Typical values of z-resolution for different Digital Breast Tomosynthesis systems evaluated in a multicenter study.

PURPOSE: The aim of the present study, conducted by a working group of the Italian Association of Medical Physics (AIFM), was to define typical z-resolution values for different digital breast tomosynthesis (DBT) models to be used as a reference for quality control (QC). Currently, there are no typical values published in internationally agreed QC protocols. METHODS: To characterize the z-resolution of the DBT models, the full width at half maximum (FWHM) of the artifact spread function (ASF), a technical parameter that quantifies the signal intensity of a detail along reconstructed planes, was analyzed. Five different commercial phantoms, CIRS Model 011, CIRS Model 015, Modular DBT phantom, Pixmam 3-D, and Tomophan, were evaluated on reconstructed DBT images and 82 DBT systems (6 vendors, 9 models) in use at 39 centers in Italy were involved. RESULTS: The ASF was found to be dependent on the detail size, the DBT angular acquisition range, the reconstruction algorithm and applied image processing. In particular, a progressively greater signal spread was observed as the detail size increased and the acquisition angle decreased. However, a clear correlation between signal spread and angular range width was not observed due to the different signal reconstruction and image processing strategies implemented in the algorithms developed by the vendors studied. CONCLUSIONS: The analysis led to the identification of typical z-resolution values for different DBT model-phantom configurations that could be used as a reference during a QC program.

Biological optimization of simultaneous boost on intra-prostatic lesions (DILs): sensitivity to TCP parameters.

The aim of this investigation was to explore the potential of biological optimization in the case of simultaneous integrated boost on intra-prostatic dominant lesions (DIL) and evaluating the impact of TCP parameters uncertainty. Different combination of TCP parameters (TD50 and γ50 in the Poisson-like model), were considered for DILs and the prostate outside DILs (CTV) for 7 intermediate/high-risk prostate patients. The aim was to maximize TCP while constraining NTCPs below 5% for all organs at risk. TCP values were highly depending on the parameters used and ranged between 38.4% and 99.9%; the optimized median physical doses were in the range 94-116 Gy and 69-77 Gy for DIL and CTV respectively. TCP values were correlated with the overlap PTV-rectum and the minimum distance between rectum and DIL. In conclusion, biological optimization for selective dose escalation is feasible and suggests prescribed dose around 90-120 Gy to the DILs. The obtained result is critically depending on the assumptions concerning the higher radioresistence in the DILs. In case of very resistant clonogens into the DIL, it may be difficult to maximize TCP to acceptable levels without violating NTCP constraints.

A practical dead time correction method in planar activity quantification for dosimetry during radionuclide therapy.

AIM: Gamma camera saturation is the first quantification problem in dosimetric studies following therapeutic administrations of 131I labeled radiopharmaceuticals. A new approach for dead time correction (DTC) is here proposed. It employs planar whole-body (WB) images without the need of standard radionuclide sources or of preliminary phantom calibrations. METHODS: Step and shoot WB acquisitions of the patient are required. A program was developed to compensate for the image discontinuities ("Continuity DTC method") between two adjacent static fields of view (FOVs) caused by different dead time count losses. For its validation, authors used two 99mTc 6 GBq phantom scans after administration of six patients with 131I labeled agents with different statistics and ten clinical scans taken between 16 h and 48 h after administration of 131I labeled agents, whose activity ranged from 4 to 10 GBq. The deviation from true decay corrected counts on phantoms and the constancy of monitor point-source counts in different patients' FOVs (root mean square error and maximum deviation) served as figures of merit. The accuracy of absorbed dose calculation was also estimated by comparison with the standard source correction method, computing the area under the time activity curve (AUC) of six lesions. RESULTS: With respect to the true phantom counts, corrected images gave excellent results, giving a 6% maximum deviation. For what concerns the other figures of merit, continuity DTC reduced the average root mean square error from 36% to 2% and the mean maximum deviation from 50% to 2%, on phantom, while from 51% to 32/28% (absence/presence of triple energy window scatter correction) and from 72% to 21/14% on patients. Mean compensation of AUC gave a correction of +56% with our method, while +78% with standard source method. CONCLUSIONS: The "Continuity DTC method" is a useful tool in dosimetry during nuclear medicine treatment, showing good accuracy. Moreover, since it does not require the use of any source, it provides with several advantages in terms of practicability and applicability, with respect to the standard source method and to methods based on the count rate characteristic curve.

Individualized dosimetry in the management of metastatic differentiated thyroid cancer.

AIM: This paper analyzes the available data on the dosimetric approach and describes the use of dosimetry in the Division of Nuclear Medicine of the National Cancer Institute in Milan. Dosimetry is rarely performed when planning radio-iodine activity, although most of the available guidelines do mention this possibility, without giving any well defined indication. Aim of the present research was to validate the usefulness of dosimetry in the management of metastatic thyroid cancer. Benua (1962) set the limit of blood absorbed dose at 2 Gy to avoid hematological toxicity. Maxon (1983) determined at 80 Gy the dose to achieve complete destruction of a metastatic lesion. Dorn (2003) combined red marrow and lesion dosimetry showing that high activity administrations with less that 3 Gy to the red marrow are a safe and more effective with respect to fixed activities administrations. Lee (2008) reported 50% responses with high activity administrations based on blood dosimetry, in 47 patients which were unsuccessfully previously treated with fixed activities. Sgouros (2005) and Song (2006) introduced key parameters as Biological Effective Dose and Uniform Equivalent Dose in order to describe the effects of continuous low dose rate irradiation and non uniform activity uptake, typical of nuclear medicine treatments. METHODS: Red marrow and lesion dosimetry (planar view) were performed during the treatment, without changing the fixed activity schema. RESULTS: This experience demonstrate first of all, that dosimetry is feasible in the clinical routine, and that it can provide the clinician with important information, no matter its often quoted limited numerical accuracy. A total of 17/20 lesion doses below 80 Gy have been detected. Three/17 (doses between 40 and 80 Gy) disappeared in the follow-up scintigram. Two/17 were undetectable at computed tomography or nuclear magnetic resonance. These data suggest that repetition of treatment on a lesion drastically reduces its uptake, with a loss of therapeutic efficacy along the sequence of fixed activity administrations. CONCLUSIONS: The usefulness of dosimetry should not be assessed only on the basis of patient survival or therapeutic efficacy; the possibility to avoid useless treatments should also be considered. According to the authors, individualized dosimetry could improve the management of metastatic differentiated thyroid cancer. Even post-therapeutic dosimetry, as performed at this institution, has a significant impact on clinical decision-making. The question for the future is how to include dosimetry into the patient management framework.