Purpose Stereotactic Body Radiation Therapy (SBRT) for patients with early stage solid tumor is increasing thanks to modern technologies and integrated image-guided systems. A national study to evaluate the influence of calculation grid resolution and CT slice thickness on small fields calculated in an homogeneous phantom, is in progress. For this purpose, a preliminary investigation that collected different parameters on TPS management at Italian level, was performed. Methods Twenty-eight centres of the Italian Association of Medical Physics (AIFM) SBRT working group were invited to fill out a questionnaire expressly prepared by expert Medical Physicists using Google Forms. Questions regarded: type of accelerator, MLC characteristics, CT images scan information, Treatment Planning System (TPS) type, strategy used for SBRT plans, small fields OF values, detector/detectors and set-up adopted for measurements. Results Preliminary results from 28 centers show great heterogeneities. Most centers used LINACs with MLC width ⩽5 mm, 6MV and VMAT technique. 60% of centers used more than one detector to measure OFs and no corrective factor was adopted in small field data. 10 × 10 cm2 field size was the normalization field. OF data inserted in TPS did not come from a multicenter OF measurements. 1 × 1 cm2 was the smallest field size measured for 47% of centers. As for TPS, PB, CCC, MC, AAA and deteministic algorithms were used in 3%, 40%, 23%, 27% and 7% of centers, respectively. About 29% and 21 % of centers used 3 mm and 2 mm as CT slice thickness, respectively. 50% of centers used 2 mm calculation grid. Conclusions The influence of the dose calculation resolution and CT slice thickness on small OFs has not been studied yet. This preliminary study, considered as the starting point, highlighted a great spread of answers of the participating center and could be used as a baseline for future investigations.

Aim: Because of extremely modulated dose distributions in radiation therapy, dosimetric plan verification with a simple, fast and reliable system is a crucial step. Three different techniques were studied, i.e. Gafchromic (EBT3) films in anthropomorphic phantom (CIRS ATOM®-701), 729 ionization chamber matrix in cylindrical phantom rotating together with the gantry (Octavius®-4D), and ionization chamber in water phantom (PTW-MP1/MP3) with independent absolute point dose (APD) measurements. The aim was to characterize the three approaches and their relative agreement.

Aim: Octavius-4D is a treatment plan quality assurance tool for IMRT/VMAT verification. It is composed of a polystyrene cylindrical phantom, rotating together with the gantry, coupled with a 729 ionization chamber array. Beginning from the device commissioning, main response dependences were studied.

The goal of establishing prompt localization of the malignant spread or recurrence of a tumor has found a powerful solution in the definition of follow-up protocols, which include the indication for CT scans on an annual or semiannual basis. In the case of long-surviving patients, however, this approach will lead to a considerable integrated dose level over a period of several years after recovery from the illness. Pathologies treated primarily by surgery and/or chemotherapy have been considered, not taking into account cancers treated with adjuvant or radical radiotherapy. Given that the most likely protocols for these cancers often call for total body scans, an estimation of the consequent effective and organ doses can be performed with acceptable accuracy. The data acquired from five centers have been collected and the related effective and organ doses calculated by means of IMPACT software. Use of the effective dose concept, however, has lately become the subject of criticism, and the recently proposed Effective Risk Model has therefore also been applied. The evaluated absolute additional risk of second tumor induction ranges between 0.1% and 10%, depending primarily on age and pathology. These results depict this additional risk as an issue of significant importance for clinical practice. A revision of follow-up and scan parameter protocols, as well as the introduction of new algorithms for dose reduction, could significantly improve the risk-benefit ratio for all the pathologies studied. Health Phys. 104(1): 1-8; 2013

