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T LoSasso

Publications and source records attributed to T LoSasso.

At least 19 recordsLinked to original sources
PubMed2005

The three-dimensional scintillation dosimetry method: test for a 106Ru eye plaque applicator.

The need for fast, accurate and high resolution dosimetric quality assurance in radiation therapy has been outpacing the development of new and improved 2D and 3D dosimetry techniques. This paper summarizes the efforts to create a novel and potentially very fast, 3D dosimetry method based on the observation of scintillation light from an irradiated liquid scintillator volume serving simultaneously as a phantom material and as a dose detector medium. The method, named three-dimensional scintillation dosimetry (3DSD), uses visible light images of the liquid scintillator volume at multiple angles and applies a tomographic algorithm to a series of these images to reconstruct the scintillation light emission density in each voxel of the volume. It is based on the hypothesis that with careful design and data processing, one can achieve acceptable proportionality between the local light emission density and the locally absorbed dose. The method is applied to a Ru-106 eye plaque immersed in a 16.4 cm3 liquid scintillator volume and the reconstructed 3D dose map is compared along selected profiles and planes with radiochromic film and diode measurements. The comparison indicates that the 3DSD method agrees, within 25% for most points or within approximately 2 mm distance to agreement, with the relative radiochromic film and diode dose distributions in a small (approximately 4.5 mm high and approximately 12 mm diameter) volume in the unobstructed, high gradient dose region outside the edge of the plaque. For a comparison, the reproducibility of the radiochromic film results for our measurements ranges from 10 to 15% within this volume. At present, the 3DSD method is not accurate close to the edge of the plaque, and further than approximately 10 mm (<10% central axis depth dose) from the plaque surface. Improvement strategies, considered important to provide a more accurate quick check of the dose profiles in 3D for brachytherapy applicators, are discussed.

Brachytherapy↗
PubMed2001

Treatment planning and delivery of intensity-modulated radiation therapy for primary nasopharynx cancer.

PURPOSE: To implement intensity-modulated radiation therapy (IMRT) for primary nasopharynx cancer and to compare this technique with conventional treatment methods. METHODS AND MATERIALS: Between May 1998 and June 2000, 23 patients with primary nasopharynx cancer were treated with IMRT delivered with dynamic multileaf collimation. Treatments were designed using an inverse planning algorithm, which accepts dose and dose-volume constraints for targets and normal structures. The IMRT plan was compared with a traditional plan consisting of phased lateral fields and a three-dimensional (3D) plan consisting of a combination of lateral fields and a 3D conformal plan. RESULTS: Mean planning target volume (PTV) dose increased from 67.9 Gy with the traditional plan, to 74.6 Gy and 77.3 Gy with the 3D and IMRT plans, respectively. PTV coverage improved in the parapharyngeal region, the skull base, and the medial aspects of the nodal volumes using IMRT and doses to all normal structures decreased compared to the other treatment approaches. Average maximum cord dose decreased from 49 Gy with the traditional plan, to 44 Gy with the 3D plan and 34.5 Gy with IMRT. With the IMRT plan, the volume of mandible and temporal lobes receiving more than 60 Gy decreased by 10-15% compared to the traditional and 3D plans. The mean parotid gland dose decreased with IMRT, although it was not low enough to preserve salivary function. CONCLUSION: Lower normal tissue doses and improved target coverage, primarily in the retropharynx, skull base, and nodal regions, were achieved using IMRT. IMRT could potentially improve locoregional control and toxicity at current dose levels or facilitate dose escalation to further enhance locoregional control.

Algorithms↗
PubMed1998

Physical and dosimetric aspects of a multileaf collimation system used in the dynamic mode for implementing intensity modulated radiotherapy.

The use of a multileaf collimator in the dynamic mode to perform intensity modulated radiotherapy became a reality at our institution in 1995. Unlike treatment with static fields using a multileaf collimator, there are significant dosimetric issues which must be assessed before dynamic therapy can be implemented. We have performed a series of calculations and measurements to quantify head scatter for small fields, collimator transmission, and the transmission through rounded leaf ends. If not accounted for, these factors affect the delivered dose to the prostate by 5%-20% for a typical plan. Data obtained with ion chambers and radiographic film are presented for both 6 and 15 MV x-ray beams. The impact on the delivered dose of the mechanical accuracy of the multileaf collimator, achieved during leaf position calibration and maintained during dose delivery, is also discussed.

