M Evans

SU‐FF‐T‐387: Rotational Total Skin Electron Irradiation Using a Commercially Available Linear Accelerator with a High Dose Rate Total Body Electron Mode

Purpose: During the past 25 years, 185 patients have been treated with a rotational total skin electron irradiation (RTSEI) technique in our center. To modernize the technique we recently transferred it from a linac (Varian Clinac‐18) with a custom modified beam line to a linac (Varian 21EX) with a commercially available electron mode intended for total skin electron irradiation. Methods and Materials: The new technique uses a “high dose rate” mode and a “high dose per Monitor Unit” mode in conjunction with a custom‐made flattening filter to produce a uniform beam at an extended SSD of 378 cm. The accessory tray holds the custom‐made flattening filter and automatically selects the beam energy (6 MeV) and high dose rate (888 MU/min) while moving the collimators to the maximum 40 × 40 cm2field size. Beam parameters are monitored using the record‐and‐verify (VARIS) system. Results: Reference dosimetry for the stationary and rotational electron fields was performed to allow delivery of the prescription dose using the linacs transmission ionization chamber. Patients are treated on a rotating platform with a high dose rate rotational electron beam having a z max at the skin surface, an R 50 at 15 mm and a bremsstrahlung contamination of the order of 3%. The nominal dose rate to water at z max(surface) for the rotational technique was determined to be 24.1 cGy/1000 MU, and beam delivery is monitored with a secondary Farmer‐type ionization chamber located near the patient in the treatment field. Conclusions:Treatment times with the rotational total skin electron irradiation technique at an SSD of 378 cm for a daily dose of 2.0 Gy are of the order of 9.5 minutes and to date we have treated 15 patients with this technique.

Sci‐Sat AM (2) Therapy‐01: A rotational total skin electron irradiation technique using a linear accelerator with a commercially available high dose rate total body electron mode

We have transferred and modernized our 25‐year‐old rotational total skin electron irradiation (RTSEI) technique from a linac (Varian Clinac‐18) with a custom modified beam line to a linac (Varian 21EX) with a commercially available electron mode intended for total skin electron irradiation. The new technique uses a “high dose rate” mode and a “high dose per Monitor Unit” mode in conjunction with a custom‐made flattening filter to produce a uniform beam at an extended SSD of 378 cm. Reference dosimetry for the stationary and rotational electron fields was performed to allow delivery of the prescription dose using the linacs transmission ionization chamber. The accessory tray holds the custom‐made flattening filter and automatically selects the beam energy (6 MeV) and high dose rate (888 MU/min) while moving the collimators to the maximum 40 × 40 cm2field size. Beam parameters are monitored using the record‐and‐verify (VARIS) system. Patients are treated on a rotating platform with a high dose rate electron beam having a z max at the skin surface, an R 50 at 15 mm and a bremsstrahlung contamination of the order of 3%. The nominal dose rate to water at z max(surface) for the rotational technique was determined to be 24.1 cGy/1000 MU, and beam delivery is monitored with a secondary Farmer‐type ionization chamber located near the patient in the treatment field. Treatment times for a daily dose of 2.0 Gy are of the order of 9.5 minutes and to date we have treated 15 patients with this technique.

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.

Poster — Thur Eve — 45: Commissioning of the Varian ECLIPSE eMC algorithm for clinical electron treatment planning

Fast electron Monte Carlo systems have been developed commercially, and implemented for clinical practice in radiation therapy clinics. In this work the Varian eMC (electron Monte Carlo) algorithm was commissioned for clinical electron beams of energies between 6 MeV and 20 MeV. Beam outputs, PDDs and profiles were measured for 29 regular and irregular cutouts using the IC-10 (Wellhöfer) ionization chamber. Detailed percentage depth dose comparisons showed that the agreement between measurement and eMC for different characteristic points on the PDD are generally less than 1 mm and always less than 2 mm, with the eMC calculated values being lower than the measured values. Of the 145 measured output factors, 19 cases fail a ±2% agreement but only 8 cases fail a ±3% agreement between calculation and measurement. Comparison of central axis dose distributions for two electron energies (9, and 20 MeV) for a 10 × 10 cm2 field, centrally shielded with Pb of width 0 cm (open), 1, 2 and 3 cm, shows agreement to within 3% except near the surface. Comparison of central axis dose distributions for 9 MeV in heterogeneous phantoms including bone and lung inserts showed agreement of 1 mm and 3 mm respectively with measured TLD data. The overall agreement between measurement and eMC calculation has enabled us to begin implementing this calculation model for clinical use.

SU‐E‐T‐57: Comprehensive Web‐Based QA in Radiation Oncology

Purpose: To describe a framework for comprehensive web‐based QA in medical physics and radiation oncology through centralized access to multiple databases. We believe that this form of centralized QA represents the future direction for data analysis and visualization in increasingly data‐driven radiation oncology and medical physics practises. Methods and Materials: Our paperless radiation oncology clinic, equipped with a centralized webserver running PERL scripts for access to the Varian ARIA record‐and‐verify database and a custom MySQL database for medical physics QA data, adverse event reports and in‐house clinical trial data. Results: Our planned framework for comprehensive electronic QA is outlined and some early results showing the power of the system for analysingtreatment times and for statistical process control are presented. Conclusions: Comprehensive web‐based QA represents the future direction for data archiving and visualization in the medical physics and radiation oncology community. We present our framework for web‐based QA and demonstrate its power with some early results.