Background: Intensity-modulated radiation therapy (IMRT) was suggested as a suitable technique to protect the rectal wall, while maintaining a satisfactory planning target volume (PTV) irradiation in the case of high-dose radiotherapy of prostate cancer. However, up to now, few investigations tried to estimate the expected benefit with respect to conventional three-dimensional (3D) conformal radiotherapy (CRT).Purpose: Estimating the expected clinical gain coming from both 1D and 2D IMRT against 3DCRT, in the case of prostate cancer by mean of radiobiological models. In order to enhance the impact of IMRT, the case of concave-shaped PTV including prostate and seminal vesicles (P + SV) was considered.Materials and methods: Five patients with concave-shaped PTV including, P + SV were selected. Two different sets of constraints were applied during planning: in the first one a quite large inhomogeneity of the dose distribution within the PTV was accepted (set (a)); in the other set (set (b)) a greater homogeneity was required. Tumor control probability (TCP) and normal tissue control probability (NTCP) indices were calculated through the Webb-Nahum and the Lyman-Kiitcher models, respectively. Considering a dose interval from 64.8 to 100.8 Gy, the value giving a 5% NTCP for the rectum was found (D(NTCPrectum = 5%)) using two different methods, and the corresponding TCP(NTCPrectum = 5%) and NTCP(NTCPrectum = 5%) for the other critical structures were derived. With the first method, the inverse optimization of the plans was performed just at a fixed 75.6 Gy ICRU dose; with the second method (applied to 2/5 patients) inverse treatment plannings were re-optimized at many dose levels (from 64.8 to 108 Gy with 3.6 Gy intervals). In this case, three different values of alpha/beta (10, 3, 1.5)were used for TCP calculation. The 3DCRT plan consisted of a 3-fields technique; in the IMRT plans, five equi-spaced beams were applied. The Helios Inverse Planning software from Varian was used for both the 2D IMRT and the 1D IMRT inverse optimization, the last one being performed fixing only one Available pair of leaves for modulation. A previously proposed forward 1 D IMRT 'class solution' technique was also considered, keeping the same irradiation geometry of the inversely optimized IMRT techniques.Results: With the first method, the average gains in TCP(NTCPrectum = 5%) of the 2D IMRT technique, with respect 3DCRT, were 10.3 and 7.8%, depending on the choice of the DVHs constraints during the inverse optimization procedure (set (a) and set (b), respectively). The average gain (ATCP(NTCPrectum = 5%)) coming from the inverse ID IMRT optimization was 5.0%, when fixing the set (b) DVHs constraints. Concerning the forward ID IMRT optimization, the average gain in TCP(NTCPrectum = 5%) was 4.5%. The gain was found to be correlated with the degree of overlapping between rectum and PTV. When comparing 2D IMRT and ID IMRT, in the case of the more realistic set (b) constraints, DeltaTCP(NTCPrectum = 5%) was always less than 3%, excepting one patient with a very large overlap region. Basing our choice on this result, the second method was applied to this patient and one of the remaining. Through the inverse re-optimization of the treatment plans at each dose level, the gain in TCP(NTCPrectum = 5%) of the inverse 2D technique was significantly higher than the ones obtained by applying the first method (concerning the two patients: +6.1% and +2.4%), while no significant benefit was found for inverse ID. The impact of changing the alpha/beta ratio was less evident in the patient with the lower gain in TCP(NTCPrectum = 5%).Conclusions: The expected benefit due to IMRT with respect to 3DCRT seems to be relevant when the overlap between PTV and rectum is high. Moreover, the difference between the inverse 2D and the simpler inverse or forward ID IMRT techniques resulted in being relatively modest, with the exception of one patient, having a very large overlap between rectum and PTV. Optimizing the inverse planning at each dose level to find TCP(NTCPrectum = 5%) can improve the performances of inverse 2D IMRT, against a significant increase of the time for planning. These results suggest the importance of selecting the patients that could have significant benefit from the application of IMRT. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.