Biophysical Phenomena↗
PubMed1997

Planning, delivery, and quality assurance of intensity-modulated radiotherapy using dynamic multileaf collimator: a strategy for large-scale implementation for the treatment of carcinoma of the prostate.

PURPOSE: To improve the local control of patients with adenocarcinoma of the prostate we have implemented intensity modulated radiation therapy (IMRT) to deliver a prescribed dose of 81 Gy. This method is based on inverse planning and the use of dynamic multileaf collimators (DMLC). Because IMRT is a new modality, a major emphasis was on the quality assurance of each component of the process and on patient safety. In this article we describe in detail our procedures and quality assurance program. METHODS AND MATERIALS: Using an inverse algorithm, we have developed a treatment plan consisting five intensity-modulated (IM) photon fields that are delivered with DMLC. In the planning stage, the planner specifies the number of beams and their directions, and the desired doses for the target, the normal organs and the "overlap" regions. Then, the inverse algorithm designs intensity profiles that best meet the specified criteria. A second algorithm determines the leaf motion that would produce the designed intensity pattern and produces a DMLC file as input to the MLC control computer. Our quality assurance program for the planning and treatment delivery process includes the following components: 1) verification of the DMLC field boundary on localization port film, 2) verification that the leaf motion of the DMLC file produces the planned dose distribution (with an independent calculation), 3) comparison of dose distribution produced by DMLC in a flat phantom with that calculated by the treatment planning computer for the same experimental condition, 4) comparison of the planned leaf motions with that implemented for the treatment (as recorded on the MLC log files), 5) confirmation of the initial and final positions of the MLC for each field by a record-and-verify system, and 6) in vivo dose measurements. RESULTS: Using a five-field IMRT plan we have customized dose distribution to conform to and deliver 81 Gy to the PTV. In addition, in the overlap regions between the PTV and the rectum, and between the PTV and the bladder, the dose is kept within the tolerance of the respective organs. Our QA checks show acceptable agreement between the planned and the implemented leaf motions. Correspondingly, film and TLD dosimetry indicates that doses delivered agrees with the planned dose to within 2%. As of September 15, 1996, we have treated eight patients to 81 Gy with IMRT. CONCLUSION: For complex planning problems where the surrounding normal tissues place severe constraints on the prescription dose, IMRT provides a powerful and efficient solution. Given a comprehensive and rigorous quality-assurance program, the intensity-modulated fields can be efficaciously and accurately delivered using DMLC. IMRT treatment is now ready for routine implementation on a large scale in our clinic.

Adenocarcinoma↗
PubMed1997

Multileaf collimation in the treatment of the intact breast.

PURPOSE: To compare multileaf-collimator beam shaping and conventional metal-alloy blocking in irradiation of the intact breast after breast-conserving surgery. MATERIALS AND METHODS: Transmission dose was measured in a phantom, by using both types of blocking with 6- and 15-MV photon beams. Buildup data were collected, using a 6-MV photon beam, comparing open fields to various beam modifiers. RESULTS: Contralateral breast transmission was reduced with the multileaf collimator from 4% to 1%. With the jaws and multileaf collimator, the primary beam component of 0.5% was eliminated. Buildup data for the multileaf collimator most closely resembled the surface dose when a blocking tray is not used and were slightly lower than those for the conventional mounted blocks. This relates to the use of the lower of the two sets of wedges on the multileaf collimator, which is closer to the patient and thus enhances the surface dose relative to the dose with an open field and no wedge. CONCLUSION: Multileaf-collimator blocking for primary breast treatment is similar to conventional blocking, and the transition from one technique to another should be uneventful. The transmission dose to the contralateral breast is decreased with the multileaf collimator.

Breast Neoplasms↗
PubMed1996

Conformal radiation treatment of prostate cancer using inversely-planned intensity-modulated photon beams produced with dynamic multileaf collimation.

