An easy method to account for light scattering dose dependence in radiochromic films.

PURPOSE: To date no detector can offer the unbeatable characteristics of film dosimetry in terms of spatial resolution and this is why it has been chosen by many institutions for treatment verification and, in that respect, radiochromic films are becoming increasingly popular due to their advantageous properties. It is the aim of this work to suggest an easy method to overcome one of the drawbacks in radiochromic film dosimetry associated with the scanning device, namely, the nonuniform dose dependent response, mainly due to the light scattering effect. METHODS: The suggested procedure consists of building four correction matrices by sequentially scanning one, two, three, and four unexposed blank films. The color level of these four matrices is compatible with four points in the calibration curve dose range. Therefore, the dose dependent correction to the scanned irradiated film will be obtained by interpolating between the four correction matrices. RESULTS: The validity of the suggested method is checked against an ion chamber 2D array. The use of the proposed flattening correction improves considerably the dose agreement when compared with the cases in which no correction is applied. CONCLUSIONS: The method showed to be fast and easy and practically overcomes the dependence on the dose of light scattering of flatbed scanners.

Clinical implementation of combined modulated electron and photon beams with conventional MLC for accelerated partial breast irradiation.

PURPOSE: To report the clinical implementation of a novel external beam radiotherapy technique for accelerated partial breast irradiation treatments based on combined electron and photon modulated beams radiotherapy (MERT+IMRT) with conventional MLC. MATERIALS AND METHODS: A group of patients was selected to test the viability of the technique. The prescribed dose was 38.5Gy, following a hypofractionated schema, and the structures were defined following the NSABP-B39/RTOG-0413 protocol. The plans were calculated with an in-house Monte Carlo based planning system to consider explicitly the particle interactions with the MLC. An ad-hoc breast phantom was designed for a specific QA protocol. A reduced SSD was used for electron beams. Toxicity and cosmetic effects were assessed at every follow-up visit. RESULTS: All the plans achieved the dosimetric objectives and fulfilled the specific quality assurance protocol. Treatment delivery did not entail additional drawbacks for the clinical routine. Moderate or severe grade of toxicity was not reported, and the cosmetic results were comparable to those obtained with other APBI techniques. CONCLUSIONS: Results showed that MERT+IMRT with the MLC is a feasible and secure technique, and easy to be extended to other centers with the implementation of the adequate software for planning.

Additional dose constraints for analytical beam weighting optimization in IMRT.

This work presents an improvement to an algorithm for analytical beam weighting optimization where a flexible objective function, which considers 'importance factors' for each anatomical region and 'allowed deviations' from the prescribed dose, is defined. This upgrading allows forcing the mean value of the dose distribution to be the desired value, by using Lagrange multipliers. A real case is presented to show the effect of this change.

IMRT treatment planning:- a comparative inter-system and inter-centre planning exercise of the ESTRO QUASIMODO group.

BACKGROUND AND PURPOSE: The purpose of this work was a comparison of realistic IMRT plans based on the same CT-image data set and a common predefined set of dose objectives for the planning target volume and the organs at risk. This work was part of the larger European QUASIMODO IMRT verification project. MATERIALS AND METHODS: Eleven IMRT plans were produced by nine different European groups, each applying a representative set of clinically used IMRT treatment planning systems. The plans produced were to be deliverable in a clinically acceptable treatment time with the local technical equipment. All plans were characterized using a set of different quality measures such as dose-volume histograms, number of monitor units and treatment time. RESULTS: Only one plan was able to fulfil all dose objectives strictly; six plans failed some of the objectives but were still considered to be clinically acceptable; four plans were not able to reach the objectives. Additional quality scores such as the number of monitor units and treatment time showed large variations, which mainly depend on the delivery technique. CONCLUSION: The presented planning study showed that with nearly all presently available IMRT planning and delivery systems comparable dose distributions could be achieved if the planning goals are clearly defined in advance.

Micro ionization chamber dosimetry in IMRT verification: clinical implications of dosimetric errors in the PTV.

