Hans Marijnissen

LIGHT DOSIMETRY FOR PHOTODYNAMIC THERAPY BY WHOLE BLADDER WALL IRRADIATION

Abstract In Photodynamic Therapy (PDT) there is a need for accurate quantitative light dosimetry. This has become particularly apparent in the treatment of superficial bladder cancer, either by focal or by whole bladder wall irradiation. We have studied this problem using a spherical model of the bladder, consisting of two concentric thin‐walled plastic spheres, 8 and 10 cm in diameter. The inner sphere was filled with water or with a light‐scattering medium. The space between the spheres was filled with an optically tissue equivalent liquid. An isotropic light source was placed at the center of the spheres. Light energy fluence rates (light “dose rates”) during PDT of the bladder simulated in this manner, were measured using a specially developed miniature light detector and were also calculated using a mathematical model. These data were confirmed by measurements in vivo (dog bladder). In the case of focal irradiation at a wavelength of 630 nm, the ratio (R) between the true light fluence rate at the bladder surface and the fluence rate due to the primary light beam is somewhat larger than 1, depending on the diameter of the primary beam. The maximum ratio is 2, for a beam diameter of several centimeters. In the case of whole bladder wall PDT, R is larger than 5. This is due to the strong scattering of (red) light by tissue and indicates that the intensity of the primary beam, which is usually reported, is not a good measure of the true fluence rate for whole bladder wall PDT. When the light source is moved away from the center of the spheres, the rate of change of the fluence rate at the bladder wall is more than a factor of 2 larger when the bladder cavity is filled with a light‐scattering suspension, as compared with water. The use of a light‐scattering medium may therefore not be advantageous to achieve a uniform light distribution across the bladder wall.

Construction, quality assurance and calibration of spherical isotropic fibre optic light diffusers

Spherical isotropic fibre optic light diffusers are used in photodynamic therapy either as a light source or as a light detector. The construction of light diffusers using different materials is described, viz. an optical method involving local polymerization of a dental fissure sealant, which is referred to as the Henderson method, and a second method using plastic or ceramic pre-fabricated spheres. Quality tests necessary for reliable clinical use are presented for the mechanical strength, output power and isotropy. The maximum pull-off force and blow-off output power for the different kinds of diffusers were determined. The calibration procedures are given for measurement of the output power and wavelength of the light emitted by a diffuser and for measurement of the fluence rate by a light-detecting diffuser, using a compact integrating sphere device. With all types of diffusers described, an isotropy can be obtained of better than ± 20% measured over a 320° angle for spheres as small as ≈ 1 mm. Larger ceramic diffusers are particularly suitable for delivering high output powers. A 3-mm-diameter ceramic diffuser mounted on a 600-μm-core fibre can emit up to ≈ 5 W of continuous wave (CW) visible light in air. Diffusers used for light detection can measure the light fluence rate in tissue with ≈15% accuracy or better if calibration factors are determined for each individual probe.

Monte carlo simulations for endobronchial photodynamic therapy: The influence of variations in optical and geometrical properties and of realistic and eccentric light sources

Light dosimetry for endobronchial photodynamic therapy is not very advanced to date. This study investigates the dependency of the fluence rate distribution in the bronchial wall on several parameters.

Dosimetric validation of a commercial Monte Carlo based IMRT planning system.

PURPOSE Recently a commercial Monte Carlo based IMRT planning system (Monaco version 1.0.0) was released. In this study the dosimetric accuracy of this new planning system was validated. METHODS Absolute dose profiles, depth dose curves, and output factors calculated by Monaco were compared with measurements in a water phantom. Different static on-axis and off-axis fields were tested at various source-skin distances for 6, 10, and 18 MV photon beams. Four clinical IMRT plans were evaluated in a water phantom using a linear diode detector array and another six IMRT plans for different tumor sites in solid water using a 2D detector array. In order to evaluate the accuracy of the dose engine near tissue inhomogeneities absolute dose distributions were measured with Gafchromic EBT film in an inhomogeneous slab phantom. For an end-to-end test a four-field IMRT plan was applied to an anthropomorphic lung phantom with a simulated tumor peripherally located in the right lung. Gafchromic EBT film, placed in and around the tumor area, was used to evaluate the dose distribution. RESULTS Generally, the measured and the calculated dose distributions agreed within 2% dose difference or 2 mm distance-to-agreement. But mainly at interfaces with bone, some larger dose differences could be observed. CONCLUSIONS Based on the results of this study, the authors concluded that the dosimetric accuracy of Monaco is adequate for clinical introduction.

SU‐GG‐T‐352: Dosimetric Validation of a Commercial Monte‐Carlo Based IMRT Planning

Purpose: Recently, the inverse planning planning system Monaco (CMS Inc) version 1.0.0 was installed in our department. The Monaco system incorporates a two‐stage IMRT optimization procedure and a version of the XVMC Monte Carlo dose calculation algorithm. In the first stage the fluence distribution of the IMRT beams are optimized using hard (‘biological’) constraints for the organs at risk. In the second stage, the beams are segmented and their weights optimized while still satisfy the hard constraints. In this study we investigated the dosimetric accuracy of the XVMC dose engine implemented in MONACO. Method and Materials: For an ELEKTA(r) linac (6, 10 and 18 MV) depth dose curves, dose profiles and output factors were measured in a waterphantom using source‐to‐surface distances (SSD) between 80 and 100 cm. Symmetric fields ranging from 2×2 to 30×30 cm and off‐axis fields of 2×2 3×3 cm and 2×10 cm were used. In addition, GAFCHROMIC(r) film measurements were performed in anthropomorphic phantoms to validate the dose accuracy in inhomogeneous media. Furthermore, the dosimetric accuracy of IMRT fields for prostate and head‐and‐neck cancer treatments was evaluated. All fields were simulated in Monaco using a grid size of 2 mm and variances of 0.5 and 1%. Results: The agreement between measured and calculated dose distributions was generally within 2%. Only in the build‐up region larger dose differences were observed, especially for the highest photon energy. No impact of the SSD on the dose accuracy was observed. Calculation times for a 10×10 cm field and a 0.5 or 1% variance were 75 and 23 minutes, respectively. Conclusion: A very good agreement was observed between Monaco Monte Carlo dose calculations and measurements, allowing clinical introduction.