[Indications and limits of ablative therapies in prostate cancer]. / Indications et limites actuelles des traitements ablatifs dans le cancer de la prostate.

OBJECTIVE: To perform a state of the art about indications and limits of ablative therapies for localized prostate cancer. METHODS: A review of the scientific literature was performed in Medline database (http://www.ncbi.nlm.nih.gov) and Embase (http://www.embase.com) using different associations of keywords. Publications obtained were selected based on methodology, language and relevance. After selection, 107 articles were analysed. RESULTS: The objective to combine reduction of side effects and oncological control has induced recent development of several ablative therapies. Beyond this heterogeneity, some preferential indications appear: unilateral cancer of low risk (but with significant volume, excluding active surveillance) or intermediate risk (excluding majority of grade 4); treatment targeted the index lesion, by quarter or hemi-ablation, based on biopsy and mpMRI. In addition, indications must considered specific limits of each energy, such as gland volume and tumor localization. CONCLUSION: Based on new imaging and biopsy, ablative therapies will probably increased its role in the future in management of localize prostate cancer. The multiple ongoing trials will certainly be helpful to better define their indications and limits.

Hematopoiesis in the equine fetal liver suggests immune preparedness.

We investigated how the equine fetus prepares its pre-immune humoral repertoire for an imminent exposure to pathogens in the neonatal period, particularly how the primary hematopoietic organs are equipped to support B cell hematopoiesis and immunoglobulin (Ig) diversity. We demonstrated that the liver and the bone marrow at approximately 100 days of gestation (DG) are active sites of hematopoiesis based on the expression of signature messenger RNA (mRNA) (c-KIT, CD34, IL7R, CXCL12, IRF8, PU.1, PAX5, NOTCH1, GATA1, CEBPA) and protein markers (CD34, CD19, IgM, CD3, CD4, CD5, CD8, CD11b, CD172A) of hematopoietic development and leukocyte differentiation molecules, respectively. To verify Ig diversity achieved during the production of B cells, V(D)J segments were sequenced in primary lymphoid organs of the equine fetus and adult horse, revealing that similar heavy chain VDJ segments and CDR3 lengths were most frequently used independent of life stage. In contrast, different lambda light chain segments were predominant in equine fetal compared to adult stage, and surprisingly, the fetus had less restricted use of variable gene segments to construct the lambda chain. Fetal Igs also contained elements of sequence diversity, albeit to a smaller degree than that of the adult horse. Our data suggest that the B cells produced in the liver and bone marrow of the equine fetus generate a wide repertoire of pre-immune Igs for protection, and the more diverse use of different lambda variable gene segments in fetal life may provide the neonate an opportunity to respond to a wider range of antigens at birth.

Sci-Thur PM: YIS - 04: Forcing lateral electron disequilibrium to spare lung tissue: A novel technique for SBRT of small lung tumours.

Stereotactic body radiation therapy(SBRT), a technique that uses tightly conformed Megavoltage(MV) x-ray fields, improves local control of lung cancer. However, small MV x-ray fields can cause lateral electron disequilibrium(LED), which reduces the dose within lung. These effects are difficult to predict and are presently a cause of alarm for the radiotherapy community. Previously, we developed The Relative Depth Dose Factor(RDDF), which is an indicator of the extent of LED (RDDF < 1). We propose a positive application of LED for lung sparing in SBRT: LED can be exploited to irradiate a small tumor while greatly reducing the dose in surrounding lung tissue. The Monte Carlo code, DOSXYZnrc, was employed to calculate dose within a cylindrical lung phantom. The phantom's diameter and height were set to 25 cm, and consisted of water and lung (density = 0.25g/cm3 ) shells surrounding a small water tumor (volume = 0.8 cm3 ). Two 180° 6MV arcs were focused onto the tumor with field sizes of 1×1cm2 (RDDF∼0.5) and 3×3cm2 (RDDF∼1). Analyzing dose results, the 1×1cm2 arc reduced dose within lung and water tissues by 70% and 80% compared to the 3×3cm2 arc. Although, central tumor dose was also reduced by 15% using the 1×1cm2 arc, these reductions can be offset by escalating the prescription dose appropriately. Using the RDDF as a guideline, it's possible to design a SBRT treatment plan that reduces lung dose while maintaining relatively high tumor dose levels. Clinical application requires an accurate dose algorithm and may lower SBRT dose-induced toxicity levels in patients.

