M. Bucciolini

A pencil beam algorithm for proton dose calculations

The sharp lateral penumbra and the rapid fall-off of dose at the end of range of a proton beam are among the major advantages of proton radiation therapy. These beam characteristics depend on the position and characteristics of upstream beam-modifying devices such as apertures and compensating boluses. The extent of separation, if any, between these beam-modifying devices and the patient is particularly critical in this respect. We have developed a pencil beam algorithm for proton dose calculations which takes accurate account of the effects of materials upstream of the patient and of the air gap between them and the patient. The model includes a new approach to picking the locations of the pencil beams so as to more accurately model the penumbra and to more effectively account for the multiple-scattering effects of the media around the point of interest. We also present a faster broad-beam version of the algorithm which gives a reasonably accurate penumbra. Predictions of the algorithm and results from experiments performed in a large-field proton beam are presented. In general the algorithm agrees well with the measurements.

Verification of IMRT fields by film dosimetry

In intensity modulated radiation therapy (IMRT) the aim of an accurate conformal dose distribution is obtained through a complex process. This ranges from the calculation of the optimal distribution of fluence by the treatment planning system (TPS), to the dose delivery through a multilamellar collimator (MLC), with several segments per beam in the step and shoot approach. The above-mentioned consideration makes mandatory an accurate dosimetric verification of the IM beams. A high resolution and integrating dosimeter, like the radiographic film, permits one to simultaneously measure the dose in a matrix of points, providing a good means of obtaining dose distributions. The intrinsic limitation of film dosimetry is the sensitivity dependence on the field size and on the measurement depth. However, the introduction of a scattered radiation filter permits the use of a single calibration curve for all field sizes and measurement depths. In this paper the quality control procedure developed for dosimetric verification of IMRT technique is reported. In particular a system of film dosimetry for the verification of a 6 MV photon beam has been implemented, with the introduction of the scattered radiation filter in the clinical practice that permits one to achieve an absolute dose determination with a global uncertainty within 3.4% (1 s.d.). The film has been calibrated to be used both in perpendicular and parallel configurations. The work also includes the characterization of the Elekta MLC. Ionimetric independent detectors have been used to check single point doses. The film dosimetry procedure has been applied to compare the measured absolute dose distributions with the ones calculated by the TPS, both for test and clinical plans. The agreement, quantified by the gamma index that seldom reaches the 1.5 value, is satisfying considering that the comparison is performed between absolute doses.

Diamond detector versus silicon diode and ion chamber in photon beams of different energy and field size

The aim of this work was to test the suitability of a PTW diamond detector for nonreference condition dosimetry in photon beams of different energy (6 and 25 MV) and field size (from 2.6 cm x 2.6 cm to 10 cm x 10 cm). Diamond behavior was compared to that of a Scanditronix p-type silicon diode and a Scanditronix RK ionization chamber. Measurements included output factors (OF). percentage depth doses (PDD) and dose profiles. OFs measured with diamond detector agreed within 1% with those measured with diode and RK chamber. Only at 25 MV, for the smallest field size, RK chamber underestimated OFs due to averaging effects in a pointed shaped beam profile. Agreement was found between PDDs measured with diamond detector and RK chamber for both 6 MV and 25 MV photons and down to 5 cm x 5 cm field size. For the 2.6 cm x 2.6 cm field size, at 25 MV, RK chamber underestimated doses at shallow depth and the difference progressively went to zero in the distal region. PDD curves measured with silicon diode and diamond detector agreed well for the 25 MV beam at all the field sizes. Conversely, the nontissue equivalence of silicon led, for the 6 MV beam, to a slight overestimation of the diode doses in the distal region, at all the field sizes. Penumbra and field width measurements gave values in agreement for all the detectors but with a systematic overestimate by RK measurements. The results obtained confirm that ion chamber is not a suitable detector when high spatial resolution is required. On the other hand, the small differences in the studied parameters, between diamond and silicon systems, do not lead to a significant advantage in the use of diamond detector for routine clinical dosimetry.

Accelerated Partial Breast Irradiation With IMRT: New Technical Approach and Interim Analysis of Acute Toxicity in a Phase III Randomized Clinical Trial

PURPOSE To evaluate with a randomized clinical trial the possibility of treating the index quadrant with external intensity-modulated radiotherapy (IMRT) in a selected group of patients with early-stage breast cancer and to analyze the acute toxicity. METHODS AND MATERIALS From September 2005, a randomized Phase III clinical trial has been conducted to compare conventional (tangential field) fractionated whole breast treatment (Arm A) with accelerated partial breast irradiation plus intensity-modulated radiotherapy (Arm B). For intensity-modulated radiotherapy, the clinical target volume was drawn with a uniform 1-cm margin around the surgical clips in three dimensions. The ipsilateral and contralateral breast, ipsilateral and contralateral lung, heart, and spinal cord were contoured as organs at risk. All the regions of interest were contoured according to the International Commission on Radiation Units and Measurements reports 50 and 62 recommendations. RESULTS In September 2008, 259 patients were randomized and treated. The mean clinical target volume in Arm B was 44 cm(3) and the mean planning target volume was 123 cm(3). The mean value of the ratio between the planning target volume and the ipsilateral breast volume was 21%. The rate of Grade 1 and Grade 2 acute skin toxicity was 22% and 19% in Arm A (Radiation Therapy Oncology Group scale), respectively. The tolerance in Arm B was excellent with only 5% Grade 1 and 0.8% Grade 2 acute skin toxicity. The planning constraints were fully satisfied in most patients. In a very few cases, this was not possible because of very unfavorable anatomy. Quality assurance procedures were performed according to our internal quality assurance protocol, with excellent results. CONCLUSION In the present preliminary analysis, we have demonstrated that accelerated partial breast irradiation is feasible, with very low acute toxicity.

