M Carlone

Twenty-year review of radiotherapy for vaginal cancer: An institutional experience

OBJECTIVEnTo evaluate clinical outcome, prognostic factors and chronic morbidity with radiotherapy for vaginal cancer treatment.nnnMATERIALS AND METHODSn68 patients with vaginal cancer treated by radical or adjuvant radiotherapy (RT) were selected. Five with rare subtypes of histopathology and 8 with adenocarcinoma were excluded from this study. 76.4% of the remainder had early-stage diseases (stage I: 14, II: 28, III: 9, and IV: 4). The patients in the years from which they were treated were almost evenly distributed (1st 5 years: 13, 2nd: 14, 3rd: 16, and 4th: 12). There were four treatment groups: external beam radiotherapy (EBRT) alone (n=18), brachytherapy (BT) alone (n=4), EBRT and BT (n=30), and surgery plus RT (n=3).nnnRESULTSnMedian follow-up was 50.3 months ranging from 3 to 213 months. 5-year overall survival (OS) was 55.6%, disease-specific survival (DSS) was 77.3%, disease-free survival was 74.2%, and local control was 87.7%. Independent prognostic factors for DSS and OS were tumor stage, site and size (p<0.05). Late radiation toxicity was minimal in the bladder (4.6%) and bowel (4.6%). Vaginal morbidity was observed in 35 patients (63.6%). It was lowest in the BT alone (0%), and highest in the EBRT and BT group (82.1%), especially for those received more than 70 Gy (p=0.05, Odds ratio=4.64, 95% confidence interval: 1.01-21.65).nnnCONCLUSIONnThis retrospective review suggested that tumor stage, site, and size were important prognostic factors in patients with vaginal cancer. Higher radiation dose was associated with more frequent vaginal toxicity.

TH‐C‐M100J‐01: Magnetic Shielding of a Coupled MRI‐Linac System

Purpose: To investigate the optimum requirements of magnetically shielding a Linac from an MRI coupled to it for the purpose of real‐time image‐guided adaptive radiotherapy. In addition, to investigate the effects on the MRIsmagnetic field homogeneity due to the presence of the required shielding. Method and Materials:Finite element method was used (COMSOL MultiPhysics) to model a Linac coupled to a 0.2 T bi‐planar MRI system, and to calculate the magnetic fields in its vicinity. Associated shielding was optimized by varying sheets of shielding material (eg, iron, Mu Metal) to keep the MRIs magnetic fringe fields within the waveguide to less than 0.5 Gauss. Results: We found that to reduce the MRIs magnetic fringe fields within the waveguide from 20 Gauss (unshielded waveguide) to the specific requirement of less than 0.5 Gauss, we require a 5 cm thick iron plate between the Linac and the MRI, and a 1 mm Mu Metal wrapping around the waveguide. An additional 5 cm plate is placed on the opposing side to improve magnetic field symmetry. The magnets field inhomogenieties resulting from the shielding were calculated to be less than 142 ppm, and thus easily “shimmable”. Conclusion:Magnetic shielding is needed for operating a Linac coupled to a 0.2T MRI — system. We have optimized the shielding required to reduce the MRIs magnetic fringe fields within the waveguide to less than 0.5 Gauss by using simple and realizable passive shielding. Field inhomogeneities due to the presence of our shielding are sufficiently small that they can be easily corrected though conventional shimming techniques.

SU‐GG‐T‐502: A Fast Algorithm to Predict Dose Distribution Perturbations in An MRI‐Linac Hybrid System

Purpose: The hybrid MRI‐linac unit under construction at our institute consists of a unique rotating bi‐planar (RBP) geometry where a linac is coupled to the open end of a 0.2 T permanent magnet and the entire structure rotates about the patient as a unit. Monte Carlo(MC) simulations that incorporate the influence of the magnetic field in this geometry have demonstrated small perturbations to patient specific dose distributions. This work takes advantage of the constant direction of the net Lorentz force in the RBP geometry to derive an electron density‐scaled dose shifting algorithm that will quickly and accurately incorporate magnetic fieldeffects into the treatment planning process. Method and Materials:MC simulations of the projection of a 6MV photon beam onto a slab phantom with a central slab of variable electron density were performed using DOSXYZnrc with macros invoked to account for the changes in electron velocity due to the Lorentz force for each transport step. Dose profiles at 0.2 T were shifted in the direction of the Lorentz force until a minimum difference with 0.0 T profiles was observed. From this we derived an electron density scaled‐shift function. This function was applied to a four field brain plan calculated by the treatment planning system and the results were compared to full MC simulations.Results: The shift correction reasonably reproduced magnetic fieldeffects on dose profiles for relative electron densities down to 0.3. The shift correction accurately accounted for dose perturbations within the patients body for the brain plan. Conclusion:Magnetic field induced dose perturbations for the MRI‐linac system at our centre result in a shift of dose in the net Lorentz force direction, which can be quickly accounted for using a shift of the dose distribution based on the relative electron density of the local medium.