Sci-Fri PM: Delivery - 02: CT image guidance strategies for dose-adaptive IMRT of the prostate.

On-line CT imaging in the radiotherapy room has become the norm for targeted intensity-modulated radiotherapy (IMRT), enabling precise adjustments of the daily patient setup based on soft tissue visualization. Corrections for plasticity of the anatomy and dose deformation are within technological reach but will require more on-line resources. We have developed a computer model that allows exploration of "what if" scenarios for assessing the benefits of Image Guidance strategies in terms of the multi-fraction dose distribution and DVH metrics (Target D95 and rectum V70). In this work we report on changes in anatomy and resultant dose distribution as observed in 35 daily megavoltage CT (MVCT) scans of the pelvis during prostate therapy for 13 patients. Our goal is to assess the effectiveness and efficiency of various adaptive strategies involving imaging schedule with and without dose re-planning of 5-field IMRT with 18 MV x-rays. Our research questions are: To what extent do radiation dose distributions delivered to individual patients (in vivo) diverge from the planned dose distributions (in silico)? Is there a robust schedule of CT image guidance, with or without dose re-planning that will mitigate discrepancies? For prostate IMRT, we conclude that image guidance schedule can be relaxed when generous GTV margins (10/7mm) are used. Tighter margins (isotropic 5 mm) reduce the dose to the rectum as expected. However, daily re-planning may be required to maintain adequate target coverage as planned when tighter margins are used.

Sci-Fri PM: Delivery - 11: Accuracy considerations in modern radiation oncology: An update.

The most recent reviews of accuracy requirements in radiation oncology were published in the 1990s, primarily in an era that was transitioning from 2-D to 3-D conformal radiation therapy (CRT). Since then, the technology associated with radiation oncology has changed dramatically. The combination of various forms of imaging for radiation therapy planning, treatment planning software, dose delivery technology including 4-D considerations as well as in-room daily image guidance has resulted in new perspectives on accuracy considerations. The underlying hypothesis for the use of these advanced technologies is that loco-regional control of cancer remains a significant barrier to cancer cure for many common cancers and that better dose distributions will translate into better outcomes. However, further clinical gain using these new technologies may be limited by single or compounded uncertainties associated with the entire treatment process. Thus, it is important to understand what factors should be considered in determining accuracy requirements as well as the realistic expectations of uncertainties that exist within the total treatment process. The need for accuracy is based on clinical requirements such as the steepness of dose-response curves, inherent heterogeneity in patient response to treatment, and the level of accuracy that is practically achievable. Statements on accuracy are dependent on the technology used and the reality of what is practically achievable and necessary. This review highlights some of the major differences between accuracy requirements as determined in the 2-D RT and 3-D CRT era versus the modern era of intensity modulated, image-guided, 4-D radiation therapy.

Evaluation of inter-fraction prostate motion using kilovoltage cone beam computed tomography during radiotherapy.

AIMS: The success of delivering the prescribed radiation dose to the prostate while sparing adjacent sensitive tissues is largely dependent on the ability to accurately target the prostate during treatment. Kilovoltage cone beam computed tomography (CBCT) imaging can be used to monitor and compensate for inter-fraction prostate motion, but this procedure increases treatment session time and adds incidental radiation dose to the patient. We carried out a retrospective study of CBCT data to evaluate the systematic and random correction shifts of the prostate with respect to bones and external marks. MATERIALS AND METHODS: A total of 449 daily CBCT studies from 17 patients undergoing intensity-modulated radiotherapy (IMRT) for localised prostate cancer were analysed. The difference between patient set-up correction shifts applied by radiation therapists (via matching prostate position in CBCT and planning computed tomography) and shifts obtained by matching bony anatomy in the same studies was used as a measure of the daily inter-fraction internal prostate motion. RESULTS: The average systematic and random shifts in prostate positions, calculated over all fractions versus only 10 fractions, were not found to be significantly different. DISCUSSION: The measured prostate shifts with respect to bony anatomy and external marks after the first 10 imaging sessions were shown to provide adequate predictive power for defining patient-specific margins in future fractions without a need for ongoing computed tomography imaging. Different options for CBCT imaging schedule are proposed that will reduce the treatment session time and imaging dose to radiotherapy patients while ensuring appropriate prostate cover and normal tissue sparing.

