Index of content:
Volume 34, Issue 6, June 2007
- Therapy Scientific Session: Room M100E
- New Dosimeters, Treatment Techniques and Clinical Applications
TH‐C‐M100E‐01: Numerical Feasibility Study of a Novel Absorbed Dose to Water Calorimeter‐Based Standard for 192Ir HDR Brachytherapy34(2007); http://dx.doi.org/10.1118/1.2761669View Description Hide Description
Purpose: To study the feasibility of developing a new standard for absorbed dose to water based on watercalorimetry for high dose rate iridium‐192 brachytherapysources.Method and Materials: The heat conduction pattern generated in water by the Nucleotron microSelectron‐HDR brachytherapysource was simulated using Comsol Multiphysics™ software.Source self‐heating due to self‐attenuation of photons was calculated with GEANT4. A smooth, well‐behaved three‐dimensional function was fit to the entire dose distribution data using TableCurve3D™. The heat‐loss correction Kc was calculated as the ratio of the temperature in the calorimeter under ideal conditions to realistic conditions. Results: The feasibility of a watercalorimeter based absorbed dose standard is determined by a balance between the requirements to obtain sufficient signal to perform a reproducible measurement, the effects of heat loss on the measured signal, and the positioning uncertainties. Due to self‐absorption, the source equilibrium temperature was found to be above the ambient temperature by a constant amount that depends only on setup conditions and source activity. For the source inside its nylon‐12 catheter inserted into water, the steady state excess temperature per unit source activity was found to be 0.5671 K/Ci. The source temperature reached 96% of its steady state temperature after 60 s. Conduction correction factors Kc were calculated for several exposure times and at various measurement points away from the source inside the calorimeter. A total exposure time between 140 s and 240 s at a distance that receives a minimum of 1 Gy of dose was found to allow reduction of Kc to below 0.1% of unity. Conclusions:Watercalorimetry for HDR brachytherapy is feasible and total uncertainties of significantly better than 5% on the dose can be achieved with current watercalorimetry techniques and instruments.
TH‐C‐M100E‐02: Optically Stimulated Luminescence of Aluminum Oxide Detectors for Radiation Therapy Quality Assurance34(2007); http://dx.doi.org/10.1118/1.2761670View Description Hide Description
Purpose: The purpose of this experiment was to: 1) Determine if a commercially available detector system used for monitoring personnel exposure could be adapted for use as a radiation therapydosimetry system; and 2) Evaluate the system's performance as an in‐vivo dosimeter and its ability to measure absolute surface dose, isocenter dose, and normal tissues dose in a phantom as part of patient‐specific IMRTquality assurance.Method and Materials: The dosimeters were evaluated for: 1) Signal decay; 2) Field size dependence; 3) Energy dependence; and 4) Angular dependence using the Landauer, InLight MicroStar system. In‐Vivo dosimetry measurements were taken for 22 patients treated on a Varian 21EX. The Landauer system was also tested for its ability to measure absolute dose from helical tomotherapy treatments. Results: The variation between dosimeters was evaluated and found to be ±1.6%. The dosimeters appeared to over‐respond in the first 10 minutes, however, after 10 minutes the chips were within 1 percent of the steady‐state reading. Unlike other detectors, the Al2O3dosimeters showed no field size, energy, or angular dependence. The agreement between the dosimeters and the calculated doses for the in‐vivo dosimetry patients was 2.2±6.1 cGy or 3.7±2.5%. The dosimeters were also tested for their ability to measure absolute dose inside an IMRT phantom. The agreement between the dosimeters and the calculated doses was 0.1±5.3 cGy or 0.7±6.7%. Conclusion: dosimeters can be a convenient, inexpensive alternative to TLDs, MOSFETS, and Diodes. The agreement between calculated and measured doses for in‐vivo dosimetry and IMRT QA is comparable to TLDs, MOSFETS, and Diodes. The dosimeters can be quickly read and analyzed after 10 minutes (to allow time for signal decay). The dosimeters do not appear to have an energy, field size, or angular dependence. In addition, the detectors can be erased and re‐used.
