Hamza Benmakhlouf

Output correction factors for nine small field detectors in 6 MV radiation therapy photon beams: A PENELOPE Monte Carlo study

PURPOSE To determine detector-specific output correction factors,[Formula: see text], in 6 MV small photon beams for air and liquid ionization chambers, silicon diodes, and diamond detectors from two manufacturers. METHODS Field output factors, defined according to the international formalism published byAlfonso et al. [Med. Phys. 35, 5179-5186 (2008)], relate the dosimetry of small photon beams to that of the machine-specific reference field; they include a correction to measured ratios of detector readings, conventionally used as output factors in broad beams. Output correction factors were calculated with the PENELOPE Monte Carlo (MC) system with a statistical uncertainty (type-A) of 0.15% or lower. The geometries of the detectors were coded using blueprints provided by the manufacturers, and phase-space files for field sizes between 0.5 × 0.5 cm(2) and 10 × 10 cm(2) from a Varian Clinac iX 6 MV linac used as sources. The output correction factors were determined scoring the absorbed dose within a detector and to a small water volume in the absence of the detector, both at a depth of 10 cm, for each small field and for the reference beam of 10 × 10 cm(2). RESULTS The Monte Carlo calculated output correction factors for the liquid ionization chamber and the diamond detector were within about ± 1% of unity even for the smallest field sizes. Corrections were found to be significant for small air ionization chambers due to their cavity dimensions, as expected. The correction factors for silicon diodes varied with the detector type (shielded or unshielded), confirming the findings by other authors; different corrections for the detectors from the two manufacturers were obtained. The differences in the calculated factors for the various detectors were analyzed thoroughly and whenever possible the results were compared to published data, often calculated for different accelerators and using the EGSnrc MC system. The differences were used to estimate a type-B uncertainty for the correction factors. Together with the type-A uncertainty from the Monte Carlo calculations, an estimation of the combined standard uncertainty was made, assigned to the mean correction factors from various estimates. CONCLUSIONS The present work provides a consistent and specific set of data for the output correction factors of a broad set of detectors in a Varian Clinac iX 6 MV accelerator and contributes to improving the understanding of the physics of small photon beams. The correction factors cannot in general be neglected for any detector and, as expected, their magnitude increases with decreasing field size. Due to the reduced number of clinical accelerator types currently available, it is suggested that detector output correction factors be given specifically for linac models and field sizes, rather than for a beam quality specifier that necessarily varies with the accelerator type and field size due to the different electron spot dimensions and photon collimation systems used by each accelerator model.

On the Monte Carlo simulation of small-field micro-diamond detectors for megavoltage photon dosimetry

Monte Carlo (MC) calculated detector-specific output correction factors for small photon beam dosimetry are commonly used in clinical practice. The technique, with a geometry description based on manufacturer blueprints, offers certain advantages over experimentally determined values but is not free of weaknesses. Independent MC calculations of output correction factors for a PTW-60019 micro-diamond detector were made using the EGSnrc and PENELOPE systems. Compared with published experimental data the MC results showed substantial disagreement for the smallest field size simulated ([Formula: see text] mm). To explain the difference between the two datasets, a detector was imaged with x rays searching for possible anomalies in the detector construction or details not included in the blueprints. A discrepancy between the dimension stated in the blueprints for the active detector area and that estimated from the electrical contact seen in the x-ray image was observed. Calculations were repeated using the estimate of a smaller volume, leading to results in excellent agreement with the experimental data. MC users should become aware of the potential differences between the design blueprints of a detector and its manufacturer production, as they may differ substantially. The constraint is applicable to the simulation of any detector type. Comparison with experimental data should be used to reveal geometrical inconsistencies and details not included in technical drawings, in addition to the well-known QA procedure of detector x-ray imaging.

