Quantitative Assessment of Lipiodol-Related Artifact Reduction for Dual-Energy Computed Tomography After Transcatheter Arterial Chemoembolization: A Phantom Study Evaluating the Use of Metal Artifact Reduction Algorithms.

OBJECTIVE: This study used metal artifact reduction (MAR) software to examine the computed tomography (CT) number of dual-energy CT (DECT) of hepatocellular carcinoma after transcatheter arterial chemoembolization. METHODS: Hollow columnar acrylic phantoms were filled with lipiodol and inserts of 2 sizes (large and small) were used to simulate liver tumors on a Revolution GSI CT scanner. The CT numbers of a single test object were collected twice: once with and once without the MAR algorithm. Lipiodol beam-hardening artifacts were quantified by measuring CT numbers in a region of interest around the tumor-simulating insert. RESULTS: The virtual monochromatic CT numbers of large and small tumors were closely related to energy. For small tumors, CT numbers increased with energy. For large tumors, CT numbers increased with energy at 1 cm from the margin but decreased with an increase in energy at 5 cm. Regardless of the size, distance, or location of the tumor, the CT numbers fluctuated more at low energy levels. CONCLUSIONS: At 1 cm from the margin, the CT numbers with MAR were significantly different from those without MAR. Low-energy CT numbers with MAR were near reference values. Metal artifact reduction exhibited superior performance for small tumors. Tumor margin images are affected by artifacts caused by Lipiodol. However, with MAR, CT numbers can be effectively calibrated, thus enabling clinicians to more accurately evaluate hepatocellular carcinoma development and identify residual tumors and recurrent or metastatic lesions.

Validation of Virtual Monochromatic Images and Effect of Body Size Obtained Using a Rapid kVp-switching Dual-energy Computed Tomography System: A Phantom Study.

ABSTRACT: The objective of this paper is to validate virtual monochromatic computed tomography (CT) numbers and the effect of the body size of insert materials in phantoms on the findings of a dual-energy CT scanner. The material inserted in the phantom simulates human organs. This study investigated the effect of different body sizes on CT numbers to understand the accuracy of dual-energy CT. The effect of body size on virtual monochromatic CT numbers was investigated using a QRM phantom. The true monochromatic CT numbers of insert materials were calculated from coefficients obtained using NIST XCOM. The true Z eff values were supplied by phantom manufacturers or computed using Mayneord's equation. The virtual monochromatic CT numbers of insert materials in both the phantoms varied with energy. The CT numbers of materials with a Z eff of >7.42 (water Z eff ) and <7.42 decreased and increased with energy, respectively. The CT numbers were affected by phantom size as a function of energy. For water, tissues, and air, the CT numbers in the XL phantom were considerably larger than those in other phantom sizes at 40 keV. Body size affected the CT numbers, particularly for the XL size and at low energies. For all materials, the magnitude of difference between the measured and true CT numbers was related to the Z eff of the materials, potentially because the photoelectric effect is more prominent at low energies for materials with a higher Z eff . The difference in CT numbers appeared to be dependent on position. The true and measured Z eff agreed to within 6% for all the materials except the SR2 brain, for which the discrepancy was 25%.

Treatment plan comparison between stereotactic body radiation therapy techniques for prostate cancer: non-isocentric CyberKnife versus isocentric RapidArc.

PURPOSE: The aim of this study was to evaluate the feasibility and dose distribution of two different stereotactic body radiation therapy (SBRT) techniques, isocentric RapidArc (RA) and non-isocentric CyberKnife (CK), for the treatment of localized prostate cancer. METHODS: Two groups of patients (Groups 1 and 2 with ten patients per group) treated with CK were re-planned with RA. The patients were grouped according to the rectum constraint used (Group1, maximum dose for rectum; Group 2, dose-volume histogram for rectum). The prescription dose was 37.5 Gy in five fractions. The two SBRT techniques were compared by target coverage, normal tissue sparing, and dose distribution parameters. Monitor units (MUs) and the delivery time were likewise compared to assess delivery efficiency. RESULTS: The RA plans consistently exhibited superior PTV coverage and better rectum sparing at low doses in the both groups. The conformity and heterogeneity indices of the RA plans were better than the CK plans. Additionally, the RA plans resulted in fewer low-dose regions, lower MUs, and faster delivery times than the CK plans. CONCLUSIONS: The good dosimetric distribution and shorter delivery time make RA an attractive SBRT technique for the treatment of localized prostate cancer.

