Dose Delivery Errors Research Articles

SU-FF-T-294: Modeling of Tomotherapy Delivery to Predict Dose Errors Due to Tumor Motion in Lung

Purpose: To develop an approximate model to the convolution technique simulations to predict tomotherapy dose delivery errors in the presence of respiratory motion using one-dimensional breathing motion and unmodulated delivery models. Catalog the “space” of safe treatments as defined by the model. Method: The patient breathing motion data, acquired as a function of tidal volume (ml) is converted to distance (mm) with time. Moving-average of the motion data is determined where the average is taken over the time span of time taken for a point at the edge of the field to cross other edge with constant couch velocity. The time axis is converted into distance by multiplying by the couch speed. Moving difference of the moving-average of the motion data is determined over the same time span to determine the drift of the point during the treatment. The dose error ratio is then found by dividing the drift by the field size. Results: Dose error ratio for combination of delivery parameters (couch speed, field size) and breathing magnitude for 42 clinical patient motions was studied. The dose error prediction by this approximate model is compared with convolution technique simulation is presented for different delivery scenarios for two different field size. Conclusion: The results show the proposed approximate model predicts the dose errors ratio reasonably close to convolution dose error prediction approach for clinically used tomotherapy delivery parameters. The approximate prediction model breaks down for extreme conditions of breathing magnitude and couch velocity. It will allow the treatment planner to determine if Tomotherapy treatment delivery is appropriate for specific patients whose tumors move due to breathing.

Retrospective analysis of dose delivery in intra-operative high dose rate brachytherapy

Background. This study was performed to quantify the inaccuracy in clinical dose delivery due to the incomplete scatter conditions inherent in intra-operative high dose rate (IOHDR) brachytherapy. Methods. Treatment plans of 10 patients previously treated in our facility, which had irregular shapes of treated areas, were used. Treatment geometries reflecting each clinical case were simulated using a phantom assembly with no added build-up on top of the applicator. The treatment planning geometry (full scatter surrounding the applicator) was subsequently simulated for each case by adding bolus on top of the applicator. Results. For geometries representing the clinical IOHDR incomplete scatter environment, measured doses at the 5 mm and 10 mm prescription depths were lower than the corresponding prescribed doses by about 7.7% and 11.1%, respectively. Also, for the two prescription methods, an analysis of the measured dose distributions and their corresponding treatment plans showed average decreases of 1.2 mm and 2.2 mm in depth of prescription dose, respectively. Conclusions. Dosimetric calculations with the assumption of an infinite scatter environment around the applicator and target volume have shown to result in dose delivery errors that significantly decrease the prescription depth for IOHDR treatment.

Dosimetric effects of patient rotational setup errors on prostate IMRT treatments

The purpose of this work is to determine dose delivery errors that could result from systematic rotational setup errors (ΔΦ) for prostate cancer patients treated with three-phase sequential boost IMRT. In order to implement this, different rotational setup errors around three Cartesian axes were simulated for five prostate patients and dosimetric indices, such as dose–volume histogram (DVH), tumour control probability (TCP), normal tissue complication probability (NTCP) and equivalent uniform dose (EUD), were employed to evaluate the corresponding dosimetric influences. Rotational setup errors were simulated by adjusting the gantry, collimator and horizontal couch angles of treatment beams and the dosimetric effects were evaluated by recomputing the dose distributions in the treatment planning system. Our results indicated that, for prostate cancer treatment with the three-phase sequential boost IMRT technique, the rotational setup errors do not have significant dosimetric impacts on the cumulative plan. Even in the worst-case scenario with ΔΦ = 3°, the prostate EUD varied within 1.5% and TCP decreased about 1%. For seminal vesicle, slightly larger influences were observed. However, EUD and TCP changes were still within 2%. The influence on sensitive structures, such as rectum and bladder, is also negligible. This study demonstrates that the rotational setup error degrades the dosimetric coverage of target volume in prostate cancer treatment to a certain degree. However, the degradation was not significant for the three-phase sequential boost prostate IMRT technique and for the margin sizes used in our institution.