PURPOSE: To implement radiotherapy with intensity-modulated beams, based on the inverse method of treatment design and using a multileaf collimation system operating in the dynamic mode. METHODS AND MATERIALS: An algorithm, based on the inverse technique, has been integrated into the radiotherapy treatment-planning computer system in our Center. This method of computer-assisted treatment design was used to derive intensity-modulated beams to optimize the boost portion of the treatment plan for a patient with a T1c cancer of the prostate. A dose of 72 Gy (in 40 fractions) was given with a six-field plan, and an additional 9 Gy (in five fractions) with six intensity-modulated beams. The intensity-modulated fields were delivered using dynamic multileaf collimation, that is, individual leaves were in motion during radiation delivery, with the treatment machine operating in the clinical mode. Exhaustive quality assurance measurement and monitoring were carried out to ensure safe and accurate implementation. RESULTS: Dose distribution and dose-volume histogram of the "inverse method" boost plan and of the composite (72 Gy primary + 9 Gy boost) plan were judged clinically acceptable. Compared to a manually designed boost plan, the inverse treatment design gave improved conformality and increased dose homogeneity in the planning target volume. Film and ion chamber dosimetry, performed prior to the first treatment, indicated that each of the six intensity-modulated fields was accurately produced. Thermoluminescent dosimeter (TLD) measurements performed on the patient confirmed that the intended dose was delivered in the treatment. In addition, computer-aided treatment-monitoring programs assured that the multileaf collimator (MLC) position file was executed to the specified precision. In terms of the overall radiation treatment process, there will likely be labor savings in the planning and the treatment phases. CONCLUSIONS: We have placed into clinical use an integrated system of conformal radiation treatment that incorporated the inverse method of treatment design and the use of dynamic multileaf collimation to deliver intensity-modulated beams. The system can provide better treatment design, which can be implemented reliably and safely. We are hopeful that improved treatment efficacy will result.

Humans↗
PubMed1996

Dosimetric verification of intensity-modulated fields.

The optimization of intensity distributions and the delivery of intensity-modulated treatments with dynamic multi-leaf collimators (MLC) offer important improvements to three-dimensional conformal radiotherapy. In this study, a nine-beam intensity-modulated prostate plan was generated using the inverse radiotherapy technique. The resulting fluence profiles were converted into dynamic MLC leaf motions as functions of monitor units. The leaf motion pattern data were then transferred to the MLC control computer and were used to guide the motions of the leaves during irradiation. To verify that the dose distribution predicted by the optimization and planning systems was actually delivered, a homogeneous polystyrene phantom was irradiated with each of the nine intensity-modulated beams incident normally on the phantom. For each exposure, a radiographic film was placed normal to the beam in the phantom to record the deposited dose. The films were calibrated and scanned to generate 2-D isodose distributions. The dose was also calculated by convolving the incident fluence pattern with pencil beams. The measured and calculated dose distributions were compared and found to have discrepancies in excess of 5% of the central axis dose. The source of discrepancies was suspected to be the rounded edges of the leaves and the scattered radiation from the various components of the collimation system. After approximate corrections were made for these effects, the agreement between the two dose distributions was within 2%. We also studied the impact of the "tongue-and-groove" effect on dynamic MLC treatments and showed that it is possible to render this effect inconsequential by appropriately synchronizing leaf motions. This study also demonstrated that accurate and rapid delivery of realistic intensity-modulated plans is feasible using a dynamic multi-leaf collimator.

Biophysical Phenomena↗
PubMed1996

Testing of dynamic multileaf collimation.

It has been shown that intensity-modulated fields have the potential to deliver optimum dose distributions, i.e., high dose uniformity in the target and lower doses in the surrounding critical organs. One way to deliver such fields is by using dynamic multileaf collimation (DMLC). This capability is already available in research mode on some treatment machines. While much effort has been devoted to developing algorithms for DMLC, the mechanical reliability of this new treatment delivery mode has not been fully studied. In this work, we report a series of tests designed to investigate the mechanical aspects of DMLC and their implications on dosimetry. Specifically, these tests were designed to examine (1) the stability of leaf speed, (2) the effect of lateral disequilibrium on dose profiles between adjacent leaves, (3) the significance of acceleration and deceleration of leaf motion, (4) the effect of positional accuracy and rounded-end of the leaves, and (5) create a simple test pattern that may serve as a basis for routine quality assurance checks. Results of these tests are presented. The implications on dosimetry and consideration for the design of leaf motion are discussed.