TH‐C‐M100E‐03: A New Generation of Electronic Portal Imaging Devices (EPID) Using Thin‐Film CdTe for Radiation Oncology Applications34(2007); http://dx.doi.org/10.1118/1.2761671View Description Hide Description
Purpose: The commercially available EPIDs today commonly manufactured from amorphous‐Silicone (a‐Si) materials display numerous problems in imagecontrast & resolution. These are related to poor radiationhardness and low Z which is detrimental to their detecting capabilities. The CdTe‐based thin film devices with superior radiationhardness and much higher Z (∼50) have not yet found many applications in medical physics. Here we report first promising results on polycrystallinethin film CdTe‐based prototype EPID.Method and Materials: The proposed design utilizes a layer of high atomic number and density functioning as a converter transforming high energy X‐rays into the Compton electrons impeding onto CdTe detector operating in pulse mode. Such a converter can replace the standard scintillators used with a‐Si devices. We conducted Monte Carlo simulations testing the proposed structure and verified them with measurements using a prototype thin filmCdTe cells. Our modeling was amplified with a semi‐empirical algorithm accounting for the processes of device degradation, simultaneously applied to the a‐Si devices for comparative study. Results: We found that a layer of Pb less than 3 mm thick in combination with low‐Z material such as polystyrene, used for filtering out low‐energy scatter, is suitable for the converter. Detector output voltage in the range of the tenths of Volt for the typical radiation therapy dose rates allowed for a possibility of using the device without biasing. We have carried out verifying experiments with polycrystallineCdTe based cells: good agreement with our MC simulations was obtained. Conclusion:CdTe based thin film detectors have a high potential to become the next generation EPID's. They feature: small thickness <10 μm; efficient collection; exceptional radiationhardness; inexpensive deposition technology; sensitivity to low intensity radiation; excellent room temperature characteristics without biasing; small integration time < 500 ns.
TH‐C‐M100E‐04: Cerenkov Light From Phantom Cassettes in Absolute Dose Measurements Using Radiographic Film34(2007); http://dx.doi.org/10.1118/1.2761672View Description Hide Description
Purpose: To determine the impact of Cerenkov light on radiographic film response and to recommend a methodology for correcting absolute dose measurements when using bare film, but calibrated using prepackaged film. Method and Materials: The Gammex RMI Film Dosimetry Cassette Model 436‐AST (Solid Water) and two in‐house cassettes (white opaque, high‐impact polystyrene) were studied using Kodak XV or EDR2 film. Films were exposed perpendicular to 16‐MeV electron (15×15 cm2) or 6 MV x‐ray (20×20 cm2) beams. Films were oriented such that quadrant ♯1 had bare film; quadrant ♯2 had film covered by the prepackaged white paper; quadrant ♯3 had film in its prepackaged container; and quadrant ♯4 had film covered by the prepackaged carbon jacket. To account for beam asymmetry, dose response for each quadrant was normalized to that in the corresponding quadrant of a film irradiated in the carbon jacket, which blocked phantom‐produced Cerenkov light. A prepackaged film, irradiated using a multi‐exposure technique, provided the dose‐response calibration.Results: The “carbon jacket only” dose values averaged 96.1% of the “prepackaged” dose values, indicating that the prepackaged white paper produced Cerenkov light that increased film response by 4.0%. No significant difference due to radiation modality or film type was evident. The “white paper only” dose values ranged from 103.6–107.5% of the “prepackaged” dose values, indicating that Cerenkov light from the phantom material contributed to an increased film response. For white opaque, high‐impact polystyrene the “bare film” dose values ranged from 102.2–109.6% of the “prepackaged” dose values, depending on phantom and modality. For Solid Water the “bare film” dose value was 117.3% of the “prepackaged” dose values. Conclusion: When making absolute dose measurements using bare film and calibrating using prepackaged film, a correction for excess film response arising from Cerenkov light is required, and the reported quadrant method is recommended.
34(2007); http://dx.doi.org/10.1118/1.2761673View Description Hide Description
Purpose: A customized gating system was developed for a large bore CT scanner to produce gated images for use with an active breathing control (ABC) system. Operator triggering following a breath hold is replaced by an electronic gating signal from a bellows. A breathing phantom was constructed and imaged to insure accurate gating. Methods and Materials: A bellows strap wraps around the patient to monitor breathing during CT scans. A universal serial port (USB) cable provides power to the bellows sensor via the circuit box. Bellows signals route to the computer from the circuit box via another USB cable. Operators can set a threshold point in the breathing cycle that corresponds to an ABC breath‐hold to trigger the CT scanner. Softwaregenerated signals gate the CT through the circuit box. Unique to this system is an option to scan continuously while the patient is holding a breath, in contrast to commercial gated‐CT systems which take one image per breathing cycle. A breathing phantom was created using a computer programmed actuator, step‐motor, and off‐set ball. A sine wave modeled on human breathing was fed to the actuator. The moving range was 2.10 cm anterior‐posterior and 1.05 cm in lateral direction. The diameter and motion period was 6.4 cm and 5.54 sec. Phantom images taken were: stationary non‐gated, moving gated, and three un‐gated while moving. All image sets were analyzed with the Pinnacle planning system. Volumes generated from contours where used as a metric to determine imaging accuracy. Results: Target volumes of stationary and gated imaging were 146.8 and 145.5cm3, respectively (1% agreement). However, the discrepancies between stationary and all three un‐gated scans ranged from 4.84 to 5.45% with significant shape distortion.