Effects of quality assurance regulatory enforcement on performance of mammography systems: evidence from large-scale surveys in Taiwan.

OBJECTIVE: The Standards for Medical Exposure Quality Assurance in mammography systems were enacted on July 1, 2008, in Taiwan. This study aimed to evaluate the trends in performance of mammography units before and after the regulation started on the basis of annual on-site surveys from 2008 to 2010. MATERIALS AND METHODS: On-site measurements were conducted on 215, 205, and 209 mammography units in 2008, 2009, and 2010, respectively, which accounted for more than 95% of all units in Taiwan. Phantom image quality, average glandular dose (AGD), and half-value layer were evaluated on all systems. Processor conditions, compression conditions, radiation output, and computed radiography exposure indicators were assessed on units participating in mammography screening in 2008 and on all units in the later years. Evaluations of maximum compression force and automatic exposure control reproducibility were added into the protocol from 2009 onward. RESULTS: Mean phantom scores were improved significantly from 2008 to 2009 (11.63 ± 1.30 vs 12.31 ± 0.94, p < 0.05) and remained stable for 2010 (12.35 ± 0.87). Mean AGDs were 1.48 ± 0.47, 1.38 ± 0.41, and 1.37 ± 0.42 mGy over the 3 years, with a significant reduction from 2008 to 2009 (p < 0.05). For film-screen mammography systems, variations of sensitometric curves were greatly reduced in 2009 and 2010 when compared with 2008. Passing rates were increased after the regulation took effect in almost all aspects. CONCLUSION: Results from large-scale on-site surveys showed an overall improvement in performance after quality assurance in mammography was enforced in Taiwan.

Calibration of EBT2 film by the PDD method with scanner non-uniformity correction.

The EBT2 film together with a flatbed scanner is a convenient dosimetry QA tool for verification of clinical radiotherapy treatments. However, it suffers from a relatively high degree of uncertainty and a tedious film calibration process for every new lot of films, including cutting the films into several small pieces, exposing with different doses, restoring them back and selecting the proper region of interest (ROI) for each piece for curve fitting. In this work, we present a percentage depth dose (PDD) method that can accurately calibrate the EBT2 film together with the scanner non-uniformity correction and provide an easy way to perform film dosimetry. All films were scanned before and after the irradiation in one of the two homemade 2 mm thick acrylic frames (one portrait and the other landscape), which was located at a fixed position on the scan bed of an Epson 10 000XL scanner. After the pre-irradiated scan, the film was placed parallel to the beam central axis and sandwiched between six polystyrene plates (5 cm thick each), followed by irradiation of a 20 × 20 cm² 6 MV photon beam. Two different beams on times were used on two different films to deliver a dose to the film ranging from 32 to 320 cGy. After the post-irradiated scan, the net optical densities for a total of 235 points on the beam central axis on the films were auto-extracted and compared with the corresponding depth doses that were calculated through the measurement of a 0.6 cc farmer chamber and the related PDD table to perform the curve fitting. The portrait film location was selected for routine calibration, since the central beam axis on the film is parallel to the scanning direction, where non-uniformity correction is not needed (Ferreira et al 2009 Phys. Med. Biol. 54 1073-85). To perform the scanner non-uniformity calibration, the cross-beam profiles of the film were analysed by referencing the measured profiles from a Profiler™. Finally, to verify our method, the films were exposed to 60° physical wedge fields and the compositive fields, and their relative dose profiles were compared with those from the water phantom measurement. The fitting uncertainty was less than 0.5% due to the many calibration points, and the overall calibration uncertainty was within 3% for doses above 50 cGy, when the average of four films were used for the calibration. According to our study, the non-uniformity calibration factor was found to be independent of the given dose for the EBT2 film and the relative dose differences between the profiles measured by the film and the Profiler were within 1.5% after applying the non-uniformity correction. For the verification tests, the relative dose differences between the measurements by films and in the water phantom, when the average of three films were used, were generally within 3% for the 60° wedge fields and compositive fields, respectively. In conclusion, our method is convenient, time-saving and cost-effective, since no film cutting is needed and only two films with two exposures are needed.

