New method to obtain the midplane dose using portal in vivo dosimetry

Purpose: The aim of this study was to develop a method to derive the midplane dose [i.e., the two-dimensional (2D) dose distribution in the middle of a patient irradiated with high-energy photon beams] from transmission dose data measured with an electronic portal imaging device (EPID). A prerequisite for this method was that it could be used without additional patient information (i.e., independent of a treatment-planning system). Second, we compared the new method with several existing (conventional) methods that derive the midline dose from entrance and exit dose measurements. Methods and Materials: The proposed method first calculates the 2D contribution of the primary and scattered dose component at the exit side of the patient or phantom from the measured transmission dose. Then, a correction is applied for the difference in contribution for both dose components between exit side and midplane, yielding the midplane dose. To test the method, we performed EPID transmission dose measurements and entrance, midplane, and exit dose measurements using an ionization chamber in homogeneous and symmetrical inhomogeneous phantoms. The various methods to derive the midplane dose were also tested for asymmetrical inhomogeneous phantoms applying two opposing fields. A number of combinations of inhomogeneities (air, cork, and aluminum), phantom thicknesses, field sizes, and a few irregularly shaped fields were investigated, while each experiment was performed in 4-, 8-, and 18-MV open and wedged beams. Results: Our new method can be used to assess the midplane dose for most clinical situations within 2% relative to ionization chamber measurements. Similar results were found with other methods. In the presence of large asymmetrical inhomogeneities (e.g., lungs), discrepancies of about 8% have been found (for small field sizes) using our transmission dose method, owing to the absence of lateral electron equilibrium. Applying the other methods, differences between predicted and measured midplane doses were even larger, up to 10%. For large field sizes, the agreement between measured and predicted midplane dose was within 3% using our transmission dose method. Conclusions: Using our new method, midplane doses were estimated with a similar or higher accuracy compared with existing conventional methods for in vivo dosimetry. The advantage of our new method is that the midplane dose can be determined in the entire (2D) field. With our method, portal in vivo dosimetry is an accurate alternative for conventional in vivo dosimetry

In vivo portal dosimetry for head-and-neck VMAT and lung IMRT: linking γ-analysis with differences in dose-volume histograms of the PTV.

Purpose: To relate the results of gamma-analysis and dose-volume histogram (DVH) analysis of the PTV for detecting dose deviations with in vivo dosimetry for two treatment sites. Methods and materials: In vivo 3D dose distributions were reconstructed for 722 fractions of 200 head-and-neck (H&N) VMAT treatments and 183 fractions of 61 lung IMRT plans. The reconstructed and planned dose distributions in the PTV were compared using (a) the gamma-distribution and (b) the differences in D2, D50 and D98 between the two dose distributions. Using pre-defined tolerance levels, all fractions were classified as deviating or not deviating by both methods. The mutual agreement, the sensitivity and the specificity of the two methods were compared. Results: For lung IMRT, the classification of the fractions was nearly identical for gamma- and DVH-analyses of the PTV (94% agreement) and the sensitivity and specificity were comparable for both methods. Less agreement (80%) was found for H&N VMAT, while gamma-analysis was both less sensitive and less specific. Conclusions: DVH- and gamma-analyses perform nearly equal in finding dose deviations in the PTV for lung IMRT treatments; for H&N VMAT treatments, DVH-analysis is preferable. As a result of this study, a smooth transition to using DVH-analysis clinically for detecting in vivo dose deviations in the PTV is within reach. (C) 2014 Elsevier Ireland Ltd. All rights reserve

Overview of 3-year experience with large-scale electronic portal imaging device-based 3-dimensional transit dosimetry