SU‐GG‐T‐186: Simulation of Room Distortion On Measurements of RF Noise Due to a Medical Linac

Purpose: The goal of radiotherapy is to sculpt radiationdose to tumor shape. The integration of an MRI with a linac for real time image‐guidedradiotherapy(IGRT) would allow clinicians to reduce treatment margins beyond current technological limits. A problem with the integration of an MRI and a linac is the radio frequency (RF) noise produced by a linac. Our measurements indicate this noise exists in the MHz range with wavelengths on the order of 10s of meters. The purpose of this project is to investigate the effects an enclosed room has on the electric (E) and magnetic (H) field measurements on this type of RF source. Purpose: A software programming environment (MultiStripes, version 7.5 Flomerics) has been utilized to investigate the E and H fields produced from a small dipole in free space and an enclosed room, by solving Maxwells equations using the transmission line matrix method. The results from these simulations elucidate the effects of room structure and contents on the radiation pattern of an RF source. Results: Our free space simulation demonstrates the theoretically expected falloff, of the E and H fields, as a function of distance from the dipole. When modeling the dipole confined in the laboratory, our results show perturbations from the free space solution. Our measurements also contain these perturbations. These results were found to depend strongly on the material properties used. Conclusion: Measurement of RF noise for MRI‐linac integration is dependant on the configuration of the laboratory. Simulation of electromagnetic wave propagation can determine the wave perturbation due to room effects. Such simulations allow determination of a “room factor” which can be applied to laboratory measurements to determine the true strength of the RF emissions. More work is needed to validate the model.

SU‐GG‐T‐180: Measurement Instrumentation to Determine RF Noise Generated by a Medical Linac

Purpose: The goal of image‐guidedradiotherapy is to deliver the planned conformal radiationdose precisely to the target tumour volume and minimize dose to nearby normal tissue. To further this goal, integration of a linear accelerator(LINAC) with an MRI has been proposed. An obstacle to the integration of a LINAC with an MRI is the RF interference between the two devices. Our measurements indicate that LINAC induced RF occurs in the MHz range, with wavelengths on the order of 10s of meters, necessitating the RF measurements to be made in the near field. The purpose of this work is to develop and validate a method of measuring RF field patterns using standard dipole antennas both in near and far fields. Method and Materials: To validate the measurement technique, near and far field electric (E) and magnetic (H) field patterns were measured using specialized E and H field probes from two dipole radiation sources, and compared to the theoretical values. Antennas and probes were mounted in controlled geometry using a specially made wooden stand. The wave impedance (E/H) was also calculated and compared to theory. Results: For E and H fields, angular and radial field strength patterns demonstrate compliance with theory in both the near and far fields. The E field measurements oscillate around the theoretical falloff as a function radial distance, these oscillations are predominantly a result of reflections from the room structures. Our room simulations are consistent with this interpretation. Wave impedance in the near field agrees with theoretical predictions. Conclusion: A technique to measure the RF field strength in the near and far fields has been demonstrated. This technique is suitable for measuring RF field values in the vicinity of a medicalLINAC enabling quantification of RF interference between a LINAC and MRI.