Evaluation of tumor motion effects on dose distribution for hypofractionated intensity-modulated radiotherapy of non-small-cell lung cancer.

Respiration-induced tumor motion during intensity-modulated radiotherapy (IMRT) of non-small-cell lung cancer (NSCLC) could cause substantial differences between planned and delivered doses. While it has been shown that, for conventionally fractionated IMRT, motion effects average out over the course of many treatments, this might not be true for hypofractionated IMRT (IMHFRT). Numerical simulations were performed for nine NSCLC patients (11 tumors) to evaluate this problem. Dose distributions to the Clinical Target Volume (CTV) and Internal Target Volume (ITV) were retrospectively calculated using the previously-calculated leaf motion files but with the addition of typical periodic motion (i.e. amplitude 0.36-1.26cm, 3-8sec period). A typical IMHFRT prescription of 20Gy x 3 fractions was assumed. For the largest amplitude (1.26 cm), the average +/- standard deviation of the ratio of simulated to planned mean dose, minimum dose, D95 and V95 were 0.98+/-0.01, 0.88 +/- 0.09, 0.94 +/- 0.05 and 0.94 +/- 0.07 for the CTV, and 0.99 +/-0.01, 0.99 +/- 0.03, 0.98 +/- 0.02 and 1.00 +/- 0.01 for the ITV, respectively. There was minimal dependence on period or initial phase. For typical tumor geometries and respiratory amplitudes, changes in target coverage are minimal but can be significant for larger amplitudes, faster beam delivery, more highly-modulated fields, and smaller field margins.

Verification and source-position error analysis of film reconstruction techniques used in the brachytherapy planning systems.

A method was presented that employs standard linac QA tools to verify the accuracy of film reconstruction algorithms used in the brachytherapy planning system. Verification of reconstruction techniques is important as suggested in the ESTRO booklet 8: "The institution should verify the full process of any reconstruction technique employed clinically." Error modeling was also performed to analyze seed-position errors. The "isocentric beam checker" device was used in this work. It has a two-dimensional array of steel balls embedded on its surface. The checker was placed on the simulator couch with its center ball coincident with the simulator isocenter, and one axis of its cross marks parallel to the axis of gantry rotation. The gantry of the simulator was rotated to make the checker behave like a three-dimensional array of balls. Three algorithms used in the ABACUS treatment planning system: orthogonal film, 2-films-with-variable-angle, and 3-films-with-variable-angle were tested. After exposing and digitizing the films, the position of each steel ball on the checker was reconstructed and compared to its true position, which can be accurately calculated. The results showed that the error is dependent on the object-isocenter distance, but not the magnification of the object. The averaged errors were less than 1 mm within the tolerance level defined by Roué et al. ["The EQUAL-ESTRO audit on geometric reconstruction techniques in brachytherapy," Radiother. Oncol. 78, 78-83 (2006)]. However, according to the error modeling, the theoretical error would be greater than 2 mm if the objects were located more than 20 cm away from the isocenter with a 0.5 degrees reading error of the gantry and collimator angles. Thus, in addition to carefully performing the QA of the gantry and collimator angle indicators, it is suggested that the patient, together with the applicators or seeds inside, should be placed close to the isocenter as much as possible. This method could be used to test the reconstruction techniques of any planning system, and the most suitable one can be chosen for clinical use.

