R Srivastava

The value of EDR2 film dosimetry in compensator-based intensity modulated radiation therapy

Radiographic or silver halide film is a well-established 2D dosimeter with an unquestioned spatial resolution. But its higher sensitivity to low-energy photons has to be taken into consideration. Metal compensators or physical modulators to deliver intensity modulated radiation therapy (IMRT) are known to change the beam energy spectrum and to produce scattered photons and contaminating electrons. Therefore the reliability of film dosimetry in compensator-based IMRT might be questioned. Conflicting data have been reported in the literature. This uncertainty about the validity of film dosimetry in compensator-based IMRT triggered us to conduct this study. First, the effect of MCP-96 compensators of varying thickness on the depth dose characteristics was investigated using a diamond detector which has a uniform energy response. A beam hardening effect was observed at 6 MV that resulted in a depth dose increase that remained below 2% at 20 cm depth. At 25 MV, in contrast, beam softening produced a dose decrease of up to 5% at the same depth. Second, dose was measured at depth using EDR2 film in perpendicular orientation to both 6 MV and 25 MV beams for different compensator thicknesses. A film dose underresponse of 1.1% was found for a 30 mm thick block in a 25 MV beam, which realized a transmission factor of 0.243. The effect induced by the compensators is higher than the experimental error but still within the accepted overall uncertainty of film dosimetry in clinical IMRT QA. With radiographic film as an affordable QA tool, the physical compensator remains a low threshold technique to deliver IMRT.

Evaluation of a glassless photographic film scanner for high-gradient radiochromic film dosimetry

This study evaluates the performance of the Nikon Coolscan 9000 ED film scanner for high-gradient radiochromic film dosimetry. As a reference for comparison, analogue experiments were performed on the Epson Expression 10000XL flatbed scanner. Based on these results, a dosimetric protocol was established for the Nikon scanner and its overall performance for high-gradient dosimetry was evaluated. The Nikon scanner demonstrated a high sensitivity for radiochromic film dosimetry, resulting in more contrast in the digitized image. The scanners optics also demonstrated excellent stability and did not necessitate warm-up scans prior to data acquisition. Moreover, negative effects of temperature changes of the film inside the scanner were shown to be limited. None of the digitized images showed significant disturbances by moiré-patterns, by virtue of the absence of a glass plate for film positioning. However, scanner response was found to vary considerably across the reading area, requiring an optical density-dependent correction procedure to be incorporated into the scanning protocol. The main limitation of the Nikon Coolscan 9000 ED transmission scanner remains its film size restriction to 6.2 × 20 cm2. Nevertheless, its excellent characteristics render it the preferential tool for high-gradient radiochromic film dosimetry in applications limited to small film sizes, such as dosimetry in the build-up region.

A method of increasing the film intrinsic robustness of radiochromic film dosimetry

The radiochromic film, which is used, in combination with a flatbed scanner has become a widely used tool for a quantitative evaluation of radiation dose in radiation therapy. One aspect of uncertainty using the radiochromic film is the magnitude of orientation effects when the orientation of the film is not kept constant during the digitization process. The aim of this note was to investigate the impact of using a combination of two crossed sheets of EBT2 film on various aspects of radiochromic film dosimetry. First the impact on the film sensitivity was studied. We also investigated the influence on orientation effects during scanning. The results show that the double crossed film combination increases the sensitivity with a factor 1.7-2.1 and practically eliminates the effects of film orientation on the optical density read-out and the lateral correction profiles.

The effects of incidence angle on film dosimetry and their consequences in IMRT dose verification

PURPOSE The dosimetric accuracy of EDR2 radiographic film has been rigorously assessed in regular and intensity modulated beams for various incidence angles, including the parallel and perpendicular orientation. There clearly exists confusion in literature regarding the effect of film orientation. The primary aim is to clarify potential sources of the confusion and to gain physical insight into the film orientation effect with a link to radiochromic film as well. METHODS An inverse pyramid IMRT field, consisting of six regular and elongated 3 × 20 cm(2) field segments, was studied in perpendicular and parallel orientation. Assessment of film self-perturbation and intrinsic directional sensitivity were also included in the experiments. Finally, the authors investigated the orientational effect in composite beams in the two extreme orientations, i.e., perpendicular and parallel. RESULTS The study of an inverse pyramid dose profile revealed good agreement between the perpendicular film and the diamond detector within 0.5% in the low-scatter regions for both 6 and 18 MV. The parallel oriented film demonstrated a 3% under-response at 5-cm (6 MV) depth against the perpendicular orientation, but both orientations over responded equally in the central region, which received only scattered dose, at both 5- and 20-cm depths. In a regular 6-MV 5 × 5 cm(2) field, a 4.1% lower film response was observed in the parallel orientation compared to perpendicular orientation. The under response gradually increased to 6% when reducing the field size to 0.5 × 5 cm(2). On the other hand, the film showed a 1.7% lower response in parallel orientation for the large field size of 20 × 20 cm(2) at 5-cm depth but the difference disappeared at 10 cm. At 18 MV, similar but somewhat lower differences were found between the two orientations. The directional sensitivity of the film diminishes with increasing field size and depth. Surprisingly a composite IMRT beam consisting of 20 adjacent strip segments also produced a significant orientational dependence of film response, notwithstanding the large total field size of 20 × 20 cm(2). CONCLUSIONS This analysis allowed the development of a hypothesis about the physics behind the orientational dependence of film response in general and to formulate precautions when using film dosimetry in the dosimetric verification of multibeam treatments.