SU‐GG‐J‐161: Radiofrequency Interference Between a Linac and MRI

Purpose: Modern radiotherapy relies heavily on 3‐D image based guidance to reduce treatment margins and to conform the dose distribution to the target shape. Due to the poor soft‐tissue contrast and non‐real‐time nature of x‐ray and ultrasound based image guidance, several groups are integrating an MRI and linear accelerator. One of several unresolved technical issues is RF interference between the two devices. The purpose of this presentation is to show that LINACRF correlates with the operation of certain components of its pulsed power modulator (PPM). Method and Materials: Electric (E) and magnetic (H) field temporal patterns (RF signals) were measured using separate E‐field and H‐field probes on three medical LINAC configurations: klystron and PPM in the same room; klystron and PPM in different rooms; and magnetron and PPM in the same room. Time resolved high voltage power supply (HVPS) current, pulse forming network (PFN) voltage,klystron current and klystronvoltage, and magnetron current were all measured coinciding with the RF signals. All measured signals were compared in time and frequency domains. Results: For a klystron powered LINAC, correlation between the HVPS current, and E and H field signals were observed in both the time and frequency domains. No correlation was observed between RF signals and the klystron current pulse. For a magnetron powered LINAC, correlation among HVPS current, magnetron current and RF signals was observed. RF signals coincided in time with the magnetron current, but the frequency spectrum were dissimilar. Conclusion: The results suggest that charging of the PFN by the HVPS current is a mechanism of RF noise generation in the klystron based LINAC operation. For magnetron based PPMs the results further suggest the magnetron current pulse generates RF noise, however dissimilar frequency spectrum indicates an indirect and more complicateds mechanism requiring further investigation.

TH‐C‐L100J‐02: Study of RF Interference Between a Linear Accelerator and MRI

Purpose: To study methods to eliminate RF interference between a linear accelerator and MRI to facilitate the integration of a linac and MRI for real time image guidance. Methods and Materials:Linear accelerators function in a pulsed power mode by generating high current and voltage pulses with fast rise times by charging and discharging a PFN. Most of the power in the discharged pulse is contained in the DC component of the pulse, and is absorbed by the klystron/magnetron due to impedance matching at DC. However, the higher order frequencies in the pulse are not be transmitted to the load since the impedance match is only optimized at DC. These modes can resonate on the transmission line between PFN and load and transmit RF in the MHz range. This could interfere with the RF receiver of an MRIimager. Thus, it should be possible to control the frequency of RF pulsing noise by choosing an electrical delay line between the PFN and klystron/magnetron, which avoids the operating frequency of the MRI.Results: RF noises due to modulated current pulses were measured on a Varian 2100 EX linac, and suggest the resonance mechanism is correct. To further test this hypothesis, an experimental linear accelerator system is being re‐configured. RF power measurements in the range of the operating frequency of a 0.2 T MRI are difficult due to long wavelengths (10s of m). Measurement antennas are being developed using finite element methods. Early simulations results of a dipole antenna are presented. Conclusions: Initial measurements indicate that RF pulsing noise from a medical linac exhibits resonance like properties. This implies that the RF interference between a linac and MRI can be eliminated by a simple reconfiguration of the linac modulator, which would greatly simplify the integration of a linac and MRI.

SU‐FF‐T‐65: An Analytic Investigation of the Effect of Inter‐Patient Heterogeneity On Alpha/beta Ratio Estimates for Tumors

Purpose: To analytically determine the relationship between the α/β ratio that would be obtained by fitting the individual (non‐averaged) tumor control probability (TCP) model to clinical data and the α/β estimate from a fit of the population‐averaged TCP model to the same clinical dose‐response dataset. Method and Materials: Recently, Carlone et al. (Med. Phys., 2006. 33(6): p. 1634–42) published fundamental forms of the population TCP model for the limits of dominant heterogeneity in radiosensitivity, and in clonogen number. In each case, the model is parameterized by γ50 and D 50. The individual Poisson TCP model has also been expressed in terms of these geometric parameters. Since the functional forms of these TCP models are similar, approximately the same γ50 and D 50 values would be obtained for each model if they were fit to the same clinical dataset. This fact allows us to determine mapping relationships between parameter ratio estimates obtained from fits using the averaged or non‐averaged TCP model. Mapping relationships are determined for the case of dominant heterogeneity in clonogen number, and in radiosensitivity. Results: When heterogeneity in clonogen number dominates a clinical dataset, the individual and population‐averaged α/β estimates are virtually identical. However, for the case where heterogeneity in radiosensitivity dominates, the individual α/β estimate will not be the same as the corresponding population α/β estimate. Conclusion: Heterogeneity in radiosensitivity is believed to be the dominant form among clinical datasets. Hence, our analytic expressions suggest the individual α/β ratio estimate should be different from the population estimate. Because of this ambiguity, we suggest that modelling has limited value in α/β determination; only the clinical hypofractionation trials will have the ability to validate the hypothesis originally put forth by Brenner and Hall (IJROBP, 1999. 43(5): p. 1095–1101) that prostate cancer responds to fractionation as does a late responding tissue.