BACKGROUND AND PURPOSE: Absolute dose measurements for Intensity Modulated Radiotherapy (IMRT) beamlets is difficult due to the lack of lateral electron equilibrium. Recently we found that the absolute dosimetry in the penumbra region of the IMRT beamlet, can suffer from significant errors (Capote et al., Med Phys 31 (2004) 2416-2422). This work has the goal to estimate the error made when measuring the Planning Target Volume's (PTV) absolute dose by a micro ion chamber (microIC) in typical IMRT treatment. The dose error comes from the assumption that the dosimetric parameters determining the absolute dose are the same as for the reference conditions. MATERIALS AND METHODS: Two IMRT treatment plans for common prostate carcinoma case, derived by forward and inverse optimisation, were considered. Detailed geometrical simulation of the microIC and the dose verification set-up was performed. The Monte Carlo (MC) simulation allows us to calculate the delivered dose to water and the dose delivered to the active volume of the ion chamber. However, the measured dose in water is usually derived from chamber readings assuming reference conditions. The MC simulation provides needed correction factors for ion chamber dosimetry in non reference conditions. RESULTS: Dose calculations were carried out for some representative beamlets, a combination of segments and for the delivered IMRT treatments. We observe that the largest dose errors (i.e. the largest correction factors) correspond to the smaller contribution of the corresponding IMRT beamlets to the total dose delivered in the ionization chamber within PTV. CONCLUSION: The clinical impact of the calculated dose error in PTV measured dose was found to be negligible for studied IMRT treatments.

Microionization chamber for reference dosimetry in IMRT verification: clinical implications on OAR dosimetric errors.

Intensity modulated radiotherapy (IMRT) has become a treatment of choice in many oncological institutions. Small fields or beamlets with sizes of 1 to 5 cm2 are now routinely used in IMRT delivery. Therefore small ionization chambers (IC) with sensitive volumes 0.1 cm3 are generally used for dose verification of an IMRT treatment. The measurement conditions during verification may be quite different from reference conditions normally encountered in clinical beam calibration, so dosimetry of these narrow photon beams pertains to the so-called non-reference conditions for beam calibration. This work aims at estimating the error made when measuring the organ at risk's (OAR) absolute dose by a micro ion chamber (microIC) in a typical IMRT treatment. The dose error comes from the assumption that the dosimetric parameters determining the absolute dose are the same as for the reference conditions. We have selected two clinical cases, treated by IMRT, for our dose error evaluations. Detailed geometrical simulation of the microIC and the dose verification set-up was performed. The Monte Carlo (MC) simulation allows us to calculate the dose measured by the chamber as a dose averaged over the air cavity within the ion-chamber active volume (D(air)). The absorbed dose to water (D(water)) is derived as the dose deposited inside the same volume, in the same geometrical position, filled and surrounded by water in the absence of the ion chamber. Therefore, the D(water)/D(air) dose ratio is the MC estimator of the total correction factor needed to convert the absorbed dose in air into the absorbed dose in water. The dose ratio was calculated for the microIC located at the isocentre within the OARs for both clinical cases. The clinical impact of the calculated dose error was found to be negligible for the studied IMRT treatments.

Routine IMRT verification by means of an automated Monte Carlo simulation system.

PURPOSE: A tool to simulate complete intensity-modulated radiation therapy (IMRT) treatments with the Monte Carlo (MC) method has been developed. This application is based on a distribution model to employ as short processing times as possible for an operative verification. MATERIALS AND METHODS: The Clinical Primus-Siemens Linac beam was simulated with MC, using the EGS4 OMEGA-BEAM code package. An additional home-made program prepares the appropriate parameters for the code, using as input the file sent from the planning system to the linac. These parameters are adapted to the simulation code, making physical and clinical subdivisions of the global simulation of the treatment. Each resultant partition is ordered to a client personal computer in a cluster with 47 machines under a Linux environment. The verification procedure starts delivering the treatment on a plastic phantom containing an ionization chamber. If differences are less than 2%, films are inserted at selected planes in the phantom and the treatment is delivered again to evaluate the relative doses. When matching between treatment planning system (TPS), film, and MC is acceptable, a new evaluation of the patient is then performed between TPS and MC. Three different cases are shown to prove the applicability of the verification model. RESULTS: Acceptable agreement between the three methods used was obtained. The results are presented using different analysis tools. The actual time employed to simulate the total treatment in each case was no more than 5 h, depending on the number of segments. CONCLUSIONS: The MC model presented is fully automated, and results can be achieved within the operative time limits. The procedure is a reliable tool to verify any IMRT treatment.

MLC leaf width impact on the clinical dose distribution: a Monte Carlo approach.