Conclusions: We successfully created a custom gating system for a large‐bore CT simulator.
TH‐C‐M100E‐06: Determining Optimal Respiratory Gating Parameters for Passively Scattered Synchrotron Based Proton Irradiation34(2007); http://dx.doi.org/10.1118/1.2761674View Description Hide Description
Purpose: Respiratory gated irradiation offers potential for margin reduction and dose escalation for treating moving tumors in the thorax or abdomen. Unfortunately, for synchrotron‐based proton irradiation, it may not be efficient. We have determined the optimal respiratory gating parameters for passively scattered proton irradiation on a synchrotron through a simulation study. Method and Materials: An in‐house software program was developed to investigate the interaction of the respiratory gating intervals with different synchrotron magnet excitation cycle patterns. Test data was obtained by using the recorded respiratory trace of 94 patients who underwent 4DCT. A typical magnet excitation cycle, Tcyc consists of proton acceleration, flat top and deceleration periods. Proton beam delivery occurs only during the flat top portion of each such excitation cycle. Respiratory gating was simulated at expiration for a 30% duty cycle around peak exhalation. The time required to deliver 100 MUs was estimated for the following scenarios: (a) Ungated irradiation with Tcyc set to the minimum value (2.7sec) and (b) Gated irradiation with Tcyc set to (i) the minimum value, (ii) approximately equal each patient's average respiratory cycle, and (iii) a variable value according to each individual respiratory cycle. Overall treatment time and efficiency of treatment delivery were studied in each case. Results: Average times required to deliver 100 MUs were 1.1 minutes for ungated irradiation; and 3.7 (1.7 – 6.0), 3.2 (1.6 – 7.1), 2.3 (1.4 – 3.1) minutes respectively for gated irradiations at various scenarios mentioned above. For gated irradiation, variable Tcyc mode of operation yielded least overall treatment time and greatest efficiency of proton beam delivery. Conclusion: Respiratory gated passively scattered proton delivery using a synchrotron‐based system is feasible without significantly increasing treatment time. Based on above results, variable Tcyc mode of operation offered least overall treatment time and greatest efficiency for respiratory gated irradiation.
34(2007); http://dx.doi.org/10.1118/1.2761675View Description Hide Description
Purpose: To report experience with a novel total body irradiation (TBI) technique. 3D planning techniques are used to deliver a uniform dose to a patient using a conventional linear accelerator in a standard bunker. Manually segmented intensity modulated fields are employed to provide dose compensation for contour variation, tissue heterogeneity, inverse square law effects and junction dose stability. Methods and Materials: The technique uses a conventional Elekta Synergy linear accelerator together with a custom designed floor couch. The couch, positioned 102.5 cm below the machine isocentre, provides treatment distances near 180 cm SSD. The couch is oriented in the gantry rotation plane, with couch motion along the cranial‐caudal axis enabling a match of beam divergence through patient translations and gantry rotations. Treatment is delivered by a set of 2 to 3 divergence matched abutting fields, with field modulation feathering junctions through 4 cm on the patient. Treatment plans are created using conventional beam models in Pinnacle 7.6C and whole body CT scan data. Independent plans for supine and prone orientations are constructed to deliver a uniform dose at mid‐separation throughout the patient and create a composite uniform dose. Segmentation is used to adjust the dose at mid‐plane, correcting for effects of patient thickness, inverse square law, and lung density. Results: A total of 11 patients have been treated with this new technique. Dosimetry measurements in phantom at extended distance and in‐vivo measurements have demonstrated an accurate dose delivery. Composite AP‐PA dose assessments based on contributions to uniquely identified anatomical points have shown that a dose within 10% of the prescribed dose is achieved throughout the treatment volume. Conclusions: A new TBI technique has been implemented which employs modern imaging and delivery methods to achieve a uniform patient dose. The technique utilizes standard equipment, and does not require specialized bunker design.