Effect of lateral target motion on image registration accuracy in CT-guided helical tomotherapy: a phantom study.

Optimisation of imaging modes for kilovoltage CT (kVCT) used for treatment planning and megavoltage CT (MVCT) image guidance used in ungated helical tomotherapy was investigated for laterally moving targets. Computed tomography images of the QUASAR Respiratory Motion Phantom were acquired without target motion and for lateral motion of the target, with 2-cm peak-to-peak amplitude and a period of 4 s. Reference kVCT images were obtained using a 16-slice CT scanner in standard fast helical CT mode, untagged average CT mode and various post-processed 4D-CT modes (0% phase, average and maximum intensity projection). Three sets of MVCT images with different inter-slice spacings of were obtained on a Hi-Art tomotherapy system with the phantom displaced by a known offset position. Eight radiation therapists performed co-registration of MVCT obtained with 2-, 4- and 6-mm slice spacing and kVCT studies independently for all 15 CT imaging combinations. In the investigated case, the untagged average kVCT and 4-mm slice spacing for the MVCT yielded more accurate registration in the transverse plane. The average residual uncertainty of this combination of imaging procedures was 0.61 +/- 0.16 mm in the longitudinal direction, 0.45 +/- 0.14 mm in the anterior-posterior direction and insignificant in the lateral direction. Manual registration of MVCT-kVCT study pairs is necessary to account for a target in significant lateral motion with respect to bony structures.

[Endoscopic approaches to the orbit]. / Les abords endoscopiques de l'orbite.

During the last decade, the use of endoscopic endonasal approaches to the pituitary has increased considerably. The endoscopic endonasal and transantral approaches offer a minimally invasive alternative to the classic transcranial or transconjunctival approaches to the medial aspect of the orbit. The medial wall of the orbit, the orbital apex, and the optic canal can be exposed through a middle meatal antrostomy, an anterior and posterior ethmoidectomy, and a sphenoidotomy. The inferomedial wall of the orbit can be also perfectly visualized through a sublabial antrostomy or an inferior meatal antrostomy. Several reports have described the use of an endoscopic approach for the resection or the biopsy of lesions located on the medial extraconal aspect of the orbit and orbital apex. However, the resection of intraconal lesions is still limited by inadequate instrumentation. Other indications for the endoscopic approach to the orbit are the decompression of the orbit for Graves' ophthalmopathy and traumatic optic neuropathy. However, the optimal management of traumatic optic neuropathy remains very controversial. Endoscopic endonasal decompression of the optic nerve in case of tumor compression could be a more valid indication in combination with radiation therapy. Finally, the endoscopic transantral treatment of blowout fracture of the floor of the orbit is an interesting option that avoids the eyelid or conjunctive incision of traditional approaches. The collaboration between the neurosurgeon and the ENT surgeon is mandatory and reduces the morbidity of the approach. Progress in instrumentation and optical devices will certainly make this approach promising for intraconal tumor of the orbit.

Schedule for CT image guidance in treating prostate cancer with helical tomotherapy.

The aim of this study was to determine the effect of reducing the number of image guidance sessions and patient-specific target margins on the dose distribution in the treatment of prostate cancer with helical tomotherapy. 20 patients with prostate cancer who were treated with helical tomotherapy using daily megavoltage CT (MVCT) imaging before treatment served as the study population. The average geometric shifts applied for set-up corrections, as a result of co-registration of MVCT and planning kilovoltage CT studies over an increasing number of image guidance sessions, were determined. Simulation of the consequences of various imaging scenarios on the dose distribution was performed for two patients with different patterns of interfraction changes in anatomy. Our analysis of the daily set-up correction shifts for 20 prostate cancer patients suggests that the use of four fractions would result in a population average shift that was within 1 mm of the average obtained from the data accumulated over all daily MVCT sessions. Simulation of a scenario in which imaging sessions are performed at a reduced frequency and the planning target volume margin is adapted provided significantly better sparing of organs at risk, with acceptable reproducibility of dose delivery to the clinical target volume. Our results indicate that four MVCT sessions on helical tomotherapy are sufficient to provide information for the creation of personalised target margins and the establishment of the new reference position that accounts for the systematic error. This simplified approach reduces overall treatment session time and decreases the imaging dose to the patient.