Humans↗
PubMed1995

Multileaf collimation versus alloy blocks: analysis of geometric accuracy.

PURPOSE: To compare the misalignment error due to the fabrication of custom lead alloy blocks with the displacement error introduced by the finite resolution of a multileaf collimator relative to the prescribed smooth apertures. METHODS AND MATERIALS: Treatment field apertures for randomly selected patients for four clinical sites were obtained at various stages of the block fabrication process. These apertures and the corresponding multileaf collimator (MLC) apertures for each field were superimposed with the smooth apertures prescribed by physicians. The deviations from the prescribed apertures were measured at 10 degrees intervals. Comparisons of the magnitude and frequency of errors from block fabrication with those from the geometric displacements introduced by the finite leaf width of the multileaf collimator were made. RESULTS: The degree of conformity of the multileaf collimator is treatment-site dependent as, in general, are the shapes of fields. For three of the four sites examined, the multileaf collimator apertures track the prescribed apertures at least as accurately as custom blocking when the block design, construction, mounting, and alignment on the treatment machine are considered. CONCLUSIONS: The geometric conformality of multileaf collimation is comparable to, and in some cases superior to, that of custom blocks.

Alloys↗
PubMed1994

Preclinical evaluation of the reliability of a 50 MeV racetrack microtron.

PURPOSE: A 50 MeV racetrack microtron has been installed and tested at Memorial Sloan-Kettering Cancer Center. It is designed to execute multi-segment conformal therapy automatically under computer control using scanned X ray and electron beams from 10 to 50 MeV. Prior to acceptance of the machine from the manufacturer, formal reliability testing was carried out. Only in this way could confidence be gained in its usefulness for routine 3D computer-controlled conformal therapy. MATERIALS AND METHODS: To assess reliability, a set of 25 multi-segment test cases, each consisting of 10 to 17 fixed segments, was developed. The field arrangements and modalities for some of the test cases were identical to 3D conformal treatments that were being delivered with multiple static fields on conventional linear accelerators at our institution. Other cases were designed to explore reliability under more complex sets of conditions. These cases were "treated" repeatedly during a total period of 45 hours, over 5 days. During the treatments, ion chambers attached to the head of the machine provided dosimetric data for each field. Data from sensors connected to every set-up parameter (for example, couch positions, gantry angle, collimator leaf positions, etc.) were recorded and verified by an external computer. RESULTS: While preliminary tests indicated an interlock rate of 5%, final reliability test results demonstrated an interlock fault rate of approximately 0.5%. The reproducibility of dosimetric data and geometric setup parameters was within specifications. As an example, leaf position reproducibility in the patient plane was within 0.5 mm for 97% of the setups. The times required to carry out treatments were recorded and compared with the times to carry out identical treatments on a conventional linear accelerator with cerrobend blocks. Areas where additional time savings can be achieved were identified. CONCLUSIONS: As an integral part of acceptance testing, the Scanditronix MM50 was rigorously tested for reliability. The machine successfully passed these tests, providing increased confidence in its usefulness for routine 3D conformal therapy.

Humans↗
PubMed1994

Dose calculation for photon beams with intensity modulation generated by dynamic jaw or multileaf collimations.

A dose calculation algorithm has been developed for photon beams with intensity modulation generated by dynamic jaw or multileaf collimations. First, an in-air fluence distribution is constructed based on the dynamic motion of the jaws or leaves, taking into account the variation of output with field size defined by the jaws. The fluence distribution is then convolved with the appropriate pencil beam kernel to give correction factors which are used to calculate the dose distribution for an intensity-modulated photon field. The proposed algorithm is strictly valid in homogeneous media only, patient heterogeneity correction is accounted for in an approximate manner. Dose distributions at several depths and for several field sizes were calculated for 6- and 15-MV x-ray beams for a set of standard wedges produced by dynamic jaws. Measurements were made with film and an ion chamber. Comparisons between calculated and measured data show good agreement (within 2%) for both dose profiles and wedge factors. Similar calculations and measurements were also made for a 25-MV intensity-modulated photon field produced by dynamic motion of a multileaf collimator. Agreement between calculations and measurements is also good (within 3%). The "tongue-and-groove" effect associated with a multileaf collimator design is also examined using a ring-shaped field produced by matching two component fields. The computation time for a dynamic-collimated field is the same as that for an irregular field shaped by conventional blocks. The algorithm is applicable to any pattern of jaw or multileaf motions. The strengths and remaining problems of the algorithm are discussed.