A pulse-rate dependence of dose per monitor unit and its significant effect on wedged-shaped fields delivered with variable dose rate and a moving jaw

Wedge-shaped dose distributions are delivered on some modern linear accelerators with a virtual wedge, combining variable dose rate and a moving jaw. Drift in the wedge factor and wedge angle of a 20 X 20 cm field for the 60 degree virtual wedge was found commonplace in several models of linear accelerator from one manufacturer. It was found that errors in dose delivery both on and off axis could exceed 5% if quality assurance checks are limited to 10 X 10 cm or smaller fields or wedge angles of 45 degrees or less. A procedure to easily identify and remedy the problem is presented. In each case the change was due to variation in dose per monitor unit (D/MU) with the electron beam pulse rate. The variation was traced to a pair of circuit boards in the dosimetry system, one for each output measurement channel. Wedge factors and dose profiles measured before and after board replacement on 4 accelerators, and for a set of defective boards placed on one of the accelerators, were compared. The effect was largest for the wedge with the steepest profile (60 degree wedge angle) and the largest field measured: 20 X 20 cm. In this case, a 1% variation in D/MU with a factor of 5 reduction in pulse rate corresponded to an average 0.8% change in wedge factor and 0.8% change in the off axis ratio at 8.5 cm off axis on the high dose side of the wedge field, 0.3% on the low dose side. After board replacement, wedge factors and profiles measured on the 4 machines generally agreed to 2% for the full range of wedge angles and field sizes. Quality assurance of virtual wedges is discussed in light of the new findings.

A comparison between amplitude sorting and phase-angle sorting using external respiratory measurement for 4D CT

Respiratory motion can cause significant dose delivery errors in conformal radiation therapy for thoracic and upper abdominal tumors. Four-dimensional computed tomography (4D CT) has been proposed to provide the image data necessary to model tumor motion and consequently reduce these errors. The purpose of this work was to compare 4D CT reconstruction methods using amplitude sorting and phase angle sorting. A 16-slice CT scanner was operated in ciné mode to acquire 25 scans consecutively at each couch position through the thorax. The patient underwent synchronized external respiratory measurements. The scans were sorted into 12 phases based, respectively, on the amplitude and direction (inhalation or exhalation) or on the phase angle (0-360 degrees) of the external respiratory signal. With the assumption that lung motion is largely proportional to the measured respiratory amplitude, the variation in amplitude corresponds to the variation in motion for each phase. A smaller variation in amplitude would associate with an improved reconstructed image. Air content, defined as the amount of air within the lungs, bronchi, and trachea in a 16-slice CT segment and used by our group as a surrogate for internal motion, was correlated to the respiratory amplitude and phase angle throughout the lungs. For the 35 patients who underwent quiet breathing, images (similar to those used for treatment planning) and animations (used to display respiratory motion) generated using amplitude sorting displayed fewer reconstruction artifacts than those generated using phase angle sorting. The variations in respiratory amplitude were significantly smaller (P < 0.001) with amplitude sorting than those with phase angle sorting. The subdivision of the breathing cycle into more (finer) phases improved the consistency in respiratory amplitude for amplitude sorting, but not for phase angle sorting. For 33 of the 35 patients, the air content showed significantly improved (P < 0.001) correlation with the respiratory amplitude than with the phase angle, suggesting a stronger relationship between internal motion and amplitude. Overall, amplitude sorting performed better than phase angle sorting for 33 of the 35 patients and equally well for two patients who were immobilized with a stereotactic body frame and an abdominal compression plate.

SU-FF-T-202: Effect of Motion On Dosimetry in IMRT Fields

Purpose: The clinical implementation of IMRT, particularly for tumors of the thoracic cavity, may be complicated by the effects of breathing motion. We present an experimental evaluation of the effects of a sinusoidal breathing cycle on the dose delivered by IMRT plans. Method and Materials: A series of phantoms were placed on a wheeled cart connected to a computer-controlled step motor. The motor was moved with constant angular velocity, causing approximately sinusoidal motion of the phantom in the superior-inferior direction. The amplitude of the oscillation was 1 cm. The frequency of oscillation was matched to a 4 second breathing cycle. We measured the effects of this motion in open beams and IMRT fields in using films in the transverse and coronal planes, and with an ionization chamber at isocenter. Results: Dose profiles measured in a 10 × 10 cm open beam with a film placed at a depth of 10 cm in the horizontal plane confirm that the dose measured in the moving phantom is accurately modeled by the convolution of the static dose with an analytically determined motion kernel, despite the 20% change in dose induce by the motion in the penumbra region. When IMRT fields are delivered to the moving phantom, the change in the dose at isocenter varies, but is typically less than 5%. The variation at other points in the treatment region, however, is often much larger. The convolution model accurately predicts these changes except for regions dominated by small segments. Conclusions: We have demonstrated that the effects of sinusoidal oscillatory phantom motion on dosimetry in open beams and IMRT fields can be measured experimentally, and can be accounted for using a motion kernel convolution. Motion may lead to significant errors in dose delivery, particularly for tumors in the lung and thoracic cavity.