PURPOSE: The influence of the multileaf collimator (MLC) leaf width on the dose distribution in patients treated with conformal radiotherapy and intensity-modulated radiotherapy has been analyzed. This study was based on the Monte Carlo simulation with the beams generated by a linac with the double-focused MLC. MATERIALS AND METHODS: The transmission through the leaves and the exact shape of the penumbra regions are difficult to model by treatment planning system algorithms. An accurate assessment of the dose variations due to the leaf width change can be achieved by means of Monte Carlo simulation. The BEAM/EGS4 code was used at the Hospital of the Virgen Macarena to model a Siemens PRIMUS linac, featuring an MLC with a leaf width projecting 1 cm at the isocenter. Based on this real model, a virtual head was designed while allowing for a variation of the leaf width projection. Both the real linac and the virtual linac, with leaves projecting 0.5 cm, were used to obtain the dose distributions for several treatments. A few disease sites, including the prostate, head and neck, and endometrium, were selected for the design of the conformal and intensity-modulated radiotherapy treatments with a forward planning algorithm sensitive to the different shapes of the volumes of interest. Isodose curves, differential matrix, gamma function, and the dose-volume histograms (DVHs) corresponding to both MLC models were obtained for all cases. The tumor control probability and the normal tissue complication probability were derived for those cases studied featuring the greatest differences between results for both MLCs. RESULTS: The impact on the DVHs of changing leaf width projections at the isocenter from 1.0 cm to 0.5 cm was low. Radiobiologic models showed slightly better tumor control probability/normal tissue complication probability values using the virtual MLC with a leaf width projecting 0.5 cm at isocenter in those cases presenting greater differences in the DVHs. CONCLUSIONS: The impact on the clinical dose distribution due to the MLC leaf width change is low based on the design and conditions used in this study.

Computer optimization of class solutions designed on a beam segmentation basis.

BACKGROUND AND PURPOSE: A method for analytically solving the optimization of beam weighting in radiotherapy treatments using beam segmentation is presented. PATIENTS AND METHODS: A technique has been elaborated permitting the optimization of a flexible objective function which is defined by considering 'importance factors' for each anatomical region and 'allowed deviations' from the prescribed dose. As any change in these importance factors may lead to very different solutions, a statistical tool has been developed which varies the objective function automatically and iteratively to get the best possible results compatible with the chosen class solution. In addition, the spatial symmetry found in many anatomical sites is taken advantage of. Furthermore, freeware code has been written to run this optimization approach. The guidelines to design the beam segmentation used in our institution using organ avoidance criteria, and hence the suitable class solution for different anatomical sites, are given. A treatment-planning study for three anatomical sites is presented and, for two of them, the results obtained with both the suggested and the classical inverse approach are presented. RESULTS: The work presented might be used for beam weighting optimization in any radiotherapy treatment and furthermore, the suggested procedure may successfully confront the intensity-modulated radiation therapy (IMRT) problem and the obtained dose distributions fit the clinical constraints for all anatomical sites studied. When comparing with the classical inverse approach, both results are comparable in terms of dose distribution, but the suggested technique reduces the integral dose as the total number of monitor units is lower. CONCLUSIONS: The developed code performs the optimization with a very low time cost and in addition, this process can be carried out with a conventional treatment-planning system with no need of dedicated IMRT software.

An EGSnrc Monte Carlo study of the microionization chamber for reference dosimetry of narrow irregular IMRT beamlets.