A statistical approach to infer the minimum setup distance of a well chamber to the wall or to the floor for 192Ir HDR calibration.

We propose a new method based on statistical analysis technique to determine the minimum setup distance of a well chamber used in the calibration of 192Ir high dose rate (HDR). The chamber should be placed at least this distance away from any wall or from the floor in order to mitigate the effect of scatter. Three different chambers were included in this study, namely, Sun Nuclear Corporation, Nucletron, and Standard Imaging. The results from this study indicated that the minimum setup distance varies depending on the particular chamber and the room architecture in which the chamber was used. Our result differs from that of a previous study by Podgorsak et al. [Med. Phys. 19, 1311-1314 (1992)], in which 25 cm was suggested, and also differs from that of the International Atomic Energy Agency (IAEA)-TECDOC-1079 report, which suggested 30 cm. The new method proposed in this study may be considered as an alternative approach to determine the minimum setup distance of a well-type chamber used in the calibration of 192Ir HDR.

The role of external beam in brachytherapy.

In this paper we put forward the claim that the combination of low dose-rate brachytherapy (BRT) and fractionated external-beam radiotherapy (EBRT) may, when planned to take advantage of the relative advantages of each of these two modalities, result in enhanced tumor dose with no penalties to organs at risk. The concept of iso-effective dose (IED) serves the role of common currency for fusing BRT and EBRT and, for evaluation purposes, converting back the resulting IED distribution into a biologically equivalent plan delivered by any single modality. If we accept this view, there are further questions that must be answered regarding practical matters. We show how to deal with these questions by describing an actual patient plan.

Reduced-order parameter optimization for simplifying prostate IMRT planning.

Intensity-modulated radiotherapy (IMRT) has become an effective tool for cancer treatment with radiation. However, even expert radiation planners still need to spend a substantial amount of time manually adjusting IMRT optimization parameters such as dose limits and costlet weights in order to obtain a clinically acceptable plan. In this paper, we describe two main advances that simplify the parameter adjustment process for five-field prostate IMRT planning. First, we report the results of a sensitivity analysis that quantifies the effect of each hand-tunable parameter of the IMRT cost function on each clinical objective and the overall quality of the resulting plan. Second, we show that a recursive random search over the six most sensitive parameters as an outer loop in IMRT planning can quickly and automatically determine parameters for the cost function that lead to a plan meeting the clinical requirements. Our experiments on a ten-patient dataset show that for 70% of the cases, we can automatically determine a plan in 10 min (on the average) that is either clinically acceptable or requires only minor adjustment by the planner. The outer-loop optimization can be easily integrated into a traditional IMRT planning system.

Room scatter factor modelling and measurement error analysis of 192Ir HDR calibration by a Farmer chamber.

We introduce an empirical formula to directly calculate the room scatter factor in the calibration of (192)Ir HDR using a Farmer chamber. Compared to the data of Selvam et al (2001 Phys. Med. Biol. 46 2299), our formula is accurate to within 0.3%. Our method saves time because the room scatter can be obtained in one calculation rather than being deduced through a series of setups of different source-chamber distances. It could also be cost effective because a calibration jig might be no longer necessary. We only need to position the applicator and chamber at a fixed space in air and measure its distance. We also analysed the effects of two possible errors arising from ignoring the room scatter and the measurement error of the source-chamber distance. We recommend that the source-chamber distance should be at least 10 cm.

Learning the relationship between patient geometry and beam intensity in breast intensity-modulated radiotherapy.

