Quality assurance of serial tomotherapy for head and neck patient treatments

Is it still necessary to perform measured based pre-treatment patient-specific QA for SRS HyperArc treatments?

PURPOSE

The Mobius3D system was validated as a modern secondary check dosimetry system. In particular, our objective has been to assess the suitability of the M3D as pre-treatment patient-specific Quality Assurance (QA) tool for Stereotactic Radiosurgery (SRS) HyperArc (HA) treatments. We aimed to determine whether Mobius3D could safely replace the measurements-based patient-specific QA for this type of treatment.

METHODS

30 SRS HA treatment plans for brain were selected. The dose distributions, calculated by Mobius and our routinely used algorithm (AcurosXB v.15.6), were compared using gamma analysis index and DVH parameters based on the patient's CT dataset. All 30 plans were then delivered across the ionization chamber in a homogeneous phantom and the measured dose was compared with both M3D and TPS calculated one. The plans were delivered and verified in terms of PSQA using the electronic portal imaging device (EPID) with Portal Dosimetry (PD) and myQA SRS (IBA Dosimetry) detector. Plans that achieved a global gamma passing rate (GPR) ≥ 97% based on 2%/2 mm criteria, with both Mobius3D and the conventional methods were evaluated acceptable. Finally, we assessed the capability of the M3D system to detect errors related to the position of the Multi-Leaf Collimator (MLC) in comparison to the analyzed measurement-based systems.

RESULTS

No relevant differences were observed in the comparison between the dose calculated on the CT-dataset by M3D and the TPS. Observed discrepancies are imputable to different used algorithms, but no discrepancies related to goodness of plans have been found. Average differences between calculated (M3D and TPS) vs measured dose with ionization chamber were 2.5% (from 0.41% to 3.2%) and 1.81% (from 0.66% to 2.65%), for M3D and TPS, respectively. All plans passed with a gamma passing rate > 97% using conventional PSQA methods with a gamma criterion of 2% dose difference and 2 mm distance-to-agreement. The average gamma passing rate for the M3D system was determined to be 99.4% (from 97.3% to 100%). Results from this study also demonstrated Mobius has better error detectability than conventional measurement-based systems.

CONCLUSION

Our study shows Mobius3D could be a suitable alternative to conventional measured based QA methods for SRS HyperArc treatments.

Integral Dose-Based Inverse Optimization May Reduce Side Effects in Radiotherapy of Prostate Carcinoma

PURPOSE

The purpose of this work is to apply a novel inverse optimization approach, based on utilization of quantitative imaging information in the optimization function, to prostate carcinoma.

MATERIALS AND METHODS

This new inverse optimization algorithm relies upon quantitative information derived from computed tomography (CT) imaging studies. The Hounsfield numbers of the CT voxels are converted to physical density, which in turn is used to calculate voxel mass and the corresponding integral dose, by summation over the product of dose and mass in each dose voxel. This integral dose is used for plan optimization through its global minimization. The optimization results are compared to the optimization results derived from most commonly used dose-volume-based inverse optimization, where objective functions are formed as summation over all dose voxels of the squared differences between voxel doses and user specified doses. The data from 25 prostate plans were optimized with dose-volume histogram (DVH) and integral dose (energy) minimization objective functions. The results obtained with the energy- and DVH-based optimization schemes were studied through commonly used dosimetric indices (DIs). Statistical equivalence tests were further performed to establish population-based significance results.

RESULTS

Both DVH- and energy-based plans for each case were normalized so that 95% of the planning target volume receives the prescription dose. The average differences for the rectum and bladder DIs ranged from 1.6 to 25%, where the energy-based quantities were lower. For both femoral heads, the energy-based optimization-derived doses were lower on average by 32%. The statistical tests demonstrated that the significant differences in the tallied dose indices range from 2.7% to more than 50% for rectum, bladder, and femoral heads.

CONCLUSION

For majority of the clinically relevant dosimetric quantities, energy-based inverse optimization performs better than the standard of care DVH-based optimization in prostate carcinoma. The population averaged statistically significant differences range from ~3 to ~50%. Therefore, this newly proposed optimization approach, incorporating explicitly quantitative imaging information in the inverse optimization function, holds potential for further reduction of complication rates in prostate cancer.

Mathematical Formulation of DMH-Based Inverse Optimization

Purpose: To introduce the concept of dose–mass-based inverse optimization for radiotherapy applications.