SU-FF-T-49: A QA Test to Check MLC Carriage Calibration

Purpose: The leaf span for a Varian MLC is 15 cm. Fields larger than this in the leaf motion direction must be split into multiple ports with a “carriage shift” between ports. Leaf position is a function of both carriage position and leaf position within the carriage. The carriage position is dictated by the outermost leaf position, and the carriages do not move during radiation delivery. We have developed a test to assess carriage positioning accuracy by comparing the same abutting MLC‐shaped fields both with and without carriage motion between the delivery of the two fields. Methods and Materials: A 14 cm wide × 40 cm long field was split into two 7 cm wide rectangular ports shaped by the MLC. Width is defined in the leaf motion direction. These two fields were delivered using two separate static MLC files, then using a single DMLC file. In the former case, the carriages move while the leaves remain stationary with respect to the carriages. In the latter case, the carriages remain stationary while the leaves move with respect to the carriages. Kodak XV film was taped to the collimator face for both cases and compared. Results: Both films appeared identical upon visual inspection. A quantitative analysis of the profiles was performed using the RIT software system. Comparison of the profiles revealed that the FWHM of the abutting region agreed to within 0.2 mm between the two films. Conclusion: If accuracy of leaf calibration has been demonstrated, differences between these profiles would imply carriage miscalibration. Carriage miscalibration would result in mispositioning of all leaves within the carriage, thereby causing significant dose delivery errors. This carriage calibration test could be performed routinely as part of the QA procedure for MLCs used for IMRT delivery.

SU‐FF‐T‐186: Dosimetric Effect of Patient Rotational Setup Errors On Prostate IMRT Plans

Purpose: To determine dose delivery errors that could result from systematic and random rotational setup errors for prostate patients treated with IMRT. Methods and Materials: Five prostate cancer patients treated with IMRT technique had their dose distributions re‐evaluated to assess the impact of systematic and random rotational setup errors. The IMRT treatments were delivered in three phases: 54 Gy to 95% of PTV1 (prostate and seminal vesicle plus a 10mm margin), 16.2 Gy to 95% of PTV2 (prostate plus a 5mm margin), and 5.4Gy to 95% of PTV3 (which was PTV2 excluding rectum). Rotational setup error can be equivalently simulated with matrix transformed equations by using the gantry, collimator and couch angles of treatment beams. Systematic rotational setup error ΔΦ was simulated around each of the three Cartesian axes (denoted as ΔΦLR, ΔΦAP, ΔΦSI) respectively. The dosimetric effect of random rotational setup error (Δσ) was also simulated with normal probability distribution. Various dosimetric parameters for the targets and surrounding normal organs were evaluated with respect to the IMRT plan used for treatment. Results: The cumulative dosimetric deviations of target (prostate) were within 1% when ΔΦ and Δσ were up to ±5° and 3° respectively. For OARs, when ΔΦLR=ΔΦAP=ΔΦSI=5° was simulated, the V40Gy and V70Gy of rectum deviated by 5.8±3.6% and 6.1±11.8% respectively, the V40Gy and V65Gy of bladder deviated by −1.0±2.9% and −1.9±4.4%. When ΔσLR=ΔσAP=ΔσSI=3° was simulated, the V40Gy and V70Gy of rectum deviated by −0.4±0.4% and −2.9±3.2%, the V40Gy and V65Gy of bladder deviated by 1.6±1.5% and 0.6±1.5%. Conclusions: For three phases sequential boost prostate IMRT technique, the rotational setup errors do not have a significant dosimetric impact on the cumulative target coverage when ΔΦ and Δσ were up to ±5° and 3° respectively. However, reduction of rotational setup errors can decrease doses to surrounding normal structures.