TH‐C‐M100E‐08: Evaluation of Multiple‐Isocenter IMRT Planning Technique for Field Matching with Limited Collimator Field Size34(2007); http://dx.doi.org/10.1118/1.2761676View Description Hide Description
Purpose: The Elekta Beam Modulator with fully integrated miniMLC has the precision suited for the treatment of smaller tumors. However, the maximum collimator length in the inf/sup direction is 16 cm when most of the head and neck target sizes have a larger dimension. This study used a multi‐isocenter IMRT planning technique with overlapping fields generated using inverse planning. Measurements have been taken to evaluate plans in terms of volume dose and delivery accuracy. The total monitor units for this technique were compared with a standard IMRT plan. Methods and Materials: Ten patients were planned with multiple beam arrangements; eight beam directions for the upper tumor volume without the supraclav and three to four beam arrangements for the lower tumor volume and supraclav. Treatment planning was performed on a CMS/XiO workstation. Treatment fields were delivered on a flat acrylic phantom containing EDR2 film. Isodose distributions were exported from XiO and compared to the measured data using Omni‐Pro software.Results and Conclusions: Multi‐isocenter techniques provide good coverage to the tumor volume while sparing organs at risk. Film distributions show that XiO does not model the effective tongue and groove that is evident in the film measurement. The magnitude of the tongue‐and‐groove dose decrease is on the order of up to 10%. An analysis of the gamma values shows that even with this discrepancy the agreements between plan and measurement were acceptable. The plans are not significantly degraded with small table inaccuracies on the order of 2mm. The total monitor units for multi‐centered plan increased 20% compared to single isocenter delivery.
IMRT planning techniques using two or more isocenters for this collimator show clinically acceptable results when splitting the geometry.
TH‐C‐M100E‐09: Assessment of Skin Dose for Breast Chest Wall Radiotherapy as a Function of Bolus Material34(2007); http://dx.doi.org/10.1118/1.2761677View Description Hide Description
Purpose:Skindose assessment to the chest wall is important to ensure sufficient dose to the near‐surface target volume without undue skin reaction. Bolus is often used for a portion of the treatment course, but removed if clinically necessary because of skin toxicity. This study quantifies changes to the surface dose as a function of bolus material for conventional and IMRT techniques using a thermoluminescent dosimeter(TLD) extrapolation technique. Methods and Materials: Three types of bolus materials (2mm solid, 2mm fine mesh, and 3.2mm large mesh aquaplasts) were compared with Superflab (Med‐Tec). Surface dosemeasurements were performed using the Attix (parallel‐plate) chamber in a flat solid water phantom with the bolus materials for 10×10 cm2 and 10×20 cm2 jaw fields at 00, 450 and 700 incident angles. The Attix chamber measurements were used to validate the TLD extrapolation technique (0.89, 0.38, 0.15 mm thicknesses). TLDs were used to measure the surface dose on an anthropomorphic phantom for conventional and IMRT tangential fields. Results: Surface dose increased with increasing angles and field sizes. Oblique incidence has a larger influence on the surface when no bolus is present (from 20% to 48% for 10×10 cm2). The skindose of solid 2mm aquaplast was larger than that of fine mesh aquaplast (22% for 10×10cm2−00 incidence, and 11% for 10×10cm2−700 incidence). Compared to conventional tangential fields, the skindose for IMRT decreased ∼5%. For the conventional tangential fields, skindoses of fine mesh, solid, and large mesh aquaplasts were 21%, 11% and 9% less than that of superflab, respectively. For IMRT fields, skindoses were 22%, 12% and 10% less than that of superflab, respectively. Conclusion: For chest wall radiotherapy, the bolus type can be selected to compromise near‐skin target dose vs skin tolerance dose for optimal clinical outcome.
TH‐C‐M100E‐10: A Correction Method for the MOSFET Energy Dependence Response to Therapeutic Proton Beams34(2007); http://dx.doi.org/10.1118/1.2761678View Description Hide Description
The energy‐dependence response of the metal oxide semiconductor field‐effect transistor(MOSFET) dosimeter has been investigated with regard to therapeuticproton beams. The MOSFET configurations used include the commercial standard MOSFET (TN‐502RD), the microMOSFET (TN‐502RDM) dosimeters, and a prototype MOSFET with no encapsulation. Proton beams of non‐modulated pristine and modulated Spread‐Out Bragg Peak (SOBP) of 5 cm and 10 cm widths were used with beam ranges of 8.9cm, 15.9cm and 25.9cm in water. Each MOSFET dosimeter was calibrated at the center of the modulated 10 cm SOBP proton beam with conditions of 1 cGy per MU. The MOSFET energy‐dependence response was quantitatively evaluated by the ratio of measured doses between the MOSFET and an ionization chamber in a same condition. The three dosimeters showed a similar response for the pristine proton beams at various beam ranges. This indicates that the variation in dosimeter response is dominated by the change of the linear energy transfer (LET) for the used proton beam and not by the MOSFET encapsulation thickness. The observed MOSFET trends for various pristine proton beams have been modeled by an analytical function , where Rres is the residue range (distance to the distal 90% level dose), and A, B, C are constants. A similar modeling has been performed for modulated SOBP proton beams at different widths and for various beam ranges. The k (Rres ) function has been used as a correction factor for patient dose measured by MOSFETs for corresponding residue ranges; this resulted in dose measurements with an uncertainty of less than 3.0% for various proton beams.