Intensity modulated radiotherapy (IMRT) has become an effective tool for cancer treatment with radiation. However, even expert radiation planners still need to spend a substantial amount of time adjusting IMRT optimization parameters in order to get a clinically acceptable plan. We demonstrate that the relationship between patient geometry and radiation intensity distributions can be automatically inferred using a variety of machine learning techniques in the case of two-field breast IMRT. Our experiments show that given a small number of human-expert-generated clinically acceptable plans, the machine learning predictions produce equally acceptable plans in a matter of seconds. The machine learning approach has the potential for greater benefits in sites where the IMRT planning process is more challenging or tedious.

Intensity-modulated radiotherapy as the boost or salvage treatment of nasopharyngeal carcinoma: the appropriate parameters in the inverse planning and the effect of patient's anatomic factors on the planning results.

The current study demonstrates that the large increase in normal tissue penalty often degrades target dose uniformity without a concomitant large improvement in normal tissue dose, especially in anatomically unfavorable patients. The excessively large normal tissue penalties do not improve treatment plans for patients having unfavorable geometry.

A new method of incorporating systematic uncertainties in intensity-modulated radiotherapy optimization.

Uncertainties in tumor position during intensity-modulated radiotherapy (IMRT) plan optimization are usually accounted for by adding margins to a clinical target volume (CTV), or additionally, to organs at risk (OAR). The former approach usually favors target coverage over OAR protection, whereas the latter does not account for correlation in target and OAR movement. We investigate a new approach to incorporate systematic errors in tumor and organ position. The method models a distribution of systematic errors due to setup error and organ motion with displaced replicas of volumes of interest, each representing the patient geometry for a possible systematic error, and maximizes a score function that counts the number of replicas meeting dose or biological constraints for both CTV and OAR. Dose constraints are implemented by logistic functions of Niemierko's generalized model of equivalent uniform dose (EUD). The method is applied to prostate and nasopharynx IMRT plans, in which CTV and OAR each consists of five replicas, one representing no error (the position in the planning CT) and the other four discrete systematic setup displacements in one dimension with equal probability. The resulting IMRT plans are compared with those from two other EUD-based optimizations: a standard planning target volume (PTV) approach consisting of a single replica of each OAR in the planned position and a single PTV encompassing all CTV replicas, and a PTV-PRV approach consisting of a single PTV and a single planning risk volume (PRV) for each OAR encompassing all replicas. When systematic error is present, multiple-replica optimization provides better critical organ protection while maintaining similar target coverage compared with the PTV approach, and provides better CTV-to-OAR therapeutic ratio compared with the PTV-PRV instances where there is substantial PTV-PRV overlap. The method can be used for other systematic errors due to organ motion and deformation.

Comparison of end normal inspiration and expiration for gated intensity modulated radiation therapy (IMRT) of lung cancer.

BACKGROUND AND PURPOSE: Gated delivery of radiation during part of the respiration cycle may improve the treatment of lung cancer with intensity modulated radiation therapy (IMRT). In terms of the respiration phase for gated treatment, normal end-expiration (EE) is more stable but normal end-inspiration (EI) increases lung volume. We compare the relative merit of using EI and EE in gated IMRT for sparing normal lung tissue. PATIENTS AND METHODS: Ten patients received EI and EE respiration-triggered CT scans in the treatment position. An IMRT plan for a prescription dose of 70 Gy was generated for each patient and at each respiration phase. The optimization constraints included target dose uniformity, less than 35% of the total lung receiving 20 Gy or more and maximum cord dose

Intensity-modulated radiotherapy technique for three-field breast treatment.