Materials and Methods: Mathematical derivation of the dose–mass-based formalism is presented. This mathematical representation is compared to the most commonly used dose–volume-based formulation used in inverse optimization. A simple example on digitally created phantom is presented. The phantom consists of three regions: a target surrounded by high- and low-density regions. The target is irradiated with two beams through those regions and inverse optimization with dose–volume and dose–mass-based objective functions is performed. The basic properties of the two optimization types are demonstrated on the phantom.

Results: It is demonstrated that dose–volume optimization is a special case of dose–mass optimization. In a homogenous media, dose–mass optimization turns into dose–volume optimization. The dose calculations performed on the digital phantom show that in this very simple case dose–mass optimization tends to penalize more the dose delivery through the high-density region and therefore it results in delivering more dose through the low-density region.

Conclusion: It was demonstrated that dose–mass-based optimization is mathematically more general than dose–volume-based optimization. In the case of constant density media, dose–mass optimization transforms into dose–volume optimization.

A patient-specific quality assurance study on absolute dose verification using ionization chambers of different volumes in RapidArc treatments

The recalculation of 1 fraction from a patient treatment plan on a phantom and subsequent measurements have become the norms for measurement-based verification, which combines the quality assurance recommendations that deal with the treatment planning system and the beam delivery system. This type of evaluation has prompted attention to measurement equipment and techniques. Ionization chambers are considered the gold standard because of their precision, availability, and relative ease of use. This study evaluates and compares 5 different ionization chambers: phantom combinations for verification in routine patient-specific quality assurance of RapidArc treatments. Fifteen different RapidArc plans conforming to the clinical standards were selected for the study. Verification plans were then created for each treatment plan with different chamber-phantom combinations scanned by computed tomography. This includes Medtec intensity modulated radiation therapy (IMRT) phantom with micro-ionization chamber (0.007 cm(3)) and pinpoint chamber (0.015 cm(3)), PTW-Octavius phantom with semiflex chamber (0.125 cm(3)) and 2D array (0.125 cm(3)), and indigenously made Circular wax phantom with 0.6 cm(3) chamber. The measured isocenter absolute dose was compared with the treatment planning system (TPS) plan. The micro-ionization chamber shows more deviations when compared with semiflex and 0.6 cm(3) with a maximum variation of -4.76%, -1.49%, and 2.23% for micro-ionization, semiflex, and farmer chambers, respectively. The positive variations indicate that the chamber with larger volume overestimates. Farmer chamber shows higher deviation when compared with 0.125 cm(3). In general the deviation was found to be <1% with the semiflex and farmer chambers. A maximum variation of 2% was observed for the 0.007 cm(3) ionization chamber, except in a few cases. Pinpoint chamber underestimates the calculated isocenter dose by a maximum of 4.8%. Absolute dose measurements using the semiflex ionization chamber with intermediate volume (0.125 cm(3)) shows good agreement with the TPS calculated among the detectors used in this study. Positioning is very important when using smaller volume chambers because they are more sensitive to geometrical errors within the treatment fields. It is also suggested to average the dose over the sensitive volume for larger-volume chambers. The ionization chamber-phantom combinations used in this study can be used interchangeably for routine RapidArc patient-specific quality assurance with a satisfactory accuracy for clinical practice.

Hypoxia imaging with [F-18] FMISO-PET in head and neck cancer: potential for guiding intensity modulated radiation therapy in overcoming hypoxia-induced treatment resistance

BACKGROUND AND PURPOSE

Positron emission tomography (PET) imaging with [F-18] fluoromisonidazole (FMISO) has been validated as a hypoxic tracer. Head and neck cancer exhibits hypoxia, inducing aggressive biologic traits that impart resistance to treatment. Delivery of modestly higher radiation doses to tumors with stable areas of chronic hypoxia can improve tumor control. Advanced radiation treatment planning (RTP) and delivery techniques such as intensity modulated radiation therapy (IMRT) can deliver higher doses to a small volume without increasing morbidity. We investigated the utility of co-registered FMISO-PET and CT images to develop clinically feasible RTPs with higher tumor control probabilities (TCP).

MATERIALS AND METHODS

FMISO-PET images were used to determine hypoxic sub-volumes for boost planning. Example plans were generated for 10 of the patients in the study who exhibited significant hypoxia. We created an IMRT plan for each patient with a simultaneous integrated boost (SIB) to the hypoxic sub-volumes. We also varied the boost for two patients.

