L. Azario

In vivo dosimetry by an aSi‐based EPID

A method for the in vivo determination of the isocenter dose, Diso, and mid-plane dose, Dm, using the transmitted signal St measured by 25 central pixels of an aSi-based EPID is here reported. The method has been applied to check the conformal radiotherapy of pelvic tumors and supplies accurate in vivo dosimetry avoiding many of the disadvantages associated with the use of two diode detectors (at the entrance and exit of the patient) as their periodic recalibration and their positioning. Irradiating water-equivalent phantoms of different thicknesses, a set of correlation functions F(w, l) were obtained by the ratio between St and Dm as a function of the phantom thickness, w, for a different field width, l. For the in vivo determination of Diso and Dm values, the water-equivalent thickness of the patients (along the beam central axis) was evaluated by means of the treatment planning system that uses CT scans calibrated in terms of the electron densities. The Diso and Dm values experimentally determined were compared with the stated doses D(iso,TPS) and D(m,TPS), determined by the treatment planning system for ten pelvic treatments. In particular, for each treatment four fields were checked in six fractions. In these conditions the agreement between the in vivo dosimetry and stated doses at the isocenter point were within 3%. Comparing the 480 dose values obtained in this work with those obtained for 30 patients tested with a similar method, which made use of a small ion-chamber positioned on the EPIDs to obtain the transmitted signal, a similar agreement was observed. The method here proposed is very practical and can be applied in every treatment fraction, supplying useful information about eventual patient dose variations due to the incorrect application of the quality assurance program based on the check of patient setup, machine setting, and calculations.

PTW‐diamond detector: Dose rate and particle type dependence

In this paper the suitability of a PTW natural diamond detector (DD) for relative and reference dosimetry of photon and electron beams, with dose per pulse between 0.068 mGy and 0.472 mGy, was studied and the results were compared with those obtained by a stereotactic silicon detector (SFD). The results show that, in the range of the examined dose per pulse the DD sensitivity changes up to 1.8% while the SFD sensitivity changes up to 4.5%. The fitting parameter, delta, used to correct the dose per pulse dependence of solid state detectors, was delta = 0.993 +/- 0.002 and delta = 1.025 +/- 0.002 for the diamond detector and for the silicon diode, respectively. The delta values were found to be independent of particle type of two conventional beams (a 10 MV x-ray beam and a 21 MeV electron beam). So if delta is determined for a radiotherapy beam, it can be used to correct relative dosimetry for other conventional radiotherapy beams. Moreover the diamond detector shows a calibration factor which is independent of beam quality and particle type, so an empirical dosimetric formalism is proposed here to obtain the reference dosimetry. This formalism is based on a dose-to-water calibration factor and on an empirical coefficient, that takes into account the reading dependence on the dose per pulse.

The saturation loss for plane parallel ionization chambers at high dose per pulse values

The use of plane parallel ionization chambers with electron beams with high dose per pulse entails dose uncertainties due to the overestimation of the ion recombination factor, k, up to 20% if conventional dosimetric protocols are used. In this work MD-55-2 radiochromic films have been used as reference dosimeters to obtain dose to water per pulse DGAF(w) values for three Novac7 (Hitesys) electron beams of E0 = 5.8 MeV. However, the beam calibration by MD-55-2 films is time consuming and the use of plane parallel chambers is fundamental for a periodic quality control procedure. Three plane parallel chambers have been used and the general formula for the k determination has been tested using the calibration doses, DGAF(w). In particular, consistent ion recombination factors ksat(V0) (with the ion chamber polarized at V0), that follow the Boag theory, have been estimated at different dose per pulse values for the three plane parallel ionization chambers. This means that at present any ion chamber needs a specific ksat (V0) determination by using a reference dosimeter for which the response is independent of the dose rate. An accurate determination of ksat(V0), using a reference quality beam, can be used to determine the dose to water per pulse for electron beams of different quality and geometrical configuration.