Algorithms↗
PubMed1994

Beam profiles along the nonwedged direction for large wedged fields.

Beam profiles along the nonwedged direction of a wedged field produced by a linear accelerator exhibit more "sagging" than that of an open field at the same depth. For large fields, the profiles of open and wedged fields can differ by as much as 7%. The extra "sagging" of wedged profiles is mainly due to the difference in penetration between on- and off-axis rays caused by the variation of beam quality across the field. An algorithm was developed to estimate an "effective" depth such that the profile of a wedged field can be approximated by the open-field profile at the effective depth. The algorithm was verified by measured beam profiles for 6- and 15-MV x-ray beams for 15 degree, 30 degree, 45 degree, and 60 degree wedges.

Algorithms↗
PubMed1993

The use of a multi-leaf collimator for conformal radiotherapy of carcinomas of the prostate and nasopharynx.

We investigate the use of a multi-leaf collimator for conformal radiation therapy of carcinomas of the prostate and of the nasopharynx. Following verification of dose calculation algorithms for multi-leaf collimated fields using film dosimetry, we compute dose distributions for multi-field conformal treatment using fields shaped with either the multi-leaf collimator or conventional cerrobend blocks. We compare the two sets of treatment plans using graphical isodose displays, tissue specific dose volume histograms, tumor control probabilities, and normal tissue complication probabilities. We also incorporate setup errors into the calculated dose distributions to assess the effect of treatment uncertainties on the various criteria. Based on these comparisons, we conclude that for multi-field conformal radiotherapy for these two disease sites, the use of multi-leaf collimation is equivalent to that of conventional cerrobend blocks.

Adenocarcinoma↗
PubMed1993

Perspectives of multidimensional conformal radiation treatment.

We consider the present technological advancement that underlies the implementation of computer-controlled conformal radiotherapy. We also consider the developments in modern biology that may provide input to therapy planning. The concept of multidimensional conformal radiotherapy is advanced, which integrates geometrical precision and biological conformality, to optimize the treatment planning for individual patients, with a view to improve the overall success of radiotherapy.

Humans↗
PubMed1991

The role of uncertainty analysis in treatment planning.

The role of uncertainty analysis in 3-D treatment planning systems was addressed by four institutions which contracted with NCI to evaluate high energy photon external beam treatment planning. Treatment plans were developed at eight disease sites and the effects of uncertainties assessed in a number of experiments. Uncertainties which are patient-site specific included variations in the delineation of target volumes and normal tissues and the effects of positional uncertainties due to physiological motion and setup nonreproducibility. These were found to have a potentially major impact on the doses to the target volumes and to critical normal tissues which could result in significantly altered probabilities of tumor control and normal tissue complications. Other uncertainties, such as the conversion of CT data to electron densities, heterogeneities and dose calculation algorithms' weaknesses, are related to physical processes. The latter was noted to have the greatest potential contribution to uncertainty in some sites. A third category of uncertainty related to the treatment machine, the consequences of compensator misregistration, are exclusive to the site and the treatment portal. Because conventional treatment planning systems have not incorporated uncertainty analysis, tools and techniques had to be devised for this work; further development in this area is needed. Many of the analyses could not have been done without full 3-D capabilities of the planning systems, and it can be anticipated that the availability of uncertainty analysis in these systems which allow nontraditional beam arrangements will be of great value.

Humans↗
PubMed1991

Three-dimensional treatment planning for lung cancer.

The experience of four institutions involved in a three-dimensional treatment planning contract (NCI) for lung cancer is described. It was found that three-dimensional treatment planning has a significant potential for optimization of treatment plans for radiotherapy of lung cancer both for tumor coverage and sparing of critical normal tissues within the complex anatomy of the human thorax. Evaluation tools, such as dose-volume histograms, and three-dimensional isodose displays, such as multiple plane views, surface dose displays, etc., were found to be extremely valuable in evaluation and comparison of these complex plans. It is anticipated that with further developments in three-dimensional simulation and treatment delivery systems, major progress towards uncomplicated local regional control of lung cancer may be forthcoming.

Humans↗