RESULT

A significant (mean 17%, median 15%) improvement in TCP is predicted when the modest additional boost dose to the hypoxic sub-volume is included.

CONCLUSION

Combined FMISO-PET imaging and IMRT planning permit delivery of higher doses to hypoxic regions, increasing the predicted TCP (mean 17%) without increasing expected complications.

Magnetic resonance imaging for adaptive cobalt tomotherapy: A proposal

Magnetic resonance imaging (MRI) provides excellent soft tissue contrast for oncology applications. We propose to combine a MRI scanner with a helical tomotherapy (HT) system to enable daily target imaging for improved conformal radiation dose delivery to a patient. HT uses an intensity-modulated fan-beam that revolves around a patient, while the patient slowly advances through the plane of rotation, yielding a helical beam trajectory. Since the use of a linear accelerator to produce radiation may be incompatible with the pulsed radiofrequency and the high and pulsed magnetic fields required for MRI, it is proposed that a radioactive Cobalt-60 ((60)Co) source be used instead to provide the radiation. An open low field (0.25 T) MRI system is proposed where the tomotherapy ring gantry is located between two sets of Helmholtz coils that can generate a sufficiently homogenous main magnetic field.It is shown that the two major challenges with the design, namely acceptable radiation dose rate (and therefore treatment duration) and moving parts in strong magnetic field, can be addressed. The high dose rate desired for helical tomotherapy delivery can be achieved using two radiation sources of 220TBq (6000Ci) each on a ring gantry with a source to axis-of-rotation distance of 75 cm. In addition to this, a dual row multi-leaf collimator (MLC) system with 15 mm leaf width at isocentre and relatively large fan beam widths between 15 and 30 mm per row shall be employed. In this configuration, the unit would be well-suited for most pelvic radiotherapy applications where the soft tissue contrast of MRI will be particularly beneficial. Non-magnetic MRI compatible materials must be used for the rotating gantry. Tungsten, which is non-magnetic, can be used for primary collimation of the fan-beam as well as for the MLC, which allows intensity modulated radiation delivery. We propose to employ a low magnetic Cobalt compound, sycoporite (CoS) for the Cobalt source material itself.Rotational delivery is less susceptible to problems related to the use of a low energy megavoltage photon source while the helical delivery reduces the negative impact of the relatively large penumbra inherent in the use of Cobalt sources for radiotherapy. On the other hand, the use of a (60)Co source ensures constant dose rate with gantry rotation and makes dose calculation in a magnetic field as easy as the range of secondary electrons is limited.The MR-integrated Cobalt tomotherapy unit, dubbed 'MiCoTo,' uses two independent physical principles for image acquisition and treatment delivery. It would offer excellent target definition and will allow following target motion during treatment using fast imaging techniques thus providing the best possible input for adaptive radiotherapy. As an additional bonus, quality assurance of the radiation delivery can be performed in situ using radiation sensitive gels imaged by MRI.

The impact of linac output variations on dose distributions in helical tomotherapy

It has been suggested for quality assurance purposes that linac output variations for helical tomotherapy (HT) be within +/-2% of the long-term average. Due to cancellation of systematic uncertainty and averaging of random uncertainty over multiple beam directions, relative uncertainties in the dose distribution can be significantly lower than those in linac output. The sensitivity of four HT cases with respect to linac output uncertainties was assessed by scaling both modeled and measured systematic and random linac output uncertainties until a dose uncertainty acceptance criterion failed. The dose uncertainty acceptance criterion required the delivered dose to have at least a 95% chance of being within 2% of the planned dose in all of the voxels in the treatment volume. For a random linac output uncertainty of 5% of the long-term mean, the maximum acceptable amplitude of the modeled, sinusoidal, systematic component of the linac output uncertainty for the four cases was 1.8%. Although the measured linac output variations represented values that were outside of the +/-2% tolerance, the acceptance criterion did not fail for any of the four cases until the measured linac output variations were scaled by a factor of almost three. Thus, the +/-2% tolerance in linac output variations for HT is a more conservative tolerance than necessary.