SU-FF-T-206: Effects of Static Dosimetric Leaf Gap On MLC-Based Small Beam Dose Distribution for Intensity Modulated Radiosurgery

Purpose: To evaluate the impact of static dosimetric leaf gap on MLC‐based small beam dose distribution and to compare the calculated results from planning system with actual measurement for intensity modulated radiosurgery.Methods and Materials: We determined the optimal dosimetric static leaf gap by comparing the profiles of MLC based small beam with those of the collimated fields. The applied leaf gaps were 0, 1, and 2 mm for comparison. The doughnut shaped PTV (6.1 cm3) and inner OAR (0.3 cm3) were delineated for delicate intensity modulated radiosurgery test plan. For the test, Millennium 120 leaf MLC and Eclipse Radiation Therapy Planning system (Varian) were used. For the measurement of dose, we used radiosurgery head phantom (model 605, CIRS, Norfolk, Virginia). Results: We found that 2 mm gap was optimal for the MLC based small beam. The maximum dose differences at the inside PTV, outside PTV, and inner OAR were 22.3%, 20.2%, and 35.2% for the 0 mm leaf gap, 17.8%, 22.8%, and 30.8% for the 1 mm leaf gap, and 5.5%, 8.5%, and 6.3% for the 2 mm leaf gap, respectively. In a humanoid head phantom study, the final dose distribution from the Eclipse planning system was significantly different from the measured values. The planned results were similar, while the measured showed large differences in dose according to the leaf gaps (range: 1.3 – 12.7%). Conclusion: An inadequate determination of the dosimetric static leaf gap during the RTP configuration can make errors from the final dose calculation, which can sometimes be confused with unwanted QA results of IMRS. An appropriate dosimetric leaf gap setting is critical during the commissioning of an inverse planning system and an incorrect setting can produce large dose delivery errors particularly in the delicate IMRS treatment.

Dose Delivery Accuracy of Therapeutic Photon and Electron Beams at Low Monitor Unit Settings

Dose delivery accuracy at low monitor units (LMU) was evaluated for photon and electron beams. Knowledge of this study is required for few dosimetric applications and to know the dose delivered to the patient when the treatment is delivered with few monitor units (MU). Dose measurements were carried out for photon and electron beams with 0.6 cm(3) PTW ion chamber in white polystyrene phantom at D(max) with a field size of 10 x 10 cm(2) at 100 cm FSD. The relative dose, which is the ratio of dose delivered per MU at the testing to that of the calibration condition, was found out. Significant deviation (+20% to +25%) in dose delivery was noticed for photon and electron beams (+39% to +45%) at LMU settings. Slightly higher inaccuracy in dose delivery was noticed for 6-MV compared to 18-MV photons. The deviation in dose delivery for electron beams was found to be energy-independent and the pattern of variation was similar for all electron energies. The dose delivery accuracy at LMU settings has to be ascertained before implementing conformal and IMRT (intensity- modulated radiotherapy) techniques. When there is dose nonlinearity, the treatment delivered with multiple small MU settings can result in significant error in dose delivery.

The Effect of Intravenous Contrast on Intensity-Modulated Radiation Therapy Dose Calculations for Head and Neck Cancer

The objective of this study was to determine if the use of intravenous contrast results in clinically important errors in intensity-modulated radiation therapy (IMRT) dose calculations for head and neck radiotherapy treatment planning. Nonionic, iodinated intravenous contrast (Iohexol) was administered during the treatment planning computed tomography (CT) scan of 5 patients with head and neck cancer of varying disease sites. The potential effect of intravenous contrast was studied by changing the density of the contrast-enhanced vessels. An inverse IMRT plan was generated from an unmanipulated "normal contrast" planning scan. We then applied the same planning parameters to a "no contrast" planning scan. The effect of intravenous contrast was quantified by calculating the percent change of dose in a variety of target and normal structures. To evaluate a worst-case scenario situation, this comparison was repeated by assigning the vessels the maximum density in our planning system ("maximum contrast" density plan). Dose differences between a planning set of images using intravenous contrast and a set of images without contrast were less than 0.2% for all relevant target volumes and critical structures. A worst-case scenario in which normal contrast was overridden with "maximum contrast" led to small dose differences, generally less than 0.5%. Planning head and neck IMRT from CT scans that contain intravenous contrast does not result in clinically important errors in dose delivery.

TU-C-T-6C-01: Introduction: “How much complexity is necessary for IMRT?