Spectral distribution of particle fluence in small field detectors and its implication on small field dosimetry

Purpose: Correction factors for the relative dosimetry of narrow megavoltage photon beams have recently been determined in several publications. These corrections are required because of the several small‐field effects generally thought to be caused by the lack of lateral charged particle equilibrium (LCPE) in narrow beams. Correction factors for relative dosimetry are ultimately necessary to account for the fluence perturbation caused by the detector. For most small field detectors the perturbation depends on field size, resulting in large correction factors when the field size is decreased. In this work, electron and photon fluence differential in energy will be calculated within the radiation sensitive volume of a number of small field detectors for 6 MV linear accelerator beams. The calculated electron spectra will be used to determine electron fluence perturbation as a function of field size and its implication on small field dosimetry analyzed. Methods: Fluence spectra were calculated with the user code PenEasy, based on the PENELOPE Monte Carlo system. The detectors simulated were one liquid ionization chamber, two air ionization chambers, one diamond detector, and six silicon diodes, all manufactured either by PTW or IBA. The spectra were calculated for broad (10 cm × 10 cm) and narrow (0.5 cm × 0.5 cm) photon beams in order to investigate the field size influence on the fluence spectra and its resulting perturbation. The photon fluence spectra were used to analyze the impact of absorption and generation of photons. These will have a direct influence on the electrons generated in the detector radiation sensitive volume. The electron fluence spectra were used to quantify the perturbation effects and their relation to output correction factors. Results: The photon fluence spectra obtained for all detectors were similar to the spectrum in water except for the shielded silicon diodes. The photon fluence in the latter group was strongly influenced, mostly in the low‐energy region, by photoabsorption in the high‐Z shielding material. For the ionization chambers and the diamond detector, the electron fluence spectra were found to be similar to that in water, for both field sizes. In contrast, electron spectra in the silicon diodes were much higher than that in water for both field sizes. The estimated perturbations of the fluence spectra for the silicon diodes were 11–21% for the large fields and 14–27% for the small fields. These perturbations are related to the atomic number, density and mean excitation energy (I‐value) of silicon, as well as to the influence of the “extracameral”‘ components surrounding the detector sensitive volume. For most detectors the fluence perturbation was also found to increase when the field size was decreased, in consistency with the increased small‐field effects observed for the smallest field sizes. Conclusions: The present work improves the understanding of small‐field effects by relating output correction factors to spectral fluence perturbations in small field detectors. It is shown that the main reasons for the well‐known small‐field effects in silicon diodes are the high‐Z and density of the “extracameral” detector components and the high I‐value of silicon relative to that of water and diamond. Compared to these parameters, the density and atomic number of the radiation sensitive volume material play a less significant role.

Increased Survival Using Delayed Gamma Knife Radiosurgery for Recurrent High-Grade Glioma: A Feasibility Study

OBJECTIVE The current study retrospectively assessed delayed gamma knife radiosurgery (GKRS) in the management of high-grade glioma recurrences. METHODS A total of 55 consecutive patients with high-grade glioma comprising 68 World Health Organization (WHO) III and WHO IV were treated with GKRS for local recurrences between 2001 and 2007. All patients had undergone microsurgery and radiochemotherapy, considered as standard therapy for high-grade glioma. Complete follow-up was available in all patients; median follow-up was 17.2 months (2.5-114.2 months). Median tumor volume was 5.2 mL, prescription dose was 20 Gy (14-22 Gy), and median max dose was 45 Gy (30-77.3 Gy). RESULTS The patients with WHO III tumors showed a median survival of 49.6 months with and a 2-year survival of 90%. After GKRS of the recurrences, these patients showed a median survival of 24.2 months and a 2-year survival of 50%. The patients with WHO IV tumors had a median survival of 24.5 months with a 2-year survival of 51.4%. After the recurrence was treated with GKRS, the median survival was 11.3 months and a 2-year survival: 22.9% for the WHO IV patients. CONCLUSION The current study shows a survival benefit for high-grade glioma recurrences when GKRS was administered after standard therapy. This is a relevant improvement compared with earlier studies that had had not been able to provide a beneficial effect timing radiosurgery in close vicinity to EBRT.

Implication of using MRI co-registered with CT in Leksell Gamma Knife® dose planning for patients with vestibular schwannoma.