Design, development, and implementation of the radiological physics center's pelvis and thorax anthropomorphic quality assurance phantoms

The Radiological Physics Center (RPC) developed two heterogeneous anthropomorphic quality assurance phantoms for use in verifying the accuracy of radiation delivery: one for intensity-modulated radiation therapy (IMRT) to the pelvis and the other for stereotactic body radiation therapy (SBRT) to the thorax. The purpose of this study was to describe the design and development of these two phantoms and to demonstrate the reproducibility of measurements generated with them. The phantoms were built to simulate actual patient anatomy. They are lightweight and water-fillable, and they contain imageable targets and organs at risk of radiation exposure that are of similar densities to their human counterparts. Dosimetry inserts accommodate radiochromic film for relative dosimetry and thermoluminesent dosimetry capsules for absolute dosimetry. As a part of the commissioning process, each phantom was imaged, treatment plans were developed, and radiation was delivered at least three times. Under these controlled irradiation conditions, the reproducibility of dose delivery to the target TLD in the pelvis and thorax phantoms was 3% and 0.5%, respectively. The reproducibility of radiation-field localization was less than 2.5 mm for both phantoms. Using these anthropomorphic phantoms, pelvic IMRT and thoracic SBRT radiation treatments can be verified with a high level of precision. These phantoms can be used to effectively credential institutions for participation in specific NCI-sponsored clinical trials.

Water-equivalent dosimeter array for small-field external beam radiotherapy

With the increasing complexity of dose patterns external beam radiotherapy, there is a great need for new types of dosimeters. We studied the first prototype of a new dosimeter array consisting of water-equivalent plastic scintillating fibers for dose measurement in external beam radiotherapy. We found that this array allows precise, rapid dose evaluation of small photon fields. Starting with a dosimeter system constructed with a single scintillating fiber coupled to a clear optical fiber and read using a charge coupled device camera, we looked at the dosimeter's spatial resolution under small radiation fields and angular dependence. Afterward, we analyzed the camera's light collection to determine the maximum array size that could be built. Finally, we developed a prototype made of ten scintillating fiber detectors to study the behavior and precision of this system in simple dosimetric situations. The scintillation detector showed no measurable angular dependence. Comparison of the scintillation detector and a small-volume ion chamber showed agreement except for 1 x 1 and 0.5 x 5.0 cm (2) fields where the output factor measured by the scintillator was higher. The actual field of view of the camera could accept more than 4000 scintillating fiber detectors simultaneously. Evaluation of the dose profile and depth dose curve using a prototype with ten scintillating fiber detectors showed precise, rapid dose evaluation even with placement of more than 75 optical fibers in the field to simulate what would happen in a larger array. We concluded that this scintillating fiber dosimeter array is a valuable tool for dose measurement in external beam radiotherapy. It possesses the qualities necessary to evaluate small and irregular fields with various incident angles such as those encountered in intensity-modulated radiotherapy, radiosurgery, and tomotherapy.

Tomotherapy and other innovative IMRT delivery systems

Fixed-field treatments, delivered using conventional clinical linear accelerators fitted with multileaf collimators, have rapidly become the standard form of intensity-modulated radiotherapy (IMRT). Several innovative nonstandard alternatives also exist, for which delivery and treatment planning systems are now commercially available. Three of these nonstandard IMRT approaches are reviewed here: tomotherapy, robotic linear accelerators (CyberKnife, Accuray Inc., Sunnyvale, CA), and standard linear accelerators modulated by jaws alone or by their jaws acting together with a tertiary beam-masking device. Rationales for the nonstandard IMRT approaches are discussed, and elements of their delivery system designs are briefly described. Differences between fixed-field IMRT dose distributions and the distributions that can be delivered by using the nonstandard technologies are outlined. Because conventional linear accelerators are finely honed machines, innovative design enhancement of one aspect of system performance often limits another facet of machine capability. Consequently the various delivery systems may prove optimal for different types of treatment, with specific machine designs excelling for disease sites with specific target volume and normal structure topologies. However it is likely that the delivery systems will be distinguished not just by the optimality of the dose distributions they deliver, but also by factors such as the efficiency of their treatment process, the integration of their onboard imaging systems into that process, and their ability to measure and minimize or compensate for target movement, including the effects of respiratory motion.