IMRT treatment planning and delivery systems have developed rapidly over the past 5 years. Every manufacturer of linear accelerators now offers IMRT delivery systems using multileaf collimators and some sort of automated delivery scheme. Most treatment planning systems now offer “inverse” planning algorithms that optimize fluence and dose distributions given a set of dose constraints for tumors and normal tissues. For some accelerator manufacturers, the output of the planning system is a set of multileaf shapes, or “segments” with associated intensities, which combine to give a variable intensity distribution for each beam angle. This delivery system is referred to as static multileaf IMRT (or SMLC). For others, the intensity distribution is continuous and the leaves move with variable speed across the field with the beam remaining on. This is called dynamic multileaf IMRT, or DMLC. SMLC tends to be somewhat time consuming relative to conventional delivery, as there is overhead due to communication between the MLC controller and the accelerator. For DMLC, very little time is lost relative to conventional beam delivery times. Most IMRT planning systems have a mechanism for controlling the complexity of the resulting plan, which is determined by the number of beam angles and the number of intensity levels specified in the plan. Highly complex plans are those that end up with large numbers of beam segments and small monitor units per segment, translating into rapid dose gradients within the patient. These are presumably desirable when a tumor is immediately adjacent to a very sensitive normal tissue and the dose falloff must be rapid outside the tumor border. On the other hand, dose delivery errors are expected to be greater with highly complex plans, since the dosimetry of small fields and the linearity of the dose at small numbers of monitor units, are rather uncertain. This fact drives us to limit the complexity as much as reasonably possible. Since DMLC does not have a significant time penalty for high complexity, there is a tendency to plan highly complex IMRT for these delivery systems. For SMLC, the constraints of treatment time tend to drive the planner to choose less complex solutions. The question to be addressed in this Symposium is how much complexity is necessary for IMRT. Clearly, the answer is site-dependent, but even at a given site, the answer is not clear. Today's speakers will describe the incentives for simplicity (time and dose accuracy), methods of controlling the complexity and the tradeoffs between complexity and conformality.

SU‐FF‐T‐385: Tolerance Levels for Dosimetric IMRT QA Based On Radiobiological Parameters

Purpose: Tolerance levels for dosimetric verification of IMRT plans are usually set at constant values. This QA process is not patient specific. It ignores the potential role of fraction size and treatment site in determining the clinical consequences of an error in dose calculation and/or delivery. The objective of this work is to develop dose tolerance levels for IMRT based on radiobiological parameters. Method and Materials: The linear quadratic cell survival model which accounts for the effects of both total dose and fraction size was used. A technique was developed to convert dosimetric tolerance levels between treatment schedules while maintaining a constant uncertainty in a radiobiological parameter. Specifically, a constant tolerance level in a radiobiological parameter is set, rather than in calculated/delivered dose. Calculations were performed for different treatment fractionations and α/β ratios mimicking a wide range of clinical situations. Results: Tumors with low α/β ratio (melanoma, prostate) are more sensitive to errors in total dose. Dosimetric tolerance for such tumors should be approximately 65% of those of other tumors. For such malignances, if a 3% error in calculated/delivered dose is acceptable for treatment of most tumors, this should be reduced to 2% for treatment of these more radiosensitive tumors. In addition, results show that in field late and early responding normal tissues are more sensitive to errors in dose than out of field tissues. Conclusion: Invariant tolerance levels are appropriate for a wide range of clinical IMRT schemes, especially given current uncertainties in clinical α/β values. Assuming that current dosimetric levels are appropriate for some clinical applications, the appropriate values are for all other applications are between 0.5–2 times the currently used values for IMRT QA. In conclusion, tolerance levels for dosimetric QA based on radiobiological parameters provide useful information and may be more appropriate for verifying calculated/delivered dose.