- Radiobiology: Treatment Planning and Evaluation
TH‐E‐M100E‐02: Superiority of Equivalent Uniform Dose (EUD)‐Based Optimization for Breast and Chest Wall IMRT34(2007); http://dx.doi.org/10.1118/1.2761772View Description Hide Description
Purpose: To investigate whether IMRT optimization based on generalize equivalent uniform dose1 (gEUD) objectives for target volumes and organs at risk (OAR) alike can lead to superior plans as opposed to multiple dose‐volume (DV) based objectives plans, for intact breast and postmastectomy chest wall (CW) cancer.Methods and Materials: Four IMRT plans with six or seven coplanar 6‐MV beams were prepared for a number of chest wall and breast CA patients (10 patients). The first three plans utilized our standard in‐house physician‐set of DV objectives (phys‐plan), gEUD‐based objectives for the OARs (gEUD‐plan), and multiple, “very stringent”, DV objectives for each OAR and PTV (DV‐plan), respectively. The fourth was only beam fluence optimized plan (FO‐plan), without segmentation and utilized the same objectives as in the DV‐plan. The latter plan was to be used as an “optimum” benchmark without the effects of the segmentation for deliverability. Various dosimetric quantities, such as mean dose (Dmean) for heart, contralateral breast, and contralateral lung; and V20 (volume of organ receiving 20Gy) for the ipsilateral lung were employed to evaluate our results. Results: For all patients in this study, we have seen that the gEUD‐based plans allow greater sparing of the OARs while maintaining excellent target coverage. The use of gEUD allows selective optimization of the dose for each OAR and results in a truly individualized treatment plan. Conclusions: gEUD requires a smaller number of parameters for optimization and allows exploration of a much wider space of solutions, thereby making it easier for the optimization system to balance competing requirements in search of a better solution. Thus, gEUD optimization can be used to search for or evaluate plans of different DVHs with the same gEUDs. This method can be efficiently used in routine clinical IMRTtreatment planning.
TH‐E‐M100E‐03: Optimizing the Methodology for Incorporating SPECT‐Guidance to Reduce Intensity Modulated Radiation Therapy (IMRT) Dose to Functioning Lung34(2007); http://dx.doi.org/10.1118/1.2761773View Description Hide Description
Purpose:Single photon emission computed tomography(SPECT) provides a spatial distribution map of lung perfusion. Previously, an algorithmic methodology was developed using IMRT and SPECT guidance to deliberately divert dose away from higher functioning (perfused) lung, thereby potentially reducing lung toxicity. This work aims to refine this methodology by determining the optimal number and segmentation method for incorporating different levels of functionality. Method and Materials: The lowest 15% of SPECT numbers were discarded as background noise. The remaining values were then divided equally so that each segment had the same range. IMRT treatment plans incorporating functional information were generated with the lung subdivided into varying numbers of SPECT segments. The segments ranged from 2 to the number beyond which there was no improvement in the Dose Functional Histogram (DFH) or function‐weighted lung volume above 20/30 Gy (F20 / F30). The thresholds of 20/30 Gy were chosen for their significance in predicting radiation‐induced pneumonitis. The plans generated using SPECT guidance were compared against “conventional” plans, generated with the assumption that lung function was spatially homogeneous. Results: Of all the SPECT plans generated, those created with four segments produced the most favorable results overall. The results were variable, with the four segment SPECT‐guided plans showing marginal to large improvement over conventional plans. One patient had a 42.9% and 61.7% reduction in F20 and F30 values, respectively, when compared to the conventional plan. For all patients, on average, the F20 and F30 values were reduced by 16.5% ± 18.3% and 21.1% ± 26.0%, respectively. Conclusion: A standardized intensity‐based segmentation procedure is crucial for routine use of SPECT‐guidance in IMRTlungtreatment planning. The simple procedure outlined here is valid for a range of patients. Segmenting lungSPECT into four intensity regions appears to provide the greatest benefit in reducing radiotherapy‐induced functional lung damage.