Background: In vivo dosimetry is widely considered to be an important tool for quality assurance in external radiotherapy.Introduction: In this study we report on our experience over more than 4 years in systematic in vivo dosimetry with diodes.Materials and methods: From November '94 an in vivo entrance dosimetry check was performed for every new patient irradiated at one of our treatment units (Linac 6/100, 6 MV X-rays). Diodes were calibrated in terms of entrance dose; appropriate correction factors had been previously assessed (taking SSDs, field width, wedge, oblique incidence and blocking tray into account) and were individually applied to in vivo diode readings. The in vivo measured entrance dose was compared with the expected one, with a 5% action level; if a larger deviation was found, all treatment parameters were verified, and the in vivo dosimetry check was repeated. During the period November '94-May '99, 2824 measurements on 1433 patients were collected.Results: Nine out of 1433 (0.63%) serious systematic errors (leading to a 5% or more on the delivered dose to the PTV) were detected by in vivo dosimetry; four out of nine would produce a 10% or more error if not detected. The rate of serious systematic errors detected by an independent check of treatment chart and MU calculation was found to be 1.5%, showing that less than 1/3 of the errors escapes this check. One hundred and twelve out of 1433 (7.8%) patients had more than one check: the rate of second checks was significantly higher for breast patients (31/250, 12.4%) against non-breast patients (81/1183, 6.8%, P = 0.003). A number of patients demonstrated a persistent relatively large error even after two or more checks. For almost all patients the cause of the deviation was assessed; the most frequent cause was the difficulty in correctly positioning the patient and/or the diode. When analyzing the distribution of the deviations between measured and expected entrance doses (excluding first checks in the case of repetition of the in vivo dosimetry control) the mean deviation was 0.4% with a standard deviation equal to 3.0%. The rates of deviations larger than 5 and 7% were 9.9 and 2.6%, respectively. When considering the same data taking the average deviation in the case of opposed beams, the SD became 2.6% and the rates of deviations larger than 5 and 7%, respectively, 5.2 and 0.8%. When dividing the beams according to their orientation, significantly higher rates of large deviations (>5 and 7%) were found for oblique and posterior-anterior (PA) fields against lateral and anterior-posterior (AP) fields (P < 0.05). Similarly, higher rates of large deviations were found for wedged fields against unwedged fields (P < 0.03) and for blocked fields against unblocked fields (P < 0.01). When dividing the data according to the anatomical district, accuracy was worse for breast (mean deviation 0.1%, 1 SD: 3.5%) and neck AP-PA fields (mean deviation 1%, 1 SD: 3,4%). Better accuracy was found for vertebrae (0.1%, 1 SD 2.1%) and brain patients (-0.7%, 1 SD: 2.6%). During the considered period, in vivo dosimetry was also able to promptly detect a systematic error caused by a wrong resetting of the simulator height couch indicator, with a consequent error in the estimate of patient thickness of about 4 cm.Conclusions: In our experience, systematic in vivo dosimetry demonstrated to be a valid tool for quality assurance, both in detecting systematic errors which may escape the data transfer/MU calculation check and in giving an effective way of estimating the accuracy of treatment delivery. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.

Background: In the case of concave-shaped PTVs including prostate (P) and seminal vesicles (SV), intensity-modulated radiation therapy (IMRT) should improve the therapeutic ratio of the treatment of prostate cancer.Purpose: Comparing IMRT by simple 1D modulations with conventional 3D conformal therapy (i.e. non-IMRT) in the treatment of concave-shaped PTVs including P+SV.Materials and methods: For five patients having a concave-shaped PTV (P+SV) previously treated at our Institute with conformal radiotherapy, conventional 3- and 4-fields conformal plans were compared with IMRT plans in terms of biological indices. IMRT plans were generated by using five equi-spaced beams with a partial shielding of the rectum obtainable with our single-absorber modulation technique (Fiorino C. Lev A. Fusca M. Cattaneo GM, Rudello F. Calandrino R. Dynamic beam modulation by using a single dynamic absorber. Phys. Med. Biol. 1995;40:221-240). The modulation was one-dimensional and the shape of the beams was at single minimum in correspondence with the 'core' of the rectum; the beam intensity in the minimum was set equal to 20 or 40% of the open beam intensity. All plans were simulated on the CADPLAN TPS using a pencil-beam based algorithm (with 18 MV X-rays). Tumour control probability (TCP) and normal tissue complication probabilities (NTCPs) (for rectum, bladder and femoral head) were calculated for all situations when varying the isocentre dose from 60 to 90 Gy. Dose distributions were corrected taking dose fractionation into account through the linear-quadratic model; for the TCP/NTCP estimations the Webb-Nahum and the Lyman-Kutcher models were respectively applied. Three different scores were considered: (a) increase of TCP while keeping rectum NTCP equal to 5% (TCP(5%)); (b) increase of the uncomplicated tumour control probability (P+); (c) increase of the biological-based scoring function (S+), developed by Mohan et al. (Mohan R, Mageras GS, Baldwin B, Clinically relevant optimization of 3D conformal treatments. Med. Phys, 1992;19:933-944). The impact of the uncertainty in the knowledge of the parameters of the biological models was investigated for TCP(5%).Results: (a) The average gain in TCP(5%) when considering IMRT against non-IMRT conformal plans was 7.3% (range 5.0-13.5%); (b) the average increase of P+ was 3.4% (range: 1.0-8.5%); and (c) the average increase of S+ was 5.4% (range 2.9-12.4%). The largest gain was found for one patient (patient 5) showing a significantly larger overlapping between PTV and rectum.Conclusions: Simple 1D-IMRT may clearly improve the therapeutic ratio in the treatment of concave-shaped PTVs including P and SV. In the range of clinically suitable values, the impact of the uncertainty of the parameters n and sigma(alpha) does not significantly alter the main results concerning the gain in TCP(5%). The reported gain in terms of P+ and S+ should be considered with great caution because of the intrinsic uncertainties of the model's parameters and, for bladder, because the 'true' DVH (considering variations of the shape and dimension due to variable filling) may be very different from the DVH calculated on a single CT scan. Further investigations should consider inversely-optimised 1D and 2D-IMRT plan in order to compare them in terms of cost-benefit. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.