[Endoscopy versus microsurgery: results in a consecutive series of nonfunctioning pituitary adenomas]. / Résultats comparés de la chirurgie endoscopique et de la microchirurgie dans une série consécutive de macroadénomes hypophysaires non fonctionnels.

Microsurgical removal of nonfunctioning pituitary adenomas (NFPAs) is often subtotal. Removing the blind spots as viewed through the microscope, endoscopic surgery may improve the quality of removal. Our purpose was to compare the results of the two techniques in a series of NFPA patients operated on by a single surgeon. Thirty-six patients with newly diagnosed NFPAs were operated on using a purely endoscopic procedure and 29 with a microsurgical technique. All patients were explored pre- and postoperatively (at 3 and 6 months and then every 12 months) by endocrine assays, ophthalmologic exam, and 3D MRI. The endocrine and ophthalmologic results as well as the quality of resection and the complications from the two techniques were compared. The follow-up duration and the mean tumor volume (higher in the microsurgical group) were the only differences observed between the two groups. Tumor height and the invasion of the cavernous sinus were not different. All patients with preoperative visual impairment in the endoscopic group improved, whereas in the microsurgical group 90.9% improved, 4.5% were stabilized, and 4.5% worsened (p=ns). Regarding anterior pituitary functions, 42.8% of the patients improved in the endoscopic group, 45.7% remained stable, and 11.4% worsened compared to, respectively, 31, 44,8, and 24.1% in the microsurgical group (p=ns). Gross total removal was achieved in 86.1% for the endoscopic group and in only 65.5% for the microsurgical group (p=0.075). Morbidity was similar in the two groups. This retrospective series showed that endoscopic surgery compared to microsurgery increases the quality of NFPA removal with similar morbidity.

Consistency check of planned adaptive option on helical tomotherapy.

This study aims to evaluate a new Planned Adaptive software (TomoTherapy Inc., Madison, WI) of the helical tomotherapy system by retrospective verification and adaptive re-planning of radiation treatment. Four patients with different disease sites (brain, nasal cavity, lungs, prostate) were planned in duplicate using the diagnostic planning kVCT data set and MVCT studies of the first treatment fraction with the same optimization parameters for both plan types. The dosimetric characteristics of minimum, maximum, and mean dose to the targets as well as to organs at risk were compared. Both sets of plans were used for calculation of dose distributions in a water-equivalent phantom. Corresponding measurements of these plans in phantom were carried out with the use of radiographic film and ion chamber. In the case of the lung and prostate cancer patients, changes in dosimetric parameters compared to data generated with the kVCT study alone were less than 2%. Certain changes for the nasal cavity and brain cancer patients were greater than 2%, but they were explained in part by anatomy changes that occurred during the time between kVCT and MVCT studies. The Planned Adaptive software allows for adaptive radiotherapy planning using the MVCT studies obtained by the helical tomotherapy imaging system.

Fundamental x-ray interaction limits in diagnostic imaging detectors: frequency-dependent Swank noise.