TH‐E‐M100E‐04: A Semi‐Analytical Model of Biological Effectiveness for Treatment Planning in Light Ion Radiotherapy34(2007); http://dx.doi.org/10.1118/1.2761774View Description Hide Description
Purpose: To formulate a fast and accurate model to predict the biological effectiveness of light ion beams along their penetration depths in tissue, for their applications in radiotherapy.Method and Materials: A simple model of light ion Bragg peaks is used in conjunction with a detailed probabilistic model of the subsequent biological processes in individual cells. The physical model takes into account energy loss, its straggling, and the reduction of primary particles' fluence due to nuclear reactions, but products of these reactions are not followed at all. The biological model takes into account the stochastic nature of individual particle traversals through a cell nucleus, distinguishes between two classes of DNA lesions of different severity, and can also account for cellular repair success probabilities. Results: The simple physical model enables to correctly represent the Bragg peaks of protons and light ions up to carbon. Deviations from experimental depth‐dose distributions occur for heavier ions at higher incident energies due to the neglect of nuclear reaction products. However, fragments play only a minor role with respect to biological effects of ion beams. Cell survival along the beam penetration depth for light ions up to carbon and with ranges in water up to approx. 15 cm, predicted with the combined physical and biological model, is in excellent agreement with experimental data. Conclusion: This work indicates the potential applications of the present biological model in treatment planning in radiotherapy. Further improvement might be expected if a full Bragg peak model is incorporated. The semi‐analytical nature of the present scheme enables advanced calculations that are necessary for truly biological optimization in light ion radiotherapy, including the optimization of multiple‐port intensity modulated radiotherapy with ion beams and maximization of complication‐free tumor cure.
34(2007); http://dx.doi.org/10.1118/1.2761775View Description Hide Description
Purpose: The purpose of this work was to evaluate the LET energy deposition contribution from primary proton particles versus the contribution of secondary particles. Method and Materials: The energy deposition contribution from primary proton particles versus contribution of secondary particles was calculated using HETC‐HEDS (High Energy Transport Code for Human Exploration in Deep Space). HETC‐HEDS simulates particle cascades by using Monte Carlo methods to compute the trajectories of the primary particle and all the secondary particles produced in nuclear collisions. For the initial phase of this study, the proton beam was incident on a computermodel of a detector that is being designed to measure the LET spectra for proton beams. Simulations were performed using incident proton beams between 90 and 200 MeV/Nucleon in 5 MeV/Nucleon increments incident on 10 g/cm2 of A‐150 Tissue Equivalent Plastic. In the second phase, the proton beam will be incident on a computermodel of an anthropomorphic RANDO phantom. Simulations will be performed for different patient treatments to determine the LET spectra of the protons and secondary particles. Results: The simulations showed that secondary particles contribution is of significance, and the contribution is about 50% of the total energy deposited into the Tissue Equivalent Plastic. Overall, the results suggested that secondary particles are of concern during proton radiation therapy treatments and need to be investigated, especially at low proton energy, and beyond the Bragg peak point. Conclusion: For proton therapy beams, the simulations presented show a significant contribution of about 50% of total energy deposition originates from secondary particles. Furthermore, a significant fraction of the fluence comes from secondary particles produced beyond the Bragg Peak. The authors are currently developing a model to incorporate the biological effect of the secondary particles in the proton treatment planning process.
TH‐E‐M100E‐06: A Generalized Lyman Model Incorporating Censored Time‐To‐Toxicity Data and Non‐Dosimetric Risk Factors34(2007); http://dx.doi.org/10.1118/1.2761776View Description Hide Description
Purpose: In analyzingradiation toxicity, clinical factors and occurrence times of toxicity may be of significance. Yet these factors are not incorporated into the conventional Lyman NTCP model. The purpose of this work was to generalize the Lyman model, incorporating censored time‐to‐event data and clinical risk factors in addition to dose‐volume effects, and to apply the model to analysis of radiation pneumonitis (RP). Method and Materials: Clinical data were collected from lungcancer patients treated with radiotherapy from 1999 to 2005. Grade ⩾ 3 RP was the normal tissue endpoint, censored at patient death or last follow‐up for patients without RP. Multivariate analyses were used first to identify significant clinical risk factors for RP that were independent of dosimetric quantities. A generalized version of the Lyman model was then developed that included important clinical factors and the distribution of RP times. Results: Among the 567 patients available for analysis, the crude incidence of grade ⩾ 3 RP was 21% (118 cases), occurring mainly within the first 8 months post‐therapy, when many patients succumb to disease. Smoking habits and dose‐volume parameters were found to affect the risk of RP. The generalized Lyman model provided a significantly better fit to data when smoking was taken into account (P=0.005). The estimated lung‐volume effect (the parameter n in the Lyman model) was found to be 0.54, but was not significantly different from n=1, consistent with the importance of mean lungdose on the risk of RP. The generalized model predicted NTCP values that were up to 22 percentage points different from those estimated from a fit of the standard Lyman model.Conclusion: Inclusion of non‐dosimetric factors and time‐to‐event data can significantly impact the predictions of NTCP models that have been based historically solely on dose‐volume effects.