An investigation of the operating characteristics of two PTW diamond detectors in photon and electron beams

The dosimetric properties of two PTW Riga diamond detectors type 60003 were studied in high-energy photon and electron therapy beam. Properties under study were current-voltage characteristic, polarization effect, time stability of response, dose response, dose-rate dependence, temperature stability, and beam quality dependence of the sensitivity factor. Differences were shown between the two detectors for most of the previous properties. Also, the observed behavior was, to some extent, different from what was reported in the PTW technical specifications. The necessity to characterize each diamond detector individually was addressed.

Pretreatment verification of IMRT absolute dose distributions using a commercial a-Si EPID.

A commercial amorphous silicon electronic portal imaging device (EPID) has been studied to investigate its potential in the field of pretreatment verifications of step and shoot, intensity modulated radiation therapy (IMRT), 6 MV photon beams. The EPID was calibrated to measure absolute exit dose in a water-equivalent phantom at patient level, following an experimental approach, which does not require sophisticated calculation algorithms. The procedure presented was specifically intended to replace the time-consuming in-phantom film dosimetry. The dosimetric response was characterized on the central axis in terms of stability, linearity, and pulse repetition frequency dependence. The a-Si EPID demonstrated a good linearity with dose (within 2% from 1 monitor unit), which represent a prerequisite for the application in IMRT. A series of measurements, in which phantom thickness, air gap between the phantom and the EPID, field size and position of measurement of dose in the phantom (entrance or exit) varied, was performed to find the optimal calibration conditions, for which the field size dependence is minimized. In these conditions (20 cm phantom thickness, 56 cm air gap, exit dose measured at the isocenter), the introduction of a filter for the low-energy scattered radiation allowed us to define a universal calibration factor, independent of field size. The off-axis extension of the dose calibration was performed by applying a radial correction for the beam profile, distorted due to the standard flood field calibration of the device. For the acquisition of IMRT fields, it was necessary to employ home-made software and a specific procedure. This method was applied for the measurement of the dose distributions for 15 clinical IMRT fields. The agreement between the dose distributions, quantified by the gamma index, was found, on average, in 97.6% and 98.3% of the analyzed points for EPID versus TPS and for EPID versus FILM, respectively, thus suggesting a great potential of this EPID for IMRT dosimetric applications.

Characterisation of CVD diamond dosimeters in on-line configuration

Abstract The high sensitivity and the nearly tissue equivalence of diamond make it a material suitable as detector for on-line dosimetry. The tremendous development of the CVD diamond technology allows to employ polycrystalline diamond films with controlled dimensions and a potential low cost as on-line radiation dosimeters. In this paper a characterisation and a comparison of the response of two “detector-grade” CVD diamond films under photon and electron beams are presented. The results show that both samples can be used as on-line dosimeters for applications in radiotherapy.

Characterization of a Silicon Strip Detector and a YAG:Ce Calorimeter for a Proton Computed Radiography Apparatus

Today, there is a steadily growing interest in the use of proton beams for tumor therapy, as they permit to tightly shape the dose delivered to the target reducing the exposure of the surrounding healthy tissues. Nonetheless, accuracy in the determination of the dose distribution in proton-therapy is up to now limited by the uncertainty in stopping powers, which are presently calculated from the photon attenuation coefficients measured by X-ray tomography. Proton computed tomography apparatus (pCT) has been proposed to directly measure the stopping power and reduce this uncertainty. Main problem with proton imaging is the blurring effect introduced by multiple Coulomb scattering: single proton tracking is a promising technique to face this difficulty. As a first step towards a pCT system, we designed a proton radiography (pCR) prototype based on a silicon microstrip tracker (to characterize particle trajectories) and a segmented YAG:Ce calorimeter (to measure their residual energy). Aim of the system is to detect protons with a ~1 MHz particle rate of and with kinetic energy in the range 250-270 MeV, high enough to pass through human body. Design and development of the pCR prototype, as well as the characterization of its single components, are described in this paper.

Neuronavigation accuracy dependence on CT and MR imaging parameters: a phantom-based study

Clinical benefits from neuronavigation are well established. However, the complexity of its technical environment requires a careful evaluation of different types of errors. In this work, a detailed phantom study which investigates the accuracy in a neuronavigation procedure is presented. The dependence on many different imaging parameters, such as field of view, slice thickness and different kind of sequences (sequential and spiral for CT, T1-weighted and T2-weighted for MRI), is quantified. Moreover, data based on CT images are compared to those based on MR images, taking into account MRI distortion. Finally, the contributions to global accuracy coming from image acquisition, registration and navigation itself are discussed. Results demonstrate the importance of imaging accuracy. Procedures based on CT proved to be more accurate than procedures based on MRI. In the former, values from 2 to 2.5 mm are obtained for 95% fractiles of cumulative distribution of Euclidean distances between the intended target and the reached one while, in the latter, the measured values range from 3 to 4 mm. The absence of imaging distortion proved to be crucial for registration accuracy in MR-based procedures.

Epitaxial silicon devices for dosimetry applications

A straightforward improvement of the efficiency and long term stability of silicon dosimeters has been obtained with a n+-p junction surrounded by a guard-ring structure implanted on an epitaxial p-type Si layer grown on a Czochralski substrate. The sensitivity of devices made on 50-μm-thick epitaxial Si degrades by only 7% after an irradiation with 6MeV electrons up to 1.5kGy, and shows no significant further decay up to 10kGy. These results prove the enhanced radiation tolerance and stability of epitaxial diodes as compared to present state-of-the-art Si devices.