Intensity-modulated radiotherapy in the standard management of head and neck cancer: promises and pitfalls

The purpose of this article is to review the role of intensity-modulated radiotherapy (IMRT) in the standard management of patients with head and neck cancer through a critical review of the pertinent literature. IMRT may result in a dose distribution that is more conformal than that achieved with three-dimensional conformal radiotherapy (3D CRT), allowing dose reduction to normal structures and thus decreasing toxicity and possibly enhancing locoregional control through dose escalation. Disadvantages associated with IMRT include increased risk of a marginal miss, decreased dose homogeneity, increased total body dose, and increased labor and expense. Outcomes data after IMRT are limited, and follow-up is relatively short. Locoregional control rates appear to be comparable to those achieved with 3D CRT and, depending on the location and extent of the tumor, late toxicity may be lower. Despite limited data on clinical outcomes, IMRT has been widely adopted as a standard technique in routine practice and clinical trials. The use of IMRT involves a learning curve for the practitioner and will continue to evolve, requiring continuing education and monitoring of outcomes from routine practice. Additional standards pertaining to a variety of issues, including target definitions and dose specification, need to be developed. Phase III trials will better define the role of IMRT in coming years.

Intensity-modulated radiotherapy (IMRT) for carcinoma of the maxillary sinus: A comparison of IMRT planning systems

The treatment of maxillary sinus carcinoma with forward planning can be technically difficult when the neck also requires radiotherapy. This difficulty arises because of the need to spare the contralateral face while treating the bilateral neck. There is considerable potential for error in clinical setup and treatment delivery. We evaluated intensity-modulated radiotherapy (IMRT) as an improvement on forward planning, and compared several inverse planning IMRT platforms. A composite dose-volume histogram (DVH) was generated from a complex forward planned case. We compared the results with those generated by sliding window fixed field dynamic multileaf collimator (MLC) IMRT, using sets of coplanar beams. All setups included an anterior posterior (AP) beam, and 3-, 5-, 7-, and 9-field configurations were evaluated. The dose prescription and objective function priorities were invariant. We also evaluated 2 commercial tomotherapy IMRT delivery platforms. DVH results from all of the IMRT approaches compared favorably with the forward plan. Results for the various inverse planning approaches varied considerably across platforms, despite an attempt to prescribe the therapy similarly. The improvement seen with the addition of beams in the fixed beam sliding window case was modest. IMRT is an effective means of delivering radiotherapy reliably in the complex setting of maxillary sinus carcinoma with neck irradiation. Differences in objective function definition and optimization algorithms can lead to unexpected differences in the final dose distribution, and our evaluation suggests that these factors are more significant than the beam arrangement or number of beams.

Measurement accuracy and cerenkov removal for high performance, high spatial resolution scintillation dosimetry

With highly conformal radiation therapy techniques such as intensity-modulated radiation therapy, radiosurgery, and tomotherapy becoming more common in clinical practice, the use of these narrow beams requires a higher level of precision in quality assurance and dosimetry. Plastic scintillators with their water equivalence, energy independence, and dose rate linearity have been shown to possess excellent qualities that suit the most complex and demanding radiation therapy treatment plans. The primary disadvantage of plastic scintillators is the presence of Cerenkov radiation generated in the light guide, which results in an undesired stem effect. Several techniques have been proposed to minimize this effect. In this study, we compared three such techniques-background subtraction, simple filtering, and chromatic removal-in terms of reproducibility and dose accuracy as gauges of their ability to remove the Cerenkov stem effect from the dose signal. The dosimeter used in this study comprised a 6-mm(3) plastic scintillating fiber probe, an optical fiber, and a color charge-coupled device camera. The whole system was shown to be linear and the total light collected by the camera was reproducible to within 0.31% for 5-s integration time. Background subtraction and chromatic removal were both found to be suitable for precise dose evaluation, with average absolute dose discrepancies of 0.52% and 0.67%, respectively, from ion chamber values. Background subtraction required two optical fibers, but chromatic removal used only one, thereby preventing possible measurement artifacts when a strong dose gradient was perpendicular to the optical fiber. Our findings showed that a plastic scintillation dosimeter could be made free of the effect of Cerenkov radiation.

Period between Completion of Radiation Therapy and Prosthetic Rehabilitation in Edentulous Patients: A Retrospective Study

PURPOSE

The primary purposes of this study were: (1) to describe the number and types of complications patients had before and after insertion of a removable prosthesis (i.e., denture) following radiation therapy to the head and neck and (2) to investigate whether the time between radiation therapy and denture insertion might contribute to those complications.