PURPOSE: To develop a simplified intensity-modulated radiotherapy (IMRT) algorithm for three-field breast treatment using a single isocenter setup. The algorithm aims to deliver a uniform dose throughout the breast volume. Special attention was paid to the highly divergent nature of the beam configuration. METHODS AND MATERIALS: Computed tomography (CT) image setup of the patient was acquired. On each CT slice, the computer automatically generated lines parallel to the posterior edge of the tangent field. The mid-point of each line segment that intersected the breast was determined and the dose from an open field calculated. The intensity of the divergent pencil beam corresponding to the mid-point was set to be inversely proportional to the open field dose to the mid-point. Forward dose calculation was then performed using this intensity distribution. RESULTS: A total of 15 breast cancer patients undergoing three-field IMRT who underwent planning and treatment with this algorithm were included in this study. Compared with standard wedged pair tangents, the IMRT plan produced statistically significant better dose distributions in terms of target coverage and target dose uniformity, as well as reduced dose to the contralateral breast and reduced hot spots to the ipsilateral lung. CONCLUSION: Since March 2004, the new IMRT algorithm has been used for planning and treatment of > 20 patients undergoing three-field treatment, as well as >200 patients undergoing regular two-field tangent treatment, all with excellent dose distributions throughout the breast volume.

Methodology for biologically-based treatment planning for combined low-dose-rate (permanent implant) and high-dose-rate (fractionated) treatment of prostate cancer.

PURPOSE: The combination of permanent low-dose-rate interstitial implantation (LDR-BRT) and external beam radiotherapy (EBRT) has been used in the treatment of clinically localized prostate cancer. While a high radiation dose is delivered to the prostate in this setting, the actual biologic dose equivalence compared to monotherapy is not commonly invoked. We describe methodology for obtaining the fused dosimetry of this combined treatment and assigning a dose equivalence which in turn can be used to develop desired normal tissue and target constraints for biologic-based treatment planning. METHODS AND MATERIALS: Patients treated with this regimen initially receive an I-125 implant prescribed to 110 Gy followed, 2 months later, by 50.4 Gy in 28 fractions using intensity-modulated external beam radiotherapy. Ab initio methodology is described, using clinically derived biologic parameters (alpha, beta, potential doubling time for prostate cancer cells [T(pot)], cell loss factor), for calculating tumor control probability isoeffective doses for the combined LDR and conventional fraction EBRT treatment regimen. As no such formalism exists for assessing rectal or urethral toxicity, we make use of semi-empirical expressions proposed for describing urethral and rectal complication probabilities for specific treatment situations (LDR and fractionation, respectively) and utilize the notion of isoeffective dose to extend these results to combined LDR-EBRT regimens. RESULTS: The application to treatment planning of the methodology described in this study is illustrated with real-patient data. We evaluate the effect of changing LDR and EBRT prescription doses (in a manner that remains isoeffective with 81 Gy EBRT alone or with 144 Gy LDR monotherapy) on rectal and urethral complication probabilities, and suggest that it should be possible to improve the therapeutic ratio by exploiting joint LDR-EBRT planning. CONCLUSIONS: We describe new methodology for biologically based treatment planning for patients who receive combined low-dose-rate brachytherapy and external beam radiotherapy for prostate cancer. Using relevant mathematical tools, we demonstrate the feasibility of fusing dose distributions from each treatment for this combined regimen, which can then be expressed as isoeffective dose distributions. Based on this information, dose constraints for the rectum and urethra are described which could be used for planning such combination regimens.

Comparison of intensity-modulated radiotherapy with three-dimensional conformal radiation therapy planning for glioblastoma multiforme.

This study was designed to assess the feasibility and potential benefit of using intensity-modulated radiotherapy (IMRT) planning for patients newly diagnosed with glioblastoma multiforme (GBM). Five consecutive patients with confirmed histopathologically GBM were entered into the study. These patients were planned and treated with 3-dimensional conformal radiation therapy (3DCRT) using our standard plan of 3 noncoplanar wedged fields. They were then replanned with the IMRT method that included a simultaneous boost to the gross tumor volume (GTV). The dose distributions and dose-volume histograms (DHVs) for the planning treatment volume (PTV), GTV, and the relevant critical structures, as obtained with 3DCRT and IMRT, respectively, were compared. In both the 3DCRT and IMRT plans, 59.4 Gy was delivered to the GTV plus a margin of 2.5 cm, with doses to critical structures below the tolerance threshold. However, with the simultaneous boost in IMRT, a higher tumor dose of approximately 70 Gy could be delivered to the GTV, while still maintaining the uninvolved brain at dose levels of the 3DCRT technique. In addition, our experience indicated that IMRT planning is less labor intensive and time consuming than 3DCRT planning. Our study shows that IMRT planning is feasible and efficient for radiotherapy of GBM. In particular, IMRT can deliver a simultaneous boost to the GTV while better sparing the normal brain and other critical structures.