To assess the usefulness of electronic portal imaging device (EPID)-based 3-dimensional (3D) transit dosimetry in a radiation therapy department by analyzing a large set of dose verification results. In our institution, routine in vivo dose verification of all treatments is performed by means of 3D transit dosimetry using amorphous silicon EPIDs. The total 3D dose distribution is reconstructed using a back-projection algorithm and compared with the planned dose distribution using 3D gamma evaluation. Dose reconstruction and gamma evaluation software runs automatically in our clinic, and analysis results are (almost) immediately available. If a deviation exceeds our alert criteria, manual inspection is required. If necessary, additional phantom measurements are performed to separate patient-related errors from planning or delivery errors. Three-dimensional transit dosimetry results were analyzed per treatment site between 2012 and 2014 and the origin of the deviations was assessed. In total, 4689 of 15,076 plans (31%) exceeded the alert criteria between 2012 and 2014. These alerts were patient-related and attributable to limitations of our back-projection and dose calculation algorithm or to external sources. Clinically relevant deviations were detected for approximately 1 of 430 patient treatments. Most of these errors were because of anatomical changes or deviations from the routine clinical procedure and would not have been detected by pretreatment verification. Although cone beam computed tomography scans yielded information about anatomical changes, their effect on the dose delivery was assessed quantitatively by means of 3D in vivo dosimetry. EPID-based transit dosimetry is a fast and efficient dose verification technique. It provides more useful information and is less time-consuming than pretreatment verification measurements of intensity modulated radiation therapy and volumetric modulated arc therapy. Large-scale implementation of 3D transit dosimetry is therefore a powerful method to guarantee safe dose delivery during radiation therap

Anatomy changes in radiotherapy detected using portal imaging.

BACKGROUND AND PURPOSE: Localisation images normally acquired to verify patient positioning also contain information about the patient's internal anatomy. The aim of this study was to investigate the anatomical changes observed in localisation images and examples of dosimetric consequences. PATIENTS AND METHODS: Localisation images were obtained weekly prior to radiotherapy with an electronic portal imaging device (EPID). A series of 'difference images' was created by subtracting the first localisation image from that of subsequent fractions. Images from 81 lung, 40 head and neck and 34 prostate cancer patients were classified according to the changes observed. Changes were considered relevant if the average pixel value over an area of at least 1cm(2) differed by more than 5%, to allow for variations in linac output and EPID signal. Two patients were selected to illustrate the dosimetric effects of relevant changes. Their plans were re-calculated with repeat CT scans acquired after 4 weeks of treatment and compared with the difference images of the corresponding days. RESULTS: Progressive changes were detected for 57% of lung and 37% of head and neck cancer patients studied. Random changes were observed in 37% of lung, 28% of head and neck and 82% of prostate cancer patients. For a lung case, an increase of 10.0% in EPID dose due to tumour shrinkage corresponded to an increase of 9.8% in mean lung dose. Gas pockets in the rectum region of the prostate case increased the EPID dose by 6.3%, and resulted in a decrease of the minimum dose to the planning target volume of 26.4%. CONCLUSIONS: Difference images are an efficient means of qualitatively detecting anatomical changes for various treatment sites in clinical practice. They can be used to identify changes for a particular patient, to indicate if the dose delivered to the patient would differ from planning and to detect if there is a need for re-plannin

Replacing pretreatment verification with in vivo EPID dosimetry for prostate IMRT.

PURPOSE: To investigate the feasibility of replacing pretreatment verification with in vivo electronic portal imaging device (EPID) dosimetry for prostate intensity-modulated radiotherapy (IMRT). METHODS AND MATERIALS: Dose distributions were reconstructed from EPID images, inside a phantom (pretreatment) or the patient (five fractions in vivo) for 75 IMRT prostate plans. Planned and EPID dose values were compared at the isocenter and in two dimensions using the gamma index (3%/3 mm). The number of measured in vivo fractions required to achieve similar levels of agreement with the plan as pretreatment verification was determined. The time required to perform both methods was compared. RESULTS: Planned and EPID isocenter dose values agreed, on average, within +/-1% (1 SD) of the total plan for both pretreatment and in vivo verification. For two-dimensional field-by-field verification, an alert was raised for 10 pretreatment checks with clear but clinically irrelevant discrepancies. Multiple in vivo fractions were combined by assessing gamma images consisting of median, minimum and low (intermediate) pixel values of one to five fractions. The "low" gamma values of three fractions rendered similar results as pretreatment verification. Additional time for verification was approximately 2.5 h per plan for pretreatment verification, and 15 min +/- 10 min/fraction using in vivo dosimetry. CONCLUSIONS: In vivo EPID dosimetry is a viable alternative to pretreatment verification for prostate IMRT. For our patients, combining information from three fractions in vivo is the best way to distinguish systematic errors from non-clinically relevant discrepancies, save hours of quality assurance time per patient plan, and enable verification of the actual patient treatmen