Output measurement for small field photon beams in a sandwiched phantom

Small field dosimetry is challenging for radiotherapy dosimetry. We measured output factors for small fields with EBT film in polystyrene and sandwiched phantom. A comprising a combination of polystyrene and lung equivalent slabs is named Sandwiched phantom. EBT film response is in good agreement (within 1.7%) with the diamond detector for polystyrene phantom. Due to the density variation between the sandwiched phantom layers, the difference between EBT and diamond detector responses is significantly larger than for the homogeneous polystyrene phantom.

SU-E-T-75: Application of Film Dosimetry and Comparison to Delta4 to Patient-Specific Preclinical Dosimetric Verification of RapidArc

PURPOSE Radiotherapy treatment modalities are becoming more complex in order to enable advanced patient treatments with a higher dose to irregularly shaped tumor volumes while sparing nearby organs at risk. VMAT as well as RapidArc incorporate capabilities such as variable doserate, variable gantry speed, and accurate and fast dynamic multileaf collimators (DMLC), to optimize dose conformity, delivery efficiency, accuracy and reliability. There is very little information on QA systems and techniques regarding the patient-specific QA. Therefore, we compared the results obtained with radiographic film to those from the Delta4 phantom. METHODS RapidArc treatment plans were generated using the Varian Eclipse treatment planning software version 8.9 (Varian Medical Systems, Palo Alto, CA). A single but complete (358°) RapidArc was planned. RapidArc treatment plans were delivered to the Delta4 (Delta4) cylindrical (1069 p-Si Silicon diodes) phantom from Scandidos Uppsala, Sweden, as well as to an equivalent home-made polyethylene (HD 1000) cylindrical phantom (two longitudinal halves, density 0.94 g/cm3 ) for EDR2 film dosimetry. RESULTS The correspondence between D4 and cylindrical phantom measured were analyzed in radial and longitudinal (GT) profiles. The film in the 40° plane displayed a ±3.1% agreement in radial and a 2.2% agreement in GT direction with Delta4 phantom. A small difference was found between the planes because the cylindrical phantom joining the two halves parts has some gaps which might generate the discrepancy between radial and GT direction. The film-measured dose in the isocenter of the 50° radial and GT plane showed an agreement within ±2.7% with the Delta4 phantom. CONCLUSION Film dosimetry validated the Delta4 measurements and it clearly provided more useful information than single point dose measurement. GHENT UNIVERSITY BOF08/DOS/052.

SU-E-E-02: The Affect of Backscatter and Later Phantom Dimension on Dosimetry of Radiographic Film

Purpose: The aim of this study is to determine the optimal backscatter thickness and later phantom dimension beyond the field size on the dosimetry verification with radiographic film when irradiating large volumes. Methods: The experiments were conducted for 6 and 18 MV 20 × 20 cm2 field sizes with XV2 and EDR2 films positioned at 10 cm depth inside (30 × 30 cm2) polystyrene and Virtual Water™ phantoms by varying the phantom backscattered thickness and changing the table height accordingly. A constant 100 cGy dose was delivered to EDR2 film for 6 and 18 MV by adapting the MU setting. A Farmer‐type ionization chamber was inserted centrally in a dedicated slab so that chamber position was kept at the same depth as the film. A laterally increased phantom studies were done by symmetrically increasing the lateral dimension in the gun‐target direction of the linear accelerator. Results: For 6 MV beam at 20 cm backscatter thickness, EDR2 film response in polystyrene is 20% higher than in Virtual Water™ phantom. Approximately the same difference exists between the XV2 results. The results show 11.4% and 6.4% over‐response of the XV2 film when compared to the ion chamber for 6 MV 30 × 30 cm2 and 10 × 10 cm2 field sizes respectively when the backscatter phantom thickness is 5 cm. For the same setup, measurements with EDR2 films indicate 8.5% and 1.7% over‐response. The film response on later scattered phantom study show nearly firm within 5 cm of lateral thickness and it increases when lateral thickness increases due to more multiple scatter of low energy photons. Conclusions: The backscattered phantom should not exceed more than 7cm for film accuracy. The lateral extension of the phantom should not be more than 5 cm from the field boundary in the case of large volumes.