Department of Neurosurgery, Karolinska University Hospital and Department of Clinical Neuroscience, Karolinska Institutet, Stockholm, Sweden Department Neurology, S:t Goran Hospital, Stockholm, Sweden Department of Medical Physics, Karolinska University Hospital, Stockholm, Sweden Department of Neurosurgery, Copenhagen University Hospital Rigshospitalet, Copenhagen, Denmark Department of Neuroradiology, Karolinska University Hospital, Stockholm, Sweden

Monte Carlo investigation of backscatter factors for skin dose determination in interventional neuroradiology procedures

Complex interventional and diagnostic x-ray angiographic (XA) procedures may yield patient skin doses exceeding the threshold for radiation induced skin injuries. Skin dose is conventionally determined by converting the incident air kerma free-in-air into entrance surface air kerma, a process that requires the use of backscatter factors. Subsequently, the entrance surface air kerma is converted into skin kerma using mass energy-absorption coefficient ratios tissue-to-air, which for the photon energies used in XA is identical to the skin dose. The purpose of this work was to investigate how the cranial bone affects backscatter factors for the dosimetry of interventional neuroradiology procedures. The PENELOPE Monte Carlo system was used to calculate backscatter factors at the entrance surface of a spherical and a cubic water phantom that includes a cranial bone layer. The simulations were performed for different clinical x-ray spectra, field sizes, and thicknesses of the bone layer. The results show a reduction of up to 15% when a cranial bone layer is included in the simulations, compared with conventional backscatter factors calculated for a homogeneous water phantom. The reduction increases for thicker bone layers, softer incident beam qualities, and larger field sizes, indicating that, due to the increased photoelectric crosssection of cranial bone compared to water, the bone layer acts primarily as an absorber of low-energy photons. For neurointerventional radiology procedures, backscatter factors calculated at the entrance surface of a water phantom containing a cranial bone layer increase the accuracy of the skin dose determination.

The role of radiosurgery in the acute management of fourth ventricle compression due to brain metastases

Background: Approximately 20–30% of all intracranial metastases are located in the posterior fossa. The clinical evolution hinges on factors such as tumor growth dynamics, local topographic conditions, performance status, and prompt intervention. Fourth ventricle (V4) compression with secondary life-threatening obstructive hydrocephalus remains a major concern, often requiring acute surgical intervention. We have previously reported on the application of adaptive hypofractionated Gamma Knife Radiosurgery in the acute management of critically located metastases, a technique known to us as rapid rescue radiosurgery (3R). We report the results of 3R in the management of posterior fossa lesions and ensuing V4 decompression. Case Descriptions: Four patients with V4 compression due to posterior fossa metastases were treated with 3R by three separate gamma knife radiosurgical sessions (GKRS) over a period of seven days. Mean V4 volume was 1.02 cm3 at GKRS 1, 1.13 cm3 at GKRS 2, and 1.12 cm3 at GKRS 3. Mean tumor volume during the week of treatment was 10 cm3 at both GKRS 1 and 2 and 9 cm3 at GKRS 3. On average, we achieved a tumor volume reduction of 52% and a V4 size increase of 64% at the first follow-up (4 weeks after GKRS 3). Long-term follow-up showed continued local tumor control, stable V4 volume, and absence of hydrocephalus. Conclusion: For this series, 3R was effective in terms of rapid tumor ablation, V4 decompression, and limited radiation-induced toxicity. This surgical procedure may become an additional tool in the management of intractable posterior fossa metastasis with V4 compression.

Gamma knife radiosurgery in the management of endolymphatic sac tumors

Background: Although widely regarded as rare epithelial tumors with a low grade of malignancy, endolymphatic sac tumors (ELST) often lead to disabling petrous bone destruction and significantly impairing symptoms at the time of primary diagnosis and/or recurrence. ELST is not uncommon in von Hippel Lindau (VHL) patients. Although open surgery is regarded as the best treatment option, recurrence remains a challenge, particularly when gross tumor resection (GTR) is deemed unachievable due to topographic conditions. Tumor recurrence successfully treated with fractionated radiotherapy and radiosurgery have been reported in selected cases. We present the case of a patient with recurrent ELST treated with salvage gamma knife radiosurgery (GKRS) adding a review of current literature. Case Description: A 65-year-old patient underwent GKRS of an unresectable, recurrent ELST. Tumor volumetric analysis showed almost 15% increase in tumor volume in the 4 months between the pre-GKRS magnetic resonance imaging (MRI) and the stereotactic MRI (s-MRI) at treatment. Follow-up MRI at 12 and 20 months showed significant decrease in local tumor volume, decreased contrast enhancement and no perifocal edema. The patients general and neurological status remains stable to the present day. Conclusion: In the present case, GKRS was effective in the management of a recurrent ELST over the course of 20 months. Because of ELSTs recurrence potential, long-term follow up is required. The present case as well as previous reports might suggest a possible salvage/adjunctive role of radiosurgery in the management of ELST. Further studies are deemed necessary.