Interinstitutional variations of sensitometric curves of radiographic dosimetric films

Depth and field size dependence of the sensitometric curves of radiographic films have been studied by various groups. Limited information is, however, available on the magnitude of the variations in sensitometric curves applied in clinical practice in different institutions. In this study we assessed in a systematic way the effect of the various parameters influencing the shape of the sensitometric curve: batch composition, irradiation conditions, film processing, and film scanning. Two types of film, Kodak X-Omat V and CEA TVS, were irradiated, processed, and analyzed in three different institutions. The interinstitutional variation of the sensitometric curves, expressed as the OD variation at 50 cGy, can be up to 32% and is mainly caused by differences in film processing and to a lesser degree to differences in batch composition, film scanning, and irradiation conditions. For the Kodak films, the average OD difference at 50 cGy between the three institutions is 17% as a result of differences in batch composition and 25% due to differences in processing conditions. For the CEA films these data are 6% and 24%, respectively. The long-term variation of the sensitometric curves of KODAK films in one institution was smaller than the differences in batch composition between the three institutions. The sensitometric curves of CEA films showed in one institution a large variation with time; the shape gradually varied from sigmoidal to quasilinear. By using relative OD values rather than absolute OD values, variations in sensitometric curves of KODAK films can be reduced to 2%. Consequently, one sensitometric curve is sufficient to derive relative dose values. If processing conditions are well controlled, it might therefore be advantageous to determine the absolute OD only at one or two dose values, in combination with a "universal" relative sensitometric curv

Impact of daily anatomical changes on EPID-based in vivo dosimetry of VMAT treatments of head-and-neck cancer.

Background and purpose: Target dose verification for VMAT treatments of head-and-neck (H&N) cancer using 3D in vivo EPID dosimetry is expected to be affected by daily anatomical changes. By including these anatomical changes through cone-beam CT (CBCT) information, the magnitude of this effect is investigated. Materials and methods: For 20 VMAT-treated H&N cancer patients, all plan-CTs (pCTs), 633 CBCTs and 1266 EPID movies were used to compare four dose distributions per fraction: treatment planning system (TPS) calculated dose and EPID reconstructed in vivo dose, both determined using the pCT and using the CBCT. D2, D50 and D98 of the planning target volume (PTV) were determined per dose distribution. Results: When including daily anatomical information, D2, D50 and D98 of the PTV change on average by 0.0 +/- 0.4% according to TPS calculations; the standard deviation of the difference between EPID and TPS target dose changes from 2.5% (pCT) to 2.1% (CBCT). Small time trends are seen for both TPS and EPID dose distributions when using the pCT, which disappear when including CBCT information. Conclusions: Daily anatomical changes hardly influence the target dose distribution for H&N VMAT treatments according to TPS recalculations. Including CBCT information in EPID dose reconstructions slightly improves the agreement with TPS calculations. (C) 2015 Elsevier Ireland Ltd. All rights reserve

A simple backprojection algorithm for 3D in vivo EPID dosimetry of IMRT treatments.