Technological advances in external-beam radiation therapy for the treatment of localized prostate cancer.

The relative inability of conventional radiotherapy to control localized prostate cancer results from resistance of subpopulations of tumor clonogens to dose levels of 65 to 70 Gy, the maximum feasible with traditional two-dimensional (2D) treatment planning and delivery techniques. Several technological advances have enhanced the precision and improved the outcome of external-beam radiotherapy. The three-dimensional conformal radiotherapy (3D-CRT) approach has permitted significant increases in the tumor dose to levels beyond those feasible with conventional techniques. Intensity-modulated radiotherapy (IMRT), an advanced form of conformal radiotherapy, has resulted in reduced rectal toxicity, permitting tumor dose escalation to previously unattainable levels with a concomitant improvement in local tumor control and disease-free survival. The combination of androgen deprivation and conventional-dose radiotherapy, tested mainly in patients with locally advanced disease, has also produced significant outcome improvements. Whether androgen deprivation will preclude the need for dose escalation or whether high-dose radiotherapy will obviate the need for androgen deprivation remains unknown. In some patients, both approaches may be necessary to maximize the probability of cure. In view of the favorable benefit-risk ratio of high-dose IMRT, the design of clinical trials to resolve these critical questions is essential.

The effects of intra-fraction organ motion on the delivery of intensity-modulated field with a multileaf collimator.

Intensity-modulated radiation therapy can be conveniently delivered with a multileaf collimator. With this method, the entire field is not delivered at once, but rather it is composed of many subfields defined by the leaf positions as a function of beam on time. At any given instant, only these subfields are delivered. During treatment, if the organ moves, part of the volume may move in or out of these subfields. Due to this interplay between organ motion and leaf motion the delivered dose may be different from what was planned. In this work, we present a method that calculates the effects of organ motion on delivered dose. The direction of organ motion may be parallel or perpendicular to the leaf motion, and the effect can be calculated for a single fraction or for multiple fractions. Three breast patients and four lung patients were included in this study,with the amplitude of the organ motion varying from +/- 3.5 mm to +/- 10 mm, and the period varying from 4 to 8 seconds. Calculations were made for these patients with and without organ motion, and results were examined in terms of isodose distribution and dose volume histograms. Each calculation was repeated ten times in order to estimate the statistical uncertainties. For selected patients, calculations were also made with conventional treatment technique. The effects of organ motion on conventional techniques were compared relative to that on IMRT techniques. For breast treatment, the effect of organ motion primarily broadened the penumbra at the posterior field edge. The dose in the rest of the treatment volume was not significantly affected. For lung treatment, the effect also broadened the penumbra and degraded the coverage of the planning target volume (PTV). However, the coverage of the clinical target volume (CTV) was not much affected, provided the PTV margin was adequate. The same effects were observed for both IMRT and conventional treatment techniques. For the IMRT technique, the standard deviations of ten samples of a 30-fraction calculation were very small for all patients, implying that over a typical treatment course of 30 fractions, the delivered dose was very close to the expected value. Hence, under typical clinical conditions, the effect of organ motion on delivered dose can be calculated without considering the interplay between the organ motion and the leaf motion. It can be calculated as the weighted average of the dose distribution without organ motion with the distribution of organ motion. Since the effects of organ motion on dose were comparable for both IMRT and conventional techniques, the PTV margin should remain the same for both techniques.