Adaptive hypofractionated gamma knife radiosurgery in the acute management of large thymic carcinoma brain metastases

Background: Brain metastases often lead to serious neurological impairment and life threatening states. Their acute management remains complex, particularly in the case of rare malignancies with aggressive evolution. In large single lesions, open surgery followed by radiation to the surgical cavity is widely regarded as the best approach; yet in many cases, microsurgery is not feasible due to the lesions critical location and/or the number of brain metastases present. We report the effects of adaptive hypofractionated gamma knife radiosurgery in the acute management of critically located thymic carcinoma metastases. Case Description: A 50-year-old male with metastatic thymic carcinoma was treated with radiosurgery for two large supratentorial brain metastases (M3 and M4) adjacent to eloquent areas and one smaller cerebellar metastasis (M2). M3 and M4 were treated with adaptive hypofractionated gamma knife radiosurgery, showing a dramatic volume reduction 4 weeks after treatment completion without radiation-induced side effects. Thirteen months later, two new small, threatening supratentorial lesions (M5-M6) were treated with the same technique. Interestingly, M2 (treated with standard single fraction) and M5-M6 developed local adverse radiation events. The patients general and neurological status remained next to normal by the time of paper submission. Conclusion: The application of adaptive hypofractionated radiosurgery in this acute setting proved effective in terms of rapid tumor ablation, with salvage of neurological functionality and limited toxicity. We have called the overall procedure rapid rescue radiosurgery (RRR). A systematic study of past and ongoing RRR-treatments is warranted and in progress.

TH-AB-201-09.

PURPOSE To determine the k_(Q_msr,Q)^(f_msr,f_ref) factor, introduced in the small field formalism for five common type chambers used in the calibration of Leksell Gamma-Knife Perfexion model over a range of different phantom electron densities. METHODS Five chamber types including Exradin-A16, A14SL, A14, A1SL and IBA-CC04 are modeled in EGSnrc and PENELOPE Monte Carlo codes using the blueprints provided by the manufacturers. The chambers are placed in a previously proposed water-filled phantom and four 16-cm diameter spherical phantoms made of liquid water, Solid Water, ABS and polystyrene. Dose to the cavity of the chambers and a small water volume are calculated using EGSnrc/PENELOPE codes. The calculations are performed over a range of phantom electron densities for two chamber orientations. Using the calculated dose-ratio in reference and machine specific reference field, the k_(Q_msr,Q)^(f_msr,f_ref) factor can be determined. RESULTS When chambers are placed along the symmetry axis of the collimator block (z-axis), the CC04 requires the smallest correction followed by A1SL and A16. However, when detectors are placed perpendicular to z-axis, A14SL needs the smallest and A16 the largest correction. Moreover, an increase in the phantom electron density results in a linear increase in the k_(Q_msr,Q)^(f_msr,f_ref). Depending on the chambers, the agreement between this study and a previous study performed varies between 0.05-0.70% for liquid water, 0.07-0.85% for Solid Water and 0.00-0.60% for ABS phantoms. After applying the EGSnrc-calculated k_(Q_msr,Q)^(f_msr,f_ref) factors for A16 to the previously measured dose-rates in liquid water, Solid Water and ABS normalized to the dose-rate measured with TG-21 protocol and ABS phantom, the dose-rate ratios are found to be 1.004±0.002, 0.996±0.002 and 0.998±0.002 (3σ) respectively. CONCLUSION Knowing the electron density of the phantoms, the calculated k_(Q_msr,Q)^(f_msr,f_ref) values in this work will enable users to apply the appropriate correction for their own specific phantom material. LM acknowledges partial support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council (Grant number: 432290).