TU‐EE‐A1‐01: Accuracy of Clinical Dose Delivery in IOHDR Brachytherapy

Purpose: To investigate the accuracy of clinical dose delivery in intra‐operative high dose rate (IOHDR) brachytherapy. Method and Materials: The IOHDR brachytherapy treatments of 10 patients recently treated at our facility were reconstructed. Treatment geometries reflecting each clinical scenario were simulated by a phantom assembly with no added buildup on top of the applicator. EDR2 radiographic film placed at the prescription depth recorded dose distributions for each clinical case. The treatment planning geometry (full scatter surrounding the applicator) was subsequently simulated for each case by adding bolus on top of the applicator and radiographic film was again exposed at the treatment depth. After careful determination of the film's H&amp;D curve, absolute dose distributions in the plane of the prescription depth were evaluated for both scatter environments in each clinical case. Results: For the geometries simulating the treatment planning conditions of full scatter, the average dose measured at the treatment depths was within 2% of the prescription and dose distributions were in excellent agreement with the respective treatment plan. However, for the geometry simulating treatment conditions (no added scattering material above the applicator), the dose at the prescription depth was on average 11% lower (range 8–14%) than prescribed. An analysis of the delivered dose distributions and treatment plans shows a resulting average decrease of 2 mm (range 1.2–2.4 mm) in prescription depth. Conclusion: Dosimetry calculations for IOHDR brachytherapy are typically done with treatment planning systems with dose calculation algorithms that assume an infinite scatter environment around the applicator and target volume. We have shown that this assumption leads to dose delivery errors which result in significant foreshortening of the prescription depth. It may be clinically relevant to correct for these errors by augmenting the scatter environment or, preferably, by appropriately modifying the prescription dose entered into the treatment planning system.

Effect of patient setup errors on simultaneously integrated boost head and neck IMRT treatment plans

The purpose of this study is to determine dose delivery errors that could result from random and systematic setup errors for head-and-neck patients treated using the simultaneous integrated boost (SIB)-intensity-modulated radiation therapy (IMRT) technique. Twenty-four patients who participated in an intramural Phase I/II parotid-sparing IMRT dose-escalation protocol using the SIB treatment technique had their dose distributions reevaluated to assess the impact of random and systematic setup errors. The dosimetric effect of random setup error was simulated by convolving the two-dimensional fluence distribution of each beam with the random setup error probability density distribution. Random setup errors of sigma = 1, 3, and 5 mm were simulated. Systematic setup errors were simulated by randomly shifting the patient isocenter along each of the three Cartesian axes, with each shift selected from a normal distribution. Systematic setup error distributions with Sigma = 1.5 and 3.0 mm along each axis were simulated. Combined systematic and random setup errors were simulated for sigma = Sigma = 1.5 and 3.0 mm along each axis. For each dose calculation, the gross tumor volume (GTV) received by 98% of the volume (D(98)), clinical target volume (CTV) D(90), nodes D(90), cord D(2), and parotid D(50) and parotid mean dose were evaluated with respect to the plan used for treatment for the structure dose and for an effective planning target volume (PTV) with a 3-mm margin. Simultaneous integrated boost-IMRT head-and-neck treatment plans were found to be less sensitive to random setup errors than to systematic setup errors. For random-only errors, errors exceeded 3% only when the random setup error sigma exceeded 3 mm. Simulated systematic setup errors with Sigma = 1.5 mm resulted in approximately 10% of plan having more than a 3% dose error, whereas a Sigma = 3.0 mm resulted in half of the plans having more than a 3% dose error and 28% with a 5% dose error. Combined random and systematic dose errors with sigma = Sigma = 3.0 mm resulted in more than 50% of plans having at least a 3% dose error and 38% of the plans having at least a 5% dose error. Evaluation with respect to a 3-mm expanded PTV reduced the observed dose deviations greater than 5% for the sigma = Sigma = 3.0 mm simulations to 5.4% of the plans simulated. Head-and-neck SIB-IMRT dosimetric accuracy would benefit from methods to reduce patient systematic setup errors. When GTV, CTV, or nodal volumes are used for dose evaluation, plans simulated including the effects of random and systematic errors deviate substantially from the nominal plan. The use of PTVs for dose evaluation in the nominal plan improves agreement with evaluated GTV, CTV, and nodal dose values under simulated setup errors. PTV concepts should be used for SIB-IMRT head-and-neck squamous cell carcinoma patients, although the size of the margins may be less than those used with three-dimensional conformal radiation therapy.

Quantifying IOHDR brachytherapy underdosage resulting from an incomplete scatter environment