MATERIALS AND METHODS

This research evaluated edentulous patients and those who were rendered edentulous as a result of their cancer treatment. After obtaining institutional approval following HIPAA regulations, a total of 349 charts were identified: 152 patients from Houston Veterans Administration Medical Center (HVAMC) and 197 patients from M. D. Anderson Cancer Center (MDACC). A total of 190 patients met the inclusion criteria with data available for review.

RESULTS

No significant differences were found in any of the comparisons made, except when comparing complications that occurred after the dentures were inserted and the amount of time it took for prosthetic rehabilitation. The majority of patients had no complications. The patients who received their dentures in 180 days or less had the same number of complications when compared with those patients who received their dentures in 181 to 365 days and those who had to wait longer than a year for prosthetic rehabilitation. Patients with more pre-insertion complications tended to have delayed prosthetic rehabilitation. Those patients who had complications both before and after denture insertion tended to have bilateral dosing of their radiation treatment. Patients who had received radiation therapy were 1.7 times more likely to have post-prosthesis insertion complications. The majority of patients who experienced complications before and after denture insertion had greater than 5000 cGy.

CONCLUSIONS

The numbers of complications reviewed in this retrospective analysis were considerably fewer than the number expected. There appears to be no difference in the number of pre- and post-insertion complications as a function of the time delay from oral surgical procedure to start of radiation treatment (10 to 21 days vs. 22 days or more).

Impact of multileaf collimator leaf positioning accuracy on intensity modulation radiation therapy quality assurance ion chamber measurements

Quality assurance (QA) procedures for intensity modulation radiation therapy (IMRT) usually involve an ion chamber measurement in a phantom using the beam configuration of the actual treatment plan. In our QA procedures it was observed that the degree of agreement between the measurement and the calculation could vary from plan to plan, from linac to linac, as well as over time, with a discrepancy up to 8%. In this paper we examine one aspect of the process which can contribute to such poor reproducibility, namely, the leaf end position accuracy. A series of measurements was designed to irradiate an ion chamber using small beam segments where one multileaf collimator (MLC) edge covers half of the chamber. It was shown that the reproducibility varied up to 13%, which provides a possible explanation for the observed discrepancies above. A useful tool was also developed to measure ionization signals from individual segments of an IMRT sequence. In addition, an understanding of the leaf end position variations offers some insight into the overall quality of an IMRT dose distribution.

Quality assurance of a helical tomotherapy machine

Helical tomotherapy has been developed at the University of Wisconsin, and 'Hi-Art II' clinical machines are now commercially manufactured. At the core of each machine lies a ring-gantry-mounted short linear accelerator which generates x-rays that are collimated into a fan beam of intensity-modulated radiation by a binary multileaf, the modulation being variable with gantry angle. Patients are treated lying on a couch which is translated continuously through the bore of the machine as the gantry rotates. Highly conformal dose-distributions can be delivered using this technique, which is the therapy equivalent of spiral computed tomography. The approach requires synchrony of gantry rotation, couch translation, accelerator pulsing and the opening and closing of the leaves of the binary multileaf collimator used to modulate the radiation beam. In the course of clinically implementing helical tomotherapy, we have developed a quality assurance (QA) system for our machine. The system is analogous to that recommended for conventional clinical linear accelerator QA by AAPM Task Group 40 but contains some novel components, reflecting differences between the Hi-Art devices and conventional clinical accelerators. Here the design and dosimetric characteristics of Hi-Art machines are summarized and the QA system is set out along with experimental details of its implementation. Connections between this machine-based QA work, pre-treatment patient-specific delivery QA and fraction-by-fraction dose verification are discussed.

The behavior of several microionization chambers in small intensity modulated radiotherapy fields

The aim of this work is to compare different ion chambers available for dose measurements in small fields used in intensity modulated radiotherapy. Some dosimetric aspects, related to these small radiation fields, i.e., lack of electronic lateral equilibrium and steep dose region, must be evaluated, in order to obtain an accurate technique implementation. Furthermore, the size of the sensitive volume of the chambers compared with the mapping of the beams or segments needs consideration. If the size of the chamber is too large for the flatness of the field, the measurement can deviate from the expected absorbed dose at a point. We propose a comparison of various dosimetric values between different microionization chambers with respect to a smaller dosimeter, such as the diamond detector.