Poster — Thur Eve — 41: Determination of Realistic Workload and Use Factors Using the Varian ARIA Database

Shielding design calculations used in megavoltage radiation therapy essentially reduce to determination of barrier thicknesses. A shielding barrier serves to attenuate the radiationdose from a high level produced inside a therapy room to a lower “design goal” level at a point‐of‐interest outside. The calculation must account for both radiation physics (barrier attenuation) and for the various clinic‐specific parameters that serve to modulate the design goal and the dose that impinges on the barrier. While the physical data relating to attenuation of megavoltage beams in concrete and other barrier materials is readily available (eg in NCRP Report 151 or via the NIST website), clinic‐specific parameters must be determined by the clinical physicist. Such parameters include the workload W of the treatment machine, the use factor U of the barrier in question and the occupancy T at the point‐of‐interest. In this submission we show how data archived in a clinics record‐and‐verify system can be used to compile realistic workload and use factors.

SU‐FF‐T‐627: Off‐Axis Correction Factors for the Enhanced Dynamic Wedge

Purpose: To quantify the dependence of the dosimetric wedge factor for the Varian™ Enhanced Dynamic Wedge (EDW) upon the field size, angle, energy and distance from the machine isocenter. Method and Materials: Point dose measurements in solid water and a multi‐detector ion chamber array measurements in water were used to measure the x‐ray energy deposition from two Varian™ medical accelerators over a wide range of field sizes and distances off the machine isocenter. Measurements at two energies were compared to predictions from two treatment planning programs. Results: The variance between the dose predicted by the treatment planning software and that measured for EDW rectangular fields was found to be a smooth function of the distance from the center of the wedged axis. The amplitude of the variance increases with the EDW angle and decreases with increasing energy. The variance was not observed to have any dependence on the distance along the non‐wedged axis. A correction factor algorithm was developed which reduced the variance to <1% in all cases. Conclusion: The EDW factor is nominally dependent upon the position of the stationary jaw. This work demonstrates an additional correlation between the dose variance and the beam energy, EDW angle, and length of the field along the wedged field axis. The variance increases with increasing EDW angle and decreases with increasing beam energy, becoming as large as ∼6% in extreme cases. The width of the field in the unwedged dimension did not have an effect on the variance. A correction factors procedure was able to limit the variance to <1% under all circumstances. For fields centered on the central axis or with one edge on the central axis the variance was already <1% making any additional correction clinically unnecessary.

SU‐GG‐J‐01: 3D Image Guided Brachytherapy

Purpose: Concept of image guided radiotherapy(IGRT) is on the forefront of current efforts to improve tumorcontrol by increasing dose to target volume and decreasing the dose to the nearby critical structures. Most IGRT methods handle the inter‐fractional target position uncertainties. However, intra‐fractional motion is either incorporated into the remaining PTV margin, or subsequently incorporated into forthcoming treatments based on inter‐treatment Cone Beam CT acquired images.Brachytherapy, by its nature, does not suffer from the intra‐treatment target motion uncertainties. Adding the image guidance to improve the inter‐treatment target uncertainties makes the Image Guided Brachytherapy (IGBT) one of the most sophisticated image guided radiation therapy modalities. The incorporation of a Cone Beam Simulator CT (CBSCT) in the brachytherapy suite opens the door to a step forward, toward a 3D IGBT. Method and Materials: We describe a 3D image guidance method using a CBSCT for fractionated HDR brachytherapy, based on 3D CT based treatment planning.Results: Before every treatment a Cone Beam CT(CBCT)image set is acquired with the applicator and/or catheters in its place and patient in treatment position. Planning CTimages and CBCTimages are imported into appropriate treatment planning software and co‐registration of the two image sets is performed based on previously inserted radio‐opaque clips and bony and reliable soft‐tissue landmarks. Conclusion: The full three‐dimensional patient anatomy data with respect to brachytherapy catheters prior to treatment provides additional information to further decrease uncertainty in daily target position with respect to delivered dose distribution during HDR brachytherapy treatments. Cone Beam Simulator CT may provide the ability to move from 2D to 3D Image Guided Brachytherapy.

SU‐FF‐T‐304: Characteristics of Neutron Equivalent Dose Around Medical Linear Accelerators

Purpose: To investigate the characteristics of neutron equivalent dose (NED) around medical linear accelerators (linac). Method and Materials: Two types of bubble detectors (BD-100R for fast neutrons and BDT for thermal neutrons) were used to measure the neutron equivalent dose. Most experiments were carried out using a Varian Clinac 2300C/D linac, 18 MV photon beam at two points of measurement: machine isocenter (point I) and on the isocenter axis 1 m off the isocenter (point C). Results: The NED at point I is 12.7 mSvn/100 MU (10% is from thermal neutron) and this dose decreases to 1.7 mSvn/100 MU (13%) at point C. The NED at point I increases with the increasing field size, whereas the NED at point C exhibits the maximum value for a 10×10 cm2 field. The use of a multileaf collimator (MLC) increases the NED at point C but does not show any significant effects for the NED at point I. In order to facilitate conversion of NED in air to NED in tissue so that the patient photoneutron dose can be estimated, a new quantity NTAR (neutron tissue-air ratio) is introduced and measured in this work. As well, a NED depth dose curve is determined. Inverse square law can not be applied to the NED results measured at different positions along the central beam axis. Conclusion: Photoneutrons produced by a high energy photon beam delivers to the radiotherapy patient a equivalent dose of about 1% of the treatment dose inside the treatment field and 0.1∼0.3% outside of the treatment field. The dose inside the field increases with the increasing field size, while the dose outside the field decreases with the increasing field size. NTAR provides an easy method for the conversion of NED in air to NED in tissue.