Single-Arm Phase II Study of Conformal Radiation Therapy and Temozolomide plus Fractionated Stereotactic Conformal Boost in High-Grade Gliomas

Purpose:To assess survival, local control and toxicity using fractionated stereotactic conformal radiotherapy (FSCRT) boost and temozolomide in high-grade gliomas (HGGs).Patients and Methods:Patients affected by HGG, with a CTV1(clinical target volume, representing tumor bed ± residual tumor + a margin of 5 mm) ≤ 8 cm were enrolled into this phase II study. Radiotherapy (RT, total dose 6,940 cGy) was administered using a combination of two different techniques: three-dimensional conformal radiotherapy (3D-CRT, to achieve a dose of 5,040 or 5,940 cGy) and FSCRT boost (19 or 10 Gy) tailored by CTV1diameter (≤ 6 cm and > 6 cm, respectively). Temozolomide (75 mg/m2) was administered during the first 2 or 4 weeks of RT. After the end of RT, temozolomide (150–200 mg/m2) was administered for at least six cycles. The sample size of 41 patients was assessed by the single proportion–powered analysis.Results:41 patients (36 with glioblastoma multiforme [GBM] and five with anaplastic astrocytoma [AA]) were enrolled; RTOG neurological toxicities G1–2 and G3 were 12% and 3%, respectively. Two cases of radionecrosis were observed. At a median follow-up of 44 months (range 6–56 months), global and GBM median overall survival (OS) were 30 and 28 months. The 2-year survival rate was significantly better compared to the standard treatment (63% vs. 26.5%; p < 0.00001). Median progression-free survival (PFS) was 11 months, in GBM patients 10 months.Conclusion:FSCRT boost plus temozolomide is well tolerated and seems to increase survival compared to the standard treatment in patients with HGG.ZusammenfassungZiel:Untersuchung von Uberleben, lokaler Tumorkontrolle und Toxizitat einer fraktionierten stereotaktischen konformalen Strahlentherapie (FSCRT) mit Boostbestrahlung in Kombination mit Temozolomid bei hochmalignen Gliomen (HMG).Patienten und Methodik:Patienten mit HMG und einem CTV1(klinisches Zielvolumen, d. h. Tumorbett ± Resttumor + einem Sicherheitsabstand von 5 mm) ≤ 8 cm wurden in diese Phase-II-Studie eingeschlossen. Die Strahlentherapie (Gesamtdosis 6 940 cGy) wurde als Kombination aus zwei unterschiedlichen Techniken appliziert: dreidimensionale konformale Strahlentherapie (3D-CRT, um eine Strahlendosis von 5 040 oder 5 940 cGy zu erreichen) und lokale Dosisaufsattigung mit FSCRT-Boost (19 oder 10 Gy), die auf den CTV1-Durchmesser (≤ 6 cm bzw. > 6 cm) zugeschnitten war. Temozolomid (75 mg/m2) wurde wahrend der ersten 2 oder 4 Wochen der Strahlentherapie verabreicht. Nach dem Ende der Strahlentherapie erhielten die Patienten Temozolomid (150–200 mg/m2) fur wenigstens sechs Zyklen. Die Fallzahl wurde mit Hilfe eines einfach-proportionalen Testverfahrens („single proportion-powered analysis“) bei 41 Patienten bestimmt.Ergebnisse:41 Patienten (36 mit Glioblastoma multiforme [GBM] und funf mit anaplastischem Astrozytom [AA]) wurden behandelt; Neurotoxizitat gemas RTOG-Skala G1–2 bzw. G3 wurde in 12% bzw. 3% der Patienten beobachtet. Zwei Falle von Radionekrose traten auf. Bei einer mittleren Beobachtungszeit von 44 Monaten (Range 6–56 Monate) lagen die mittlere Gesamt- und die GBM-spezifische Uberlebenszeit (OS) bei 30 und 28 Monaten. Die 2-Jahres-Uberlebensrate war signifikant besser im Vergleich zur Standardbehandlung (63% vs. 26,5%; p < 0.00001). Die mittlere progressionsfreie Uberlebenszeit (PFS) betrug 11 Monate, bei GBM-Patienten 10 Monate.Schlussfolgerung:FSCRT-Boostbestrahlung plus Temozolomid wird gut toleriert und scheint im Vergleich zur Standardbehandlung die Uberlebenszeit von Patienten mit HMG zu verbessern.