Intensity modulated radiation therapy (IMRT) has evolved toward the use of many small radiation fields, or "beamlets," to increase the resolution of the intensity map. The size of smaller beamlets can be typically about 1-5 cm2. Therefore small ionization chambers (IC) with sensitive volumes < or = 0.1 cm3 are generally used for dose verification of IMRT treatment. The dosimetry of these narrow photon beams pertains to the so-called nonreference conditions for beam calibration. The use of ion chambers for such narrow beams remains questionable due to the lack of electron equilibrium in most of the field. The present contribution aims to estimate, by the Monte Carlo (MC) method, the total correction needed to convert the IBA-Wellhöfer NAC007 micro IC measured charge in such radiation field to the absolute dose to water. Detailed geometrical simulation of the microionization chamber was performed. The ion chamber was always positioned at a 10 cm depth in water, parallel to the beam axis. The delivered doses to air and water cavity were calculated using the CAVRZ EGSnrc user code. The 6 MV phase-spaces for Primus Clinac (Siemens) used as an input to the CAVRZnrc code were derived by BEAM/EGS4 modeling of the treatment head of the machine along with the multileaf collimator [Sánchez-Doblado et al., Phys. Med. Biol. 48, 2081-2099 (2003)] and contrasted with experimental measurements. Dose calculations were carried out for two irradiation geometries, namely, the reference 10x10 cm2 field and an irregular (approximately 2x2 cm2) IMRT beamlet. The dose measured by the ion chamber is estimated by MC simulation as a dose averaged over the air cavity inside the ion-chamber (Dair). The absorbed dose to water is derived as the dose deposited inside the same volume, in the same geometrical position, filled and surrounded by water (Dwater) in the absence of the ionization chamber. Therefore, the Dwater/Dair dose ratio is a MC direct estimation of the total correction factor needed to convert the absorbed dose in air to absorbed dose to water. The dose ratio was calculated for several chamber positions, starting from the penumbra region around the beamlet along the two diagonals crossing the radiation field. For this quantity from 0 up to a 3% difference is observed between the dose ratio values obtained within the small irregular IMRT beamlet in comparison with the dose ratio derived for the reference 10x10 cm2 field. Greater differences from the reference value up to 9% were obtained in the penumbra region of the small IMRT beamlet.

Total skin electron therapy treatment verification: Monte Carlo simulation and beam characteristics of large non-standard electron fields.

Total skin electron therapy (TSET) is a complex technique which requires non-standard measurements and dosimetric procedures. This paper investigates an essential first step towards TSET Monte Carlo (MC) verification. The non-standard 6 MeV 40 x 40 cm2 electron beam at a source to surface distance (SSD) of 100 cm as well as its horizontal projection behind a polymethylmethacrylate (PMMA) screen to SSD = 380 cm were evaluated. The EGS4 OMEGA-BEAM code package running on a Linux home made 47 PCs cluster was used for the MC simulations. Percentage depth-dose curves and profiles were calculated and measured experimentally for the 40 x 40 cm2 field at both SSD = 100 cm and patient surface SSD = 380 cm. The output factor (OF) between the reference 40 x 40 cm2 open field and its horizontal projection as TSET beam at SSD = 380 cm was also measured for comparison with MC results. The accuracy of the simulated beam was validated by the good agreement to within 2% between measured relative dose distributions, including the beam characteristic parameters (R50, R80, R100, Rp, E0) and the MC calculated results. The energy spectrum, fluence and angular distribution at different stages of the beam (at SSD = 100 cm, at SSD = 364.2 cm, behind the PMMA beam spoiler screen and at treatment surface SSD = 380 cm) were derived from MC simulations. Results showed a final decrease in mean energy of almost 56% from the exit window to the treatment surface. A broader angular distribution (FWHM of the angular distribution increased from 13 degrees at SSD = 100 cm to more than 30 degrees at the treatment surface) was fully attributable to the PMMA beam spoiler screen. OF calculations and measurements agreed to less than 1%. The effect of changing the electron energy cut-off from 0.7 MeV to 0.521 MeV and air density fluctuations in the bunker which could affect the MC results were shown to have a negligible impact on the beam fluence distributions. Results proved the applicability of using MC as a treatment verification tool for complex radiotherapy techniques.

Investigation of radiosurgical beam profiles using Monte Carlo method.

An accurate determination of the penumbra of radiosurgery profiles is critical to avoid complications in organs at risk adjacent to the tumor. Conventional detectors may not be accurate enough for small field sizes. The Monte Carlo (MC) method was used to study the behavior of radiosurgical beam profiles at the penumbral region; the BEAM code was also used in this work. Two collimators (2.2- and 0.3-cm diameter) were calculated and compared with empirical measurements obtained with the detectors normally used. The differences found between film dosimetry and MC revealed a systematic error in the reading procedure. In the process, a water phantom was simulated with a layer of the same composition as that of the film. MC calculations with film differed by a small amount from those obtained with the water phantom alone. In conclusion, MC may be used as a verification tool to support dosimetrical procedures with conventional detectors, especially in very small beams such as those used in radiosurgery. Furthermore, it has been proved that the film energy dependence is negligible for fields used in radiosurgery.

Combined modulated electron and photon beams planned by a Monte-Carlo-based optimization procedure for accelerated partial breast irradiation.