A frequency-dependent x-ray Swank factor based on the "x-ray interaction" modulation transfer function and normalized noise power spectrum is determined from a Monte Carlo analysis. This factor was calculated in four converter materials: amorphous silicon (a-Si), amorphous selenium (a-Se), cesium iodide (CsI), and lead iodide (PbI2) for incident photon energies between 10 and 150 keV and various converter thicknesses. When scaled by the quantum efficiency, the x-ray Swank factor describes the best possible detective quantum efficiency (DQE) a detector can have. As such, this x-ray interaction DQE provides a target performance benchmark. It is expressed as a function of (Fourier-based) spatial frequency and takes into consideration signal and noise correlations introduced by reabsorption of Compton scatter and photoelectric characteristic emissions. It is shown that the x-ray Swank factor is largely insensitive to converter thickness for quantum efficiency values greater than 0.5. Thus, while most of the tabulated values correspond to thick converters with a quantum efficiency of 0.99, they are appropriate to use for many detectors in current use. A simple expression for the x-ray interaction DQE of digital detectors (including noise aliasing) is derived in terms of the quantum efficiency, x-ray Swank factor, detector element size, and fill factor. Good agreement is shown with DQE curves published by other investigators for each converter material, and the conditions required to achieve this ideal performance are discussed. For high-resolution imaging applications, the x-ray Swank factor indicates: (i) a-Si should only be used at low-energy (e.g., mammography); (ii) a-Se has the most promise for any application below 100 keV; and (iii) while quantum efficiency may be increased at energies just above the K edge in CsI and PbI2, this benefit is offset by a substantial drop in the x-ray Swank factor, particularly at high spatial frequencies.

[Optimization of MVCT imaging schedule in prostate cancer treatment using helical tomotherapy]. / Optimisation du processus d'imagerie de haute énergie (MVCT) pour la tomothérapie hélicoïdale des cancers de la prostate.

PURPOSE: Megavoltage CT (MVCT) study on helical tomotherapy permits to verify and correct the patient setup by coregistration with the planning kVCT. This process is time-consuming and our objective is to investigate a possibility of using a smaller number of imaging studies in the case of patients with prostate cancer. PATIENTS AND METHODS: The interfraction shifts of 20 patients (about 700 MVCT studies) treated in our institution have been recorded and analyzed. A new reference position has been calculated as an average of shifts observed during different initial number of fractions imaged. RESULTS: The analysis of the reference position obtained for the set of 20 patients as a function of the number of imaging sessions has shown that MVCT studies during first four fractions are sufficient for the majority of patients. CONCLUSION: Imaging during the first four fractions can be used to determine a reference position for patients with prostate cancer treated on helical tomotherapy. A study on Planned Adaptive (TomoTherapy Inc., Madison, WI, USA) software to evaluate the clinical significance of this scenario is currently in process in our institution.

Poster - Thurs Eve-18: Performance evaluation of MV CT imaging on the HI ART II tomotherapy unit.

The HI-ART II unit (TomoTherapy Inc., Madison WI) is a modality used by the London Regional Cancer Program (LRCP) for radiation therapy. This machine uses the same source of Megavoltage energy radiation to image (3.5 MV) and to treat (6MV) patients, combining the functionality of a traditional linear accelerator and CT simulator into one unit. Thus, it is possible to assess patient positioning and adjust for anatomy changes just prior to radiation therapy. Unfortunately, at MV energy levels, the physics of radiation interaction limits image quality, and gives rise to an inherent dose limitation concern that enhances noise levels. Therefore, we propose to quantify the image quality produced by the HI-ART II unit using techniques established for kVCT scanner technology. Our study involved the use of three standard phantoms to test image resolution, noise, uniformity, and linearity for a 512 × 512 reconstruction matrix and three scan pitch settings (0.8, 1.6, and 2.4). Results follow: linearity between MV CT number versus relative electron density was observed, noise calculations ranged from 2.15-2.51%, and a distinct central artifact was revealed during uniformity testing. The linearity between MV CT number versus relative electron density implies that MV CT images are highly suitable for dose calculations. MV CT image quality of uniform phantoms were acceptable and demonstrated noise levels higher than those produced by kVCT simulators. Further study is necessary to correct for the central artifact in MV CT images.

Sci-Fri PM: Planning-07: A low diffusion radiochromic gel dosimeter for three-dimensional radiation dosimetry.