Dynamic modulation can be affected by inaccuracies when the required acceleration is larger than the highest allowed by the mechanical characteristics of the whole apparatus. In this study, inertia effects have been investigated with regard to the single absorber 1D modulation, analysing primarily how the acceleration performed by the modulating system affects the realization of 'single absorber' fluence profiles and the type of correction which could be devised. The observed percentage deviations from desired modulation at the lowest fluence coordinate of single minimum fluence profiles, when no correction is applied, were almost negligible for 'easy' modulations of the incident fluence (i.e. slow gradients); deviations became increasingly relevant as the moving absorber executed steeper gradients (a 17.6% higher dose being delivered in the minimum position when a 0.2 modulation is required). By applying the proposed corrections, the single absorber performances were improved to a satisfactory level, with a maximum deviation from desired modulation in the minima within 1.6%.

Background and purpose: In this study the possibilities for implementing 1D tissue-deficit compensation techniques by a dynamic single absorber were investigated. This research firstly involved a preliminary examination on the accuracy of a pencil beam-based algorithm, implemented for irregularly shaped photon beams in our 3D treatment planning system (TPS) (Cadplan 2.7, Varian-Dosetek Oy), in calculating dose distributions delivered in 1D non-uniform fields. Once the reliability of the pencil beam (PB) algorithm for dose calculations in non-uniform beams was verified, we proceeded to test the feasibility of tissue-deficit compensation using our single absorber modulator. As an example, we considered a mantle field technique.Materials and methods: To evaluate the accuracy of the method employed in calculating dose distributions delivered in ID non-uniform fields, three different fluence profiles, which could be considered as a small sample representative of clinically relevant applications, were selected. The incident non-uniform fluences were simulated by the sum of simple blocked fields (i.e. with rectangular 'strip' blocks, one per beam) properly weighed by the 'modulation factors' F-i, defined in each interval of the subdivided profile as the ratio between the desired fluence and the open field fluence. Depth dose distributions in a cubic phantom were then calculated by the TPS and compared with the corresponding doses (at 5 and 10 cm acrylic depths) delivered by the single absorber modulation system. In the present application, the absorber speed profile able to compensate for the tissue deficit along the cranio-caudal direction and then homogenizing the dose distribution on a 'midline' isocentric plane with sufficient accuracy can be directly derived from anatomic data, such as the SSDs (source-skin distances) along the patient contour. The compensation can be verified through portal dosimetry techniques (using a traditional port film system).Results: The technique was tested in isocentric conditions on the humanoid RANDO phantom in a clinically suitable situation. The agreement between expected/calculated and measured incident/exit dose profiles was found to be within 4%, with deviations generally around 1-2%. As for the PB accuracy investigation for dose calculations in non-uniform fields, calculated versus measured dose profiles were found to be in good agreement, indicating a satisfactory accuracy of the method employed for dose calculation in 1D non-uniform photon beams. A better performance should be expected if the incident fluences could be directly inserted in the TPS.Conclusions: The results show that the proposed technique should be sufficiently reliable for clinical application. The main advantages are its simplicity and the possibility of application on Linacs which have no complex options for dynamic control of collimators. (C) 1998 Elsevier Science Ireland Ltd.