactions caused by the action of ionising radiation. The addition of gelling agents such as gelatin can enable these chemical reactions to be stabilised spatially to produce a 3 dimensional dose distribution. Gel dosimeters include Fricke gels and polymer gels. PRESAGE TM is an example of a dyed plastic dosimeter. It is a solid dosimeter, based on transparent polyurethane combined with the dye leucomalachite green.

5 Optical computed tomography evaluation of 3D dosimeters is possible due to the change in optical absorption induced by radiation. The Radiation Dosimetry group at NPL has purchased an IQSCAN readout system to evaluate 3D dosimeters. This is a laser scanning system coupled to a photodetector and enables a 3D reconstruction of an irradiated dosimeter. This presentation will focus on the operation of the IQSCAN 3D dosimetry system, preliminary results and future work. Practical experience of using the IPEM 2003 electron code of practice Henry Lawrence Bristol Oncology Centre From 1 January 2007 the IPEM 2003 code of practice (COP) for electron dosimetry replaces the old air kerma based IPEMB 1996 COP as the recommended protocol for the UK. The IPEM 2003 COP requires a secondary standard ionisation chamber calibrated directly in terms of absorbed dose to water. This talk examines the differences between the new and old COP and indicates that the dose measured using the 2003 COP appears to be greater than that using the 1996 COP by approximately (2±1)%, based on a survey of radiotherapy centres in the UK. The talk also compares the characteristics of the NACP (Scanditronix) and Roos (PTW) chambers recommended by the 2003 COP. This work found that that there is no practical difference between adopting the NACP (Scanditronix) or Roos (PTW) chambers as a secondary standard chamber.