PURPOSE: To develop a low diffusion radiochromic leuco crystal violet (LCV) hydrogel utilizing micelles, for three-dimensional (3-D) radiation dosimetry. METHOD AND MATERIALS: Concentrations of LCV dye, Triton X-100 and trichloroacetic acid were varied to determine the optimal gel sensitivity for optical computed tomography (CT). Using a laser optical CT scanner at λ = 594 nm, diffusion rate measurements were performed on half-irradiated (6 MV x-rays) cuvette gel samples made with and without surfactant, respectively. A cylindrical 1 L gel volume was irradiated with a 12 MeV electron beam (Varian Clinac 2100C) to a dose of 30 Gy and scanned with cone- beam optical CT at λ ∼ 590 nm (Vista™, Modus Medical Devices Inc.). RESULTS: The most radiation sensitive gel formulation was found to be: 1 mM LCV, 4 mM Triton X-100, 30 mM trichloroacetic acid and 4% gelatin. The diffusion rates of a LCV gel without and with surfactant present were about 2 and 20 times lower than the Fricke xylenol-orange gel system, respectively. Comparison of the central axis gel attenuation coefficients normalized at depth of maximum dose (dmax ) with TG21-corrected ion chamber data, were in agreement, thus, indicating energy and dose-rate independence. CONCLUSION: Radiochromic LCV micelle gels show minimal diffusion effects and a dose response that is linear, energy and dose-rate independent. Optical CT scanned LCV micelle gels are a promising system for 3-D dose verification. CONFLICT OF INTEREST: Two of the authors (JB, KJ) have a licensing agreement with Modus Medical Devices Inc. concerning the commercialization of Vista™.

3D thoracoscopic ultrasound volume measurement validation in an ex vivo and in vivo porcine model of lung tumours.

The purpose of this study was to validate the accuracy and reliability of volume measurements obtained using three-dimensional (3D) thoracoscopic ultrasound (US) imaging. Artificial "tumours" were created by injecting a liquid agar mixture into spherical moulds of known volume. Once solidified, the "tumours" were implanted into the lung tissue in both a porcine lung sample ex vivo and a surgical porcine model in vivo. 3D US images were created by mechanically rotating the thoracoscopic ultrasound probe about its long axis while the transducer was maintained in close contact with the tissue. Volume measurements were made by one observer using the ultrasound images and a manual-radial segmentation technique and these were compared with the known volumes of the agar. In vitro measurements had average accuracy and precision of 4.76% and 1.77%, respectively; in vivo measurements had average accuracy and precision of 8.18% and 1.75%, respectively. The 3D thoracoscopic ultrasound can be used to accurately and reproducibly measure "tumour" volumes both in vivo and ex vivo.

Signal and noise transfer properties of photoelectric interactions in diagnostic x-ray imaging detectors.

Image quality in diagnostic x-ray imaging is ultimately limited by the statistical properties governing how, and where, x-ray energy is deposited in a detector. This in turn depends on the physics of the underlying x-ray interactions. In the diagnostic energy range (10-100 keV), most of the energy deposited in a detector is through photoelectric interactions. We present a theoretical model of the photoelectric effect that specifically addresses the statistical nature of energy absorption by photoelectrons, K and L characteristic x rays, and Auger electrons. A cascaded-systems approach is used that employs a complex structure of parallel cascades to describe signal and noise transfer through the photoelectric effect in terms of the modulation transfer function, Wiener noise power spectrum, and detective quantum efficiency (DQE). The model was evaluated by comparing results with Monte Carlo calculations for x-ray converters based on amorphous selenium (a-Se) and lead (Pb), representing both low and high-Z materials. When electron transport considerations can be neglected, excellent agreement (within 3%) is obtained for each metric over the entire diagnostic energy range in both a-Se and Pb detectors up to 30 cycles/mm, the highest frequency tested. The cascaded model overstates the DQE when the electron range cannot be ignored. This occurs at approximately two cycles/mm in a-Se at an incident photon energy of 80 keV, whereas in Pb, excellent agreement is obtained for the DQE over the entire diagnostic energy range. However, within the context of mammography (20 keV) and micro-computed tomography (40 keV), the effects of electron transport on the DQE are negligible compared to fluorescence reabsorption, which can lead to decreases of up to 30% and 20% in a-Se and Pb, respectively, at 20 keV; and 10% and 5%, respectively, at 40 keV. It is shown that when Swank noise is identified in a Fourier model, the Swank factor must be frequency dependent. This factor decreases quickly with frequency, and in the case of a-Se and Pb, decreases by up to a factor of 3 at five cycles/mm immediately above the K edge. The frequency-dependent Swank factor is also equivalent to what we call the "photoelectric DQE," which describes signal and noise transfer through photoelectric interactions.