Application of a practical method for the isocenter point in vivo dosimetry by a transit signal

This work reports the results of the application of a practical method to determine the in vivo dose at the isocenter point, D(iso), of brain thorax and pelvic treatments using a transit signal S(t). The use of a stable detector for the measurement of the signal S(t) (obtained by the x-ray beam transmitted through the patient) reduces many of the disadvantages associated with the use of solid-state detectors positioned on the patient as their periodic recalibration, and their positioning is time consuming. The method makes use of a set of correlation functions, obtained by the ratio between S(t) and the mid-plane dose value, D(m), in standard water-equivalent phantoms, both determined along the beam central axis. The in vivo measurement of D(iso) required the determination of the water-equivalent thickness of the patient along the beam central axis by the treatment planning system that uses the electron densities supplied by calibrated Hounsfield numbers of the computed tomography scanner. This way it is, therefore, possible to compare D(iso) with the stated doses, D(iso,TPS), generally used by the treatment planning system for the determination of the monitor units. The method was applied in five Italian centers that used beams of 6 MV, 10 MV, 15 MV x-rays and (60)Co gamma-rays. In particular, in four centers small ion-chambers were positioned below the patient and used for the S(t) measurement. In only one center, the S(t) signals were obtained directly by the central pixels of an EPID (electronic portal imaging device) equipped with commercial software that enabled its use as a stable detector. In the four centers where an ion-chamber was positioned on the EPID, 60 pelvic treatments were followed for two fields, an anterior-posterior or a posterior-anterior irradiation and a lateral-lateral irradiation. Moreover, ten brain tumors were checked for a lateral-lateral irradiation, and five lung tumors carried out with three irradiations with different gantry angles were followed. One center used the EPID as a detector for the S(t) measurement and five pelvic treatments with six fields (many with oblique incidence) were followed. These last results are reported together with those obtained in the same center during a pilot study on ten pelvic treatments carried out by four orthogonal fields. The tolerance/action levels for every radiotherapy fraction were 4% and 5% for the brain (symmetric inhomogeneities) and thorax/pelvic (asymmetric inhomogeneities) irradiations, respectively. This way the variations between the total measured and prescribed doses at the isocenter point in five fractions were well within 2% for the brain treatment, and 4% for thorax/pelvic treatments. Only 4 out of 90 patients needed new replanning, 2 patients of which needed a new CT scan.

Radiochromic film dosimetry of a low energy proton beam.

In this work some dosimetric characteristics of MD-55-2 GafChromic films were studied in a low energy proton beam (21.5 MeV) directly in a water phantom. The nonlinearity of the optical density was quantified by a factor P(lin). A correction factor P(en), that accounts for optical density dependence on the energy, was empirically determined. The effects of detector thickness in depth dose measurements and of the film orientation with respect to beam direction were investigated. The results show that the MD-55-2 films provide dose measurements with the films positioned perpendicularly to the proton beam. A dosimetric formalizm is proposed to determine the dose to water at depth d, with films oriented perpendicularly to the beam axis. This formalism uses a calibration factor of the radiochromic film determined directly on the proton beam at a reference depth in water, and the P(lin) factor, that takes into account the nonlinearity of the calibration curve and the P(en) factor that, in turn takes into account the change of proton beam energy in water. The MD-55-2 films with their high spatial resolution and the quasiwater equivalent material are attractive, positioned perpendicularly along the beam axis, for the absolute dose determination of very small beam sizes and modulated proton beams.

A study of quality of bremsstrahlung spectra reconstructed from transmission measurements

A numerical method for the reconstruction of bremsstrahlung spectra has been applied to the analysis of simulated data. The method is found to be reliable in reconstructing x-ray spectra of maximum energy up to 10 MeV. Measurements of aluminum transmission data carried out for five linear accelerators have also been analyzed by this method to determine the fractional energy fluence. The values of the Spencer-Attix water/air stopping power ratio, SS.A. w,air, reported as a function of the ratios TPR20 10, are in good agreement with computed results. Quality index values were correlated to mean incident energies of x-ray beams used in radiotherapy. The experimental setup has also provided information regarding the softening effect on the off-axis beam, which can be used in clinical computer dosimetry to correct conventional zero field size tissue maximum ratio.

A preliminary dosimetric characterization of chemical vapor deposition diamond detector prototypes in photon and electron radiotherapy beams.

Three radiation detectors based on polycrystalline diamond films with different thickness and resistivity, obtained by microwave chemical vapor deposition, were tested to assess their suitability for relative dosimetry of photon and electron beams supplied by clinical linear accelerators. All samples showed a linear response as a function of the absorbed dose. The sensitivity per unit of detector sensitive volume spanned between 7 and 43 nC Gy(-1) mm(-3) with an applied electric field of 40 kV/cm. The dose rate dependence was evaluated following the Fowler theory and delta coefficient values between 0.95 and 1.00 were found for the three samples when polarized at 40 kV/cm. Percentage depth dose curves, output factors, and normalized dose profiles were determined for 6 and 10 MV photon beams and for 6 and 15 MeV electron beams. The results obtained with the diamond detectors were in good agreement with those obtained by reference detector measurements [all the data were within the experimental uncertainty of 1% (1sigma)].