SU‐FF‐T‐349: Small Field Dosimetry in Non‐Homogeneous Phantom

Purpose: Small fields has been used in radiotherapy due to development of new technology in tomotherapy, radiosurgery(SRS),intensity modulated radiotherapy(IMRT), Gamma Knife, and Cyber Knife. We study the dosimetry of small fields like output factor, profiles in homogenous and non‐homogeneous phantom. Material and Methods:Field sizes of 0.8, 1.6, 2.4, 3.2, 4.8 and 9.6 cm square (defined at isocenter level) were used for this experiment from Elekta Synergy accelerator (Elekta, Crawley, UK). Two phantoms were used in this study: polystyrene (Polystyrol 495F, BASF, Germany) 30 × 30 × 20 cm3 phantom of 1‐cm thick slabs and a sandwiched (non‐homogeneous) lung equivalent phantom, which is further described. The sandwiched phantom is a combination of nine polystyrene slabs (1‐cm thick each) of density 1.02 ± 0.02 g/cm3 positioned above and below two slabs (1.2‐cm thick each) of lung equivalent density 0.3 ± 0.02g/cm3. EBT film (International Specialty Products, NJ, USA) has been applied in this study and results were compared with diamond detector (Type 60003‐9‐0002 — PTW‐Freiburg). Results: A 4.3% and 1.1%output difference was measured in homogeneous polystyrene phantom between EBT film and diamond measurement at 0.8cm and 1.6cm square field sizes respectively for 6 MV. This value is 2.4% and 1.4% in higher photon energy of 18 MV because lateral range of electrons is the critical parameter that influences the charge particle equilibrium rather than forward range of the electrons. Due to electronic disequilibrium, this value is high (33.6%) in non‐homogeneous phantom. The profile showed that 18MV has lower response for small field than 6MV photon beam in both phantom materials. Conclusion: EBT film can be used for output factor measurements and relative dosimetry for small fields. This uncertainty in non homogeneous medium between two detectors has activated us to continue this study.

Monte Carlo benchmarking of a CCC-based treatment planning system in case of irregularly shaped fields

Advanced radiation techniques, like intensity modulated radiotherapy (IMRT) and intensity modulated arc therapy (IMAT), are based on small, elongated and highly irregular field shapes. In our study we use in-house developed Monte Carlo software to investigate the accuracy of a commercial available treatment planning system (Pinnacle, Philips Medical Systems, Best, The Netherlands) in calculating the dose distribution for irregular fields. Pinnacle calculates the dose distribution using the collapsed cone convolution (CCC) algorithm. The analysis of the considered beams showed that the differences in flattening filter, leaf tip and tongue-andgroove modeling introduce important discrepancies between Pinnacle and MC results for single irregular fields. Therefore, great attention should be given to the degree of irregularity and to the number of this type of beams included in the treatment plans. A benchmark tool, like MC simulations or measurements, should be used to verify treatment plans with high irregular beams.

SU‐GG‐T‐172: Compensator Based Intensity Modulated Radiation Therapy Dosimetry Using EDR2 Film

Purpose:Radiographic EDR2 film is the most commonly used radiographic film for dose distribution measurement,quality assurance and treatment plan verification. The compensator is an alternative way to deliver intensity modulated treatment. The EDR2 film reliability was investigated for dosimetry and QA of compensator based IMRT.Method and Materials: MC‐96 was used to fabricate the compensator blocks (10, 20, 30 and 50 mm thickness). For this study 6 and 25 MV photon beam qualities from an Elekta SL‐25 were used. The transmission factors of the blocks were measured with a Farmer type ion chamber of 0.6cc for a 10×10cm field size. The film was placed at isocentric distance between the slabs of a polystyrene phantom in a perpendicular geometry. To determine the effect of block thickness on the dose response of EDR2 film for both beam energies, dosemeasurements were made for various field sizes and offsets and compared to Farmer chamber measurements. To obtain a similar exposure of the film, transmission factors were applied to adjust the MUs. An inverse pyramid intensity modulated beam, achieved with a compensator block, was dosimetrically investigated to assess the value of the EDR2 film in a more clinical setting. Results and Discussion: The film showed a systematic under‐response of maximally 1.3% for a 50mm thick block and 25 MV (10×10 cm) beam, which is within the 3% overall uncertainty of film dosimetry for perpendicular geometry. The field offset measurement showed also a film under‐response in the order of 1.3% in the direction towards the gun, which might be due to spectral changes related to the beam bending system. The cross‐sectional dose profiles by compensator blocks showed that the film is within 1.2% to the diamond detector. EDR2 film can be safely and easily used as a 2D dose detector in compensator based IMRT.