An experimental study of recombination and polarity effect in a set of customized plane parallel ionization chambers.

Plane parallel ionization chambers are an important tool for dosimetry and absolute calibration of electron beams used for radiotherapy. Most dosimetric protocols require corrections for recombination and polarity effects, which are to be determined experimentally as they depend on chamber design and radiation quality. Both effects were investigated in electron beams from a linear accelerator (Varian 21CD) for a set of four tissue equivalent plane parallel ionization chambers customized for the present research by Standard Imaging (Madison WI). All four chambers share the same design and air cavity dimensions, differing only in the diameter of their collecting electrode and the corresponding width of the guard ring. The diameters of the collecting electrodes were 2 mm, 4 mm, 10 mm and 20 mm. Measurements were taken using electron beams of nominal energy 6 to 20 MeV in a 10 cm x 10 cm field size with a SSD of 100 cm at various depths in a Solid Water slab phantom. No significant variation of recombination effect was found with radiation quality, depth of measurement or chamber design. However, the polarity effect exceeded 5% for the chambers with small collecting electrode for an effective electron energy below 4 MeV at the point of measurement. The magnitude of the effect increased with decreasing electron energy in the phantom. The polarity correction factor calculated following AAPM protocol TG51 ranged from approximately 1.00 for the 20.0 mm chamber to less than 0.95 for the 2 mm chamber at 4.1 cm depth in a electron beam of nominally 12 MeV. By inverting the chamber it could be shown that the polarity effect did not depend on the polarity of the electrode first traversed by the electron beam. Similarly, the introduction of an air gap between the overlying phantom layer and the chambers demonstrated that the angular distribution of the electrons at the point of measurement had a lesser effect on the polarity correction than the electron energy itself. The magnitude of the absolute difference between charge collected at positive and negative polarity was found to correlate with the area of the collecting electrode which is consistent with the explanation that differences in thickness of the collecting electrodes and the number of electrons stopped in them contribute significantly to the polarity effect. Overall, the polarity effects found in the present study would have a negligible effect on electron beam calibration at a measurement depth recommended by most calibration protocols. However, the present work tested the corrections under extreme conditions thereby aiming at greater understanding of the mechanism underlying the correction factors for these chambers. This may lead to better chamber design for absolute dosimetry and electron beam characterization with less reliance on empirical corrections.

Feasibility of a fast inverse dose optimization algorithm for IMRT via matrix inversion without negative beamlet intensities.

A fast optimization algorithm is very important for inverse planning of intensity modulated radiation therapy (IMRT), and for adaptive radiotherapy of the future. Conventional numerical search algorithms such as the conjugate gradient search, with positive beam weight constraints, generally require numerous iterations and may produce suboptimal dose results due to trapping in local minima. A direct solution of the inverse problem using conventional quadratic objective functions without positive beam constraints is more efficient but will result in unrealistic negative beam weights. We present here a direct solution of the inverse problem that does not yield unphysical negative beam weights. The objective function for the optimization of a large number of beamlets is reformulated such that the optimization problem is reduced to a linear set of equations. The optimal set of intensities is found through a matrix inversion, and negative beamlet intensities are avoided without the need for externally imposed ad-hoc constraints. The method has been demonstrated with a test phantom and a few clinical radiotherapy cases, using primary dose calculations. We achieve highly conformal primary dose distributions with very rapid optimization times. Typical optimization times for a single anatomical slice (two dimensional) (head and neck) using a LAPACK matrix inversion routine in a single processor desktop computer, are: 0.03 s for 500 beamlets; 0.28 s for 1000 beamlets; 3.1 s for 2000 beamlets; and 12 s for 3000 beamlets. Clinical implementation will require the additional time of a one-time precomputation of scattered radiation for all beamlets, but will not impact the optimization speed. In conclusion, the new method provides a fast and robust technique to find a global minimum that yields excellent results for the inverse planning of IMRT.