TH‐E‐M100E‐07: The Impact of Procedure‐Induced Edema On Cell Survival and Tumor Control Probability in Permanent Prostate Brachytherapy Using 131Cs Radioactive Source34(2007); http://dx.doi.org/10.1118/1.2761777View Description Hide Description
Purpose: Procedure‐induced prostate edema can cause significant changes to the dose delivered by a permanent prostate brachytherapy. The aim of this work was to examine systematically the impact of procedure‐induced edema on the effectiveness of cell kill in permanent prostate brachytherapy using radioactive sources.Method and Materials: The concept of biologically effective dose (BED) was used to quantify the impact of prostate edema on the radiation‐induced cell kill in permanent prostate brachytherapy. The repopulation of surviving cells and the repair of sub‐lethally damaged cells during the protracted dose delivery was modeled by the cell potential doubling time and a repair half‐time, respectively. The procedure‐induced prostate edema and its resolution dynamics was described by a quantitative model reported by Waterman et. el. (Int. J. Radiat. Oncol. Biol. Phys. 41, 1069–1077, 1998). The surviving fraction of cancer cells and the potential tumor control probability was examined over a range of edema magnitudes and resolution half‐lives observed in real patients. Results: When edema was neglected in pre‐implant treatment planning, the BED of an actual implant was found to depend strongly on the edema magnitude, its resolution half‐life, and tumor potential doubling time. For a typical edema with magnitude of 50% and half‐life of 10 days, the edema‐induced reduction in BED was greater than 15% resulting in an increase of cell‐survival by more than one order of magnitude. For larger edemas with magnitude of 90% and half‐life of 20 days, the edema‐induced increase in cell survival was more than two orders of magnitude. Conclusions: Procedure‐induced edema can cause significant reduction in BED and increase in cell survival in permanent prostate brachytherapy. Depending on the initial tumor burden, the increase in cell survival could lead to significant reduction in tumor control probability for patients having moderate and large edemas.
- Tx Planning and Delivery — Clinical Planning
34(2007); http://dx.doi.org/10.1118/1.2761728View Description Hide Description
Purpose: Dose equalization along the long axis of the patient for total body irradiation requires the use of a compensator. At our institution the compensator consists of multiple layers of lead strips and is based on measurements along the patient's mid‐sagittal plane. In this study, we examine the feasibility and limitations of replacing the lead compensator with a one‐dimensional electronic compensator using fluence management tools available in our treatment planning system. Method and Materials: The patient compensator was based on body thickness, SSD and off‐axis distance of 12 mid‐sagittal patient specific points. A phantom was modeled using these measurements from a previously treated patient. A fluence map of the transmission values for the compensator used at treatment was created. Dose calculated on the phantom was compared with patient surface dose measurements. Physical limits and software limitations of this method were evaluated. Results: The maximum patient height accommodated by our treatment room and setup is 225 cm. Using a fluence map with the largest transmission factor gradient is not an issue, nor is the use of any reasonable field width that would be seen in a clinical setting. Calculated dose to the phantom using the electronic compensator was found to be on 3.4% lower (range: +2.6% to −7.8%) than diode dose measurements taken on the patient skin surface time of treatment.Conclusion: It is feasible for most patients that an electronic compensator be used. Discrepancies between calculated and measured dose can reasonably be accounted for. Further study is planned to measure the dose using an anthropomorphic phantom and also to automate the construction of patient‐specific virtual phantoms and patient‐specific optimal fluence.