The purpose of this study was to present a Monte-Carlo (MC)-based optimization procedure to improve conventional treatment plans for accelerated partial breast irradiation (APBI) using modulated electron beams alone or combined with modulated photon beams, to be delivered by a single collimation device, i.e. a photon multi-leaf collimator (xMLC) already installed in a standard hospital. Five left-sided breast cases were retrospectively planned using modulated photon and/or electron beams with an in-house treatment planning system (TPS), called CARMEN, and based on MC simulations. For comparison, the same cases were also planned by a PINNACLE TPS using conventional inverse intensity modulated radiation therapy (IMRT). Normal tissue complication probability for pericarditis, pneumonitis and breast fibrosis was calculated. CARMEN plans showed similar acceptable planning target volume (PTV) coverage as conventional IMRT plans with 90% of PTV volume covered by the prescribed dose (D(p)). Heart and ipsilateral lung receiving 5% D(p) and 15% D(p), respectively, was 3.2-3.6 times lower for CARMEN plans. Ipsilateral breast receiving 50% D(p) and 100% D(p) was an average of 1.4-1.7 times lower for CARMEN plans. Skin and whole body low-dose volume was also reduced. Modulated photon and/or electron beams planned by the CARMEN TPS improve APBI treatments by increasing normal tissue sparing maintaining the same PTV coverage achieved by other techniques. The use of the xMLC, already installed in the linac, to collimate photon and electron beams favors the clinical implementation of APBI with the highest efficiency.

Intensity- and energy-modulated electron radiotherapy by means of an xMLC for head and neck shallow tumors.

The purpose of this paper is to assess the feasibility of delivering intensity- and energy-modulated electron radiation treatment (MERT) by a photon multileaf collimator (xMLC) and to evaluate the improvements obtained in shallow head and neck (HN) tumors. Four HN patient cases covering different clinical situations were planned by MERT, which used an in-house treatment planning system that utilized Monte Carlo dose calculation. The cases included one oronasal, two parotid and one middle ear tumors. The resulting dose-volume histograms were compared with those obtained from conventional photon and electron treatment techniques in our clinic, which included IMRT, electron beam and mixed beams, most of them using fixed-thickness bolus. Experimental verification was performed with plane-parallel ionization chambers for absolute dose verification, and a PTW ionization chamber array and radiochromic film for relative dosimetry. A MC-based treatment planning system for target with compromised volumes in depth and laterally has been validated. A quality assurance protocol for individual MERT plans was launched. Relative MC dose distributions showed a high agreement with film measurements and absolute ion chamber dose measurements performed at a reference point agreed with MC calculations within 2% in all cases. Clinically acceptable PTV coverage and organ-at-risk sparing were achieved by using the proposed MERT approach. MERT treatment plans, based on delivery of intensity-modulated electron beam using the xMLC, for superficial head and neck tumors, demonstrated comparable or improved PTV dose homogeneity with significantly lower dose to normal tissues. The clinical implementation of this technique will be able to offer a viable alternative for the treatment of shallow head and neck tumors.

Modulated electron radiotherapy treatment planning using a photon multileaf collimator for post-mastectomized chest walls.

BACKGROUND AND PURPOSE: To evaluate the feasibility of using a photon MLC (xMLC) for modulated electron radiotherapy treatment (MERT) as an alternative to conventional post-mastectomy chest wall (CW) irradiation. A Monte Carlo (MC) based planning system was developed to overcome the inaccuracy of the 'pencil beam' algorithm. MC techniques are known to accurately calculate the dose distributions of electron beams, allowing the explicit simulation of electron interactions within the MLC. MATERIALS AND METHODS: Four real clinical CW cases were planned using MERT which were compared with the conventional electron treatments based on blocks and by a straightforward approach using the MLC, and not the blocks (as an intermediate step to MERT) to shape the same segments with SSD between 60 and 70 cm depending on PTV size. MC calculations were verified with an array of ionization chambers and radiochromic films in a solid water phantom. RESULTS: Tests based on gamma analysis between MC dose distributions and radiochromic film measurements showed an excellent agreement. Differences in the absolute dose measured with a plane-parallel chamber at a reference point were below 3% for all cases. MERT solution showed a better PTV coverage and a significant reduction of the doses to the organs at risk (OARs). CONCLUSION: MERT can effectively improve the current electron treatments by obtaining a better PTV coverage and sparing healthy tissues. More directly, block-shaped treatments could be replaced by MLC-shaped non-modulated segments providing similar results.