Most brachytherapy planning systems are based on a dose calculation algorithm that assumes an infinite scatter environment surrounding the target volume and applicator. Dosimetric errors from this assumption are negligible. However, in intraoperative high-dose-rate brachytherapy (IOHDR) where treatment catheters are typically laid either directly on a tumor bed or within applicators that may have little or no scatter material above them, the lack of scatter from one side of the applicator can result in underdosage during treatment. This study was carried out to investigate the magnitude of this underdosage. IOHDR treatment geometries were simulated using a solid water phantom beneath an applicator with varying amounts of bolus material on the top and sides of the applicator to account for missing tissue. Treatment plans were developed for 3 different treatment surface areas (4 x 4, 7 x 7, 12 x 12 cm(2)), each with prescription points located at 3 distances (0.5 cm, 1.0 cm, and 1.5 cm) from the source dwell positions. Ionization measurements were made with a liquid-filled ionization chamber linear array with a dedicated electrometer and data acquisition system. Measurements showed that the magnitude of the underdosage varies from about 8% to 13% of the prescription dose as the prescription depth is increased from 0.5 cm to 1.5 cm. This treatment error was found to be independent of the irradiated area and strongly dependent on the prescription distance. Furthermore, for a given prescription depth, measurements in planes parallel to an applicator at distances up to 4.0 cm from the applicator plane showed that the dose delivery error is equal in magnitude throughout the target volume. This study demonstrates the magnitude of underdosage in IOHDR treatments delivered in a geometry that may not result in a full scatter environment around the applicator. This implies that the target volume and, specifically, the prescription depth (tumor bed) may get a dose significantly less than prescribed. It might be clinically relevant to correct for this inaccuracy.

Breathing motion induced dose errors in serial tomotherapy

Motion due to breathing often results in up to several centimeters of target motion during treatment. There have been a few studies showing the dosimetric consequences of target motion during static and dynamic multileaf collimator-based IMRT delivery. However, no studies have been reported on breathing-motion induced dose errors in serial tomotherapy. It has been reported in the literature that even small misalignments in serial tomotherapy will result in large errors in dose delivery (25%/mm) where the individual serial deliveries (indices) abut.

Breathing motion induced dose errors in serial tomotherapy

Motion due to breathing often results in up to several centimeters of target motion during treatment. There have been a few studies showing the dosimetric consequences of target motion during static and dynamic multileaf collimator-based IMRT delivery. However, no studies have been reported on breathing-motion induced dose errors in serial tomotherapy. It has been reported in the literature that even small misalignments in serial tomotherapy will result in large errors in dose delivery (25%/mm) where the individual serial deliveries (indices) abut.

Quantification of respiration-induced abdominal tumor motion and its impact on IMRT dose distributions

Purpose The treatment of moving targets with intensity-modulated radiotherapy may introduce errors in dose delivery. The motion of tumors in the abdomen was studied using quantitative fluoroscopic analysis, and the effect on dose delivery to the target was studied. Methods and materials Fluoroscopy sessions for 7 patients with pancreas or liver tumors and fiducial clips were recorded, converted to digital format, and analyzed to quantify the characteristics of tumor motion. Intensity-modulated radiotherapy plans were generated for 3 patients (a total of five plans), and the dose–volume histograms for the target volume were compared between plans with and without tumor motion. Results The average magnitude of the peak-to-peak motion for the 7 patients in the craniocaudal and AP directions was 7.4 mm and 3.8 mm, respectively. The clip motion varied widely, because the maximal clip excursions were about 47% greater than the average clip excursions for each patient. The inclusion of tumor motion did not lead to a significant degradation in the target dose–volume histogram for four of five treatment plans studied. Conclusion The amount of tumor motion for most patients in this study was not large but could, in some instances, significantly degrade the planned target dose–volume histogram. For some patients, therefore, motion mitigation or intervention during treatment may be necessary.

Quantification of respiration-induced abdominal tumor motion and its impact on IMRT dose distributions

Purpose The treatment of moving targets with intensity-modulated radiotherapy may introduce errors in dose delivery. The motion of tumors in the abdomen was studied using quantitative fluoroscopic analysis, and the effect on dose delivery to the target was studied. Methods and materials Fluoroscopy sessions for 7 patients with pancreas or liver tumors and fiducial clips were recorded, converted to digital format, and analyzed to quantify the characteristics of tumor motion. Intensity-modulated radiotherapy plans were generated for 3 patients (a total of five plans), and the dose–volume histograms for the target volume were compared between plans with and without tumor motion. Results The average magnitude of the peak-to-peak motion for the 7 patients in the craniocaudal and AP directions was 7.4 mm and 3.8 mm, respectively. The clip motion varied widely, because the maximal clip excursions were about 47% greater than the average clip excursions for each patient. The inclusion of tumor motion did not lead to a significant degradation in the target dose–volume histogram for four of five treatment plans studied. Conclusion The amount of tumor motion for most patients in this study was not large but could, in some instances, significantly degrade the planned target dose–volume histogram. For some patients, therefore, motion mitigation or intervention during treatment may be necessary.