34(2007); http://dx.doi.org/10.1118/1.2761729View Description Hide Description
Purpose: To examine characteristics of electron beams collimated by an electron multileaf collimator (eMLC) for modulated electron radiotherapy (MERT). Method and Materials: An eMLC, which has 25 pairs of Tungsten leaves with 2 cm of thickness and 0.6 cm width, was made and can be attached to a medical LINAC. In this work, measurements using films and ion chamber were performed to investigate electron beam characteristics with eMLC. The source surface distance is 70 cm, at which the distance between the bottom of eMLC and the phantom surface is 5.5 cm. The beam characteristics, such as leakage and transmission, spatial resolution, beam abutment, beam penumbra and percentage depth dose (PDD) were evaluated. Except that the PDDs were measured in water, all other measurements were conducted in a solid water phantom. Results: Leakage and transmission increases from 0.24% to 3% as the electron energy increases from 6 MeV to 20 MeV. The shape of one leaf can be fairly distinguished while that of 2 or 3 leaves can be clearly seen in the dose profile on the phantom surface. The difference between an abutting field sand a single field is obvious on the phantom surface but tends to diminish at the treatment depth. The beam penumbra is of a comparable quality with that shaped by an electron cutout, and in general, it decreases with the increase of energy and increases with the increase of the depth. The therapeutic range and the bremsstrahlung magnitude obtained from the measured PDDs with eMLC are similar to the electron beams shaped by an electron applicator and cutout. Conclusions: Characteristics of the electron beams collimated by eMLC with a thin air gap (about 5.5 cm) is investigated. Acknowledging these characteristics is essential for optimizing treatment plans of the MERT.
34(2007); http://dx.doi.org/10.1118/1.2761730View Description Hide Description
Purpose: This work investigates the use of Gafchromic EBT film for dosimetry in homogeneous and heterogeneous phantoms for static and IMRT fields. This information is necessary for thorough algorithm verification for static and IMRT fields. Method and Materials: EBT film was used to measure dose in three phantom configurations. The first configuration consisted of solid water. The second configuration consisted of 6 cm thickness of lung‐equivalent slabs sandwiched between solid water slabs. Film measurements were compared to EDR film and calculations with the Dose Planning Method (DPM) Monte Carlo code. The third configuration, non‐slab, consisted of solid water with an 8×10×2 cm3 lung‐equivalent region in the solid water. EBT film was placed perpendicula and parallel to the beam within and between materials. Measurements were made for static fields and sample IMRT fields. Results: When converted to dose, EBT measurements showed good agreement with EDR film. For an example IMRT field, agreement was between 86% and 83% for EBT to EDR and EBT film to DPM calculations comparisons, respectively, using a χ evaluation of 2%/2 mm. In the non‐slab inhomogeneous phantom, the EBT film clearly showed the effect of disequilibrium at the interfaces. Conclusions: Because EBT film is not light sensitive, EBT film is a practical choice for phantom measurements that require precise placement. In addition, EBT film can be used for measurements parallel to the beam because it is less dependent on the photon energy spectrum. Tentative results indicate that EBT can be used for dosimetry for static and IMRT fields at planes perpendicular to the beam for homogeneous and heterogeneous slab geometries. Results show that the use of EBT film at interfaces is promising in providing dosimetric data to verify dose calculations in previously difficult to measure geometries.
TH‐D‐M100E‐04: Evaluation of MVCT Images Containing Lead Alloy Masks for Electron Beam Treatment Planning34(2007); http://dx.doi.org/10.1118/1.2761731View Description Hide Description
Purpose: To evaluate the accuracy of electron beam dose calculations in MVCT images containing lead alloy masks. Method and Materials: A phantom consisting of two 30×30×5‐cm3 slabs of CIRS plastic water® was imaged using kVCT (GE Lightspeed‐RT) and MVCT (TomoTherapy Hi⋅Art). The MVCT scans were taken with nine square masks of Cerrobend® (density = 9.4gcm−3) on top of the phantom. The masks contained square openings of 3×3cm2, 6×6cm2 and 10×10cm2 and had thicknesses of 6mm, 8mm and 10mm. The same collimation was simulated in the kVCT images by creating regions‐of‐interest (ROI) duplicating the sizes, shapes, and density of the masks. Using the Philips Pinnacle3treatment planning system, twelve treatment plans were created using electron energies of 6, 9, 12, and 16 MeV for each opening size. For each plan, the mask thickness appropriate for the electron energy was used and the dose distributions calculated using the kVCT and MVCT images were compared. In uniform dose regions (doses above 90% of maximum) dose differences were calculated; in high‐dose gradient regions (doses below 90% of maximum) distances‐to‐agreement (DTA) were determined. Results: In the uniform dose region, the maximum difference between the doses in the MVCT images and the doses in the kVCT image was greater than or equal to ±5% for all opening and energy combinations. In the high‐dose gradient region, almost half of the maximum DTA values exceeded 2mm. Analysis of the MVCT images showed that DTA differences were largely due to distortions in the phantom CT numbers caused by the masks. Conclusion: Although Cerrobend® produces dramatically less distortion in MVCT images compared to kVCT images,image distortion is still too great for accurate electron beam dose calculations.
Supported in part by a research agreement with TomoTherapy, Inc.