Dosimetric characterization of silicon and diamond detectors in low-energy proton beams.

The dosimetric behaviour of a Scanditronix p-type silicon diode and a PTW natural diamond detector was studied in low-energy proton beams in the 8.3-21.5 MeV range. The properties investigated were linearity, reproducibility, dose rate dependence, energy and linear energy transfer (LET) dependence. The influence of detector thickness on the results of depth dose measurements was also demonstrated. A Markus parallel plate ionization chamber was used for reference dosimetry. Silicon diode and diamond detectors showed linearity at therapeutic dose level, reproducibility better than 1% (1sigma) and sensitivity variation with dose rate and proton energy.

Whole-brain radiotherapy combined with surgery or stereotactic radiotherapy in patients with brain oligometastases: Long-term analysis

Objective:To verify whether the treatment of brain oligometastases with whole-brain radiotherapy (WBRT) plus stereotactic radiotherapy (SRT) or surgical resection results in different outcomes.Methods:Files of patients affected by brain metastases submitted to surgical resection followed by WBRT (group A) or WBRT + SRT (group B) were retrospectively selected for this study. The two treatment groups were matched for the following potential prognostic factors: WBRT schedule, age, gender, performance status, tumor type, number of brain metastases, extra-cerebral metastases, and recursive partitioning analysis class (RPA). The outcomes of patients in both groups were evaluated in terms of toxicity, local control, and overall survival.Results:Total of 97 patients were selected (56 male; 42 female) who were respectively submitted to surgical resection followed by WBRT (group A, n = 50 patients) or WBRT + SRT (Group B, n = 47 patients). Median follow-up was 95 months (range, 8–171 months). The 1-year local control rates were 46.0% and 69.0% respectively. No significant difference in local tumor control was observed between group A and B (p = 0.10). Median overall survival was 15 and 19 months in group A and B, respectively. One-year survival was 56.0% and 62%, respectively. No difference was observed in the two groups (p = 0.40).Conclusion:Surgery remains the main therapeutic approach in symptomatic patients; nevertheless, our data support the use of WBRT plus SRT in one or two brain metastases smaller than 3 cm.Zielsetzung:Zu untersuchen, ob die Behandlung von Oligohirnmetastasen mit Ganzhirnbestrahlung (whole-brain radiotherapy, WBRT) plus stereotaktischer Radiotherapie (SRT) oder chirurgischer Resektion unterschiedliche Ergebnisse bringt.Methoden:Krankenakten von Patienten mit operativ entfernten Hirnmetastasen und anschließender WBRT (Gruppe A) oder WBRT + SRT (Gruppe B) wurden retrospektiv für diese Studie ausgewählt. Die zwei Beghandlungsarme wurden für folgende prognostische Parameter gematcht: WBRT-Schema. Alter, Geschlecht, Allgemeinzustand, Art des Primärtumors, Anzahl der Hirnmetastasen, extrazerebrale Metastasen, Recursive-Partioning-Analysis-(RPA-)Klasse. Das Behandlungsergebnis für beide Patientengruppen wurde im Hinblick auf Toxizität, lokale Tumorkontrolle und Gesamtüberleben bewertet.Ergebnisse:Insgesamt wurden 97 Patienten ausgewählt (56 m; 42 w), die entweder mit operativer Entfernung und anschließender WBRT (Gruppe A, n = 50 Pat.) oder mit WBRT + SRT (Gruppe B, n = 47 Pat.) behandelt wurden. Die mediane Beobachtungszeit betrug 95 Monate (Spanne 8–171). Die lokale Tumorkontrolle nach 1 Jahr betrug 46,0% (Gruppe A) und 69,0% (Gruppe B), ohne dass dieser Unterschied statistisch signifikant wäre (p = 0,10). Das mediane Gesamtüberleben betrug jeweils 15 und 19 Monate in Gruppe A and B. Die 1-Jahres-Überlebensrate betrug jeweils 56,0% and 62%. Es wurde kein Unterschied zwischen den beiden Gruppen gefunden (p = 0,40).Schlussfolgerung:Die chirurgische Entfernung bleibt die Behandlungsoption bei symptomatischen Patienten. Unsere Daten sprechen für eine Behandlung mit WBRT plus SRT bei Vorliegen von ein oder zwei Hirnmetastasen, die kleiner als 3 cm sind.