Treatment plans are usually designed, optimized, and evaluated based on the total 3D dose distribution, motivating a total 3D dose verification. The purpose of this study was to develop a 2D transmission-dosimetry method using an electronic portal imaging device (EPID) into a simple 3D method that provides 3D dose information. In the new method, the dose is reconstructed within the patient volume in multiple planes parallel to the EPID for each gantry angle. By summing the 3D dose grids of all beams, the 3D dose distribution for the total treatment fraction is obtained. The algorithm uses patient contours from the planning CT scan but does not include tissue inhomogeneity corrections. The 3D EPID dosimetry method was tested for IMRT fractions of a prostate, a rectum, and a head-and-neck cancer patient. Planned and in vivo-measured dose distributions were within 2% at the dose prescription point. Within the 50% isodose surface of the prescribed dose, at least 97% of points were in agreement, evaluated with a 3D gamma method with criteria of 3% of the prescribed dose and 0.3 cm. Full 3D dose reconstruction on a 0.1 x 0.1 x 0.1 cm3 grid and 3D gamma evaluation took less than 15 min for one fraction on a standard PC. The method allows in vivo determination of 3D dose-volume parameters that are common in clinical practice. The authors conclude that their EPID dosimetry method is an accurate and fast tool for in vivo dose verification of IMRT plans in 3D. Their approach is independent of the treatment planning system and provides a practical safety net for radiotherap

Impact of geometrical uncertainties on 3D CRT and IMRT dose distributions for lung cancer treatment

PURPOSE: To quantify the effect of set-up errors and respiratory motion on dose distributions for non-small cell lung cancer (NSCLC) treatment. METHODS AND MATERIALS: Irradiations of 5 NSCLC patients were planned with 3 techniques, two (conformal radiation therapy (CRT) and intensity modulated radiation therapy (IMRT1)) with a homogeneous dose in the planning target volume (PTV) and a third (IMRT2) with dose heterogeneity. Set-up errors were simulated for gross target volume (GTV) and organs at risk (OARs). For the GTV, the respiration was also simulated with a periodical motion around a varying average. Two configurations were studied for the breathing motion, to describe the situations of free-breathing (FB) and respiration-correlated (RC) CT scans, each with 2 amplitudes (5 and 10 mm), thus resulting in 4 scenarios (FB_5, FB_10, RC_5 and RC_10). Five thousand treatment courses were simulated, producing probability distributions for the dosimetric parameters. RESULTS: For CRT and IMRT1, RC_5, RC_10 and FB_5 were associated with a small degradation of the GTV coverage. IMRT2 with FB_10 showed the largest deterioration of the GTV dosimetric indices, reaching 7% for Dmin at the 95% probability level. Removing the systematic error due to the periodic breathing motion was advantageous for a 10 mm respiration amplitude. The estimated probability of radiation pneumonitis and acute complication for the esophagus showed limited sensitivity to geometrical uncertainties. Dmax in the spinal cord and the parameters predicting the risk of late esophageal toxicity were associated to a probability up to 50% of violating the dose tolerances. CONCLUSIONS: Simulating the effect of geometrical uncertainties on the individual patient plan should become part of the standard pre-treatment verification procedur

The sensitivity of dose distributions for organ motion and set-up uncertainties in prostate IMRT.

BACKGROUND AND PURPOSE: To determine the effect of organ motion and set-up uncertainties on IMRT dose distributions for prostate. METHODS: For five patients, IMRT techniques were designed to irradiate the CTV (prostate plus seminal vesicles). Technique I delivered 78 Gy to PTV1 (CTV+10 mm margin). Technique II delivered 68 Gy to PTV1, and a 10 Gy boost to PTV2 (CTV+an anisotropic margin of 0 to 5 mm). Technique III delivered 68 Gy to PTV1 and simultaneously 78 Gy to PTV2. Uncertainties were simulated using population statistics of organ motion and set-up accuracy. The average TCP (TCPpop) of the CTV and average NTCP (NTCPpop) of the rectal wall were calculated. RESULTS: The planning TCP was a good predictor for TCPpop for Techniques I and II. Technique III was sensitive for geometrical uncertainties, reducing TCPpop by 0.8 to 2.4% compared to planning. NTCPpop was reduced for Technique III by a factor 2.6 compared to Technique I. For all plans, the planning NTCP was strongly correlated with NTCPpop. CONCLUSIONS: Dose distributions created with Techniques I and II are insensitive for geometrical uncertainties, while Technique III resulted in a reduction of TCPpop. This reduction can be compensated by a small dose escalation, while still resulting in an NTCPpop of the rectal wall that is lower or comparable to Technique