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

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

Independent dosimetric calculation with inclusion of head scatter and MLC transmission for IMRT

Independent verification of the MU settings and dose calculation of IMRT treatment plans is an important step in the IMRT quality assurance (QA) procedure. At present, the verification is mainly based on experimental measurements, which are time consuming and labor intensive. Although a few simplified algorithms have recently been proposed for the independent dose (or MU) calculation, head scatter has not been precisely taken into account in all these investigations and the dose validation has mainly been limited to the central axis. In this work we developed an effective computer algorithm for IMRT MU and dose validation. The technique is superior to the currently available computer-based MU check systems in that (1) it takes full consideration of the head scatter and leaf transmission effects; and (2) it allows a precise dose calculation at an arbitrary spatial point instead of merely a point on the central axis. In the algorithm the dose at an arbitrary spatial point is expressed as a summation of the contributions of primary and scatter radiation from all beamlets. Each beamlet is modulated by a dynamic modulation factor (DMF), which is determined by the MLC leaf trajectories, the head scatter, the jaw positions, and the MLC leaf transmission. A three-source model was used to calculate the head scatter distribution for irregular segments shaped by MLC and the scatter dose contributions were computed using a modified Clarkson method. The system reads in MLC leaf sequence files (or RTP files) generated by the Corvus (NOMOS Corporation, Sewickley, PA) inverse planning system and then computes the doses at the desired points. The algorithm was applied to study the dose distributions of several testing intensity modulated fields and two multifield Corvus plans and the results were compared with Corvus plans and experimental measurements. The final dose calculations at most spatial points agreed with the experimental measurements to within 3% for both the specially designed testing fields and the clinical intensity modulated field. Furthermore, excellent agreement (mostly within +/- 3.0%) was also found between our independent calculation and the ion chamber measurements at both central axis and off-axis positions for the multifield Corvus IMRT plans. These results indicate that the approach is robust and valuable for routine clinical IMRT plan validation.

EDR2 film dosimetry for IMRT verification using low-energy photon filters

Recently the EDR2 (extended dose range) film has been introduced commercially for applications in radiation therapy dosimetry. In addition to characterizing the wide dynamic range, several authors have reported a reduced energy dependence of this film compared to that of X-Omatic Verification (XV) films for megavoltage photon beams. However, those investigations were performed under limited geometrical conditions. We have investigated the dosimetric performance of EDR2 film for the verification of IMRT fields at more clinically relevant conditions by comparing the film doses with the doses measured with an ion chamber and XV films. The effects of using a low energy scattered photon filter on EDR2 film dosimetry was also studied. In contrast to previous reports our results show that EDR2 film still exhibits considerable energy dependence (a maximum discrepancy of 9%, compared with an ion chamber) at clinically relevant conditions (10 cm depth for IMRT fields). However, by using the low-energy filters the discrepancy is reduced to within 3%. Therefore, EDR2 film, in combination with the filters, is found to be a promising two-dimensional dosimeter for verification of IMRT treatment fields.

Is tomotherapy the future of IMRT?

Intensity-modulated radiotherapy (IMRT) has become established in many clinics round the world and is, arguably, technically feasible in any facility. Serial tomotherapy contributed an extensive role in its introduction into the mainstream in the second half of the 1990s. In tomotherapy, literally "slice therapy", highly conformal treatments are possible because of the advantages available within the treatment planning of the IMRT process. Currently the majority of clinics implementing IMRT are doing so using conventional clinical linear accelerators (Linacs) fitted with an integrated multileaf collimator (MLC). At this point in time we may wonder if there is any scope for further dramatic changes in this new technology. As we venture from IMRT initial implementation into image guided therapy it is clear that major changes in approach are still valid and needed. If, at each treatment fraction, we can ensure that treatments are delivered accurately by integration of volumetric imaging into on-line validation, then we can attempt higher levels of conformality. A new treatment machine, the helical tomotherapy system, is available that combines the benefits of tomotherapy with on-line volumetric imaging. In this article we will review this approach and explore its features.

Intensity-modulated radiation therapy for head and neck cancer

Intensity-modulated radiation therapy (IMRT) involves the delivery of optimized nonuniform beam intensities to the patient. In the head and neck region, there are many critical structures in close proximity to the target, with little influence from internal organ motion. Because IMRT produces tightly conformal doses and steep-dose gradients next to normal tissues, it provides the potential for organ sparing and improved tumor control. The dosimetric superiority of head and neck IMRT over conventional techniques has been demonstrated. The initial results of clinical IMRT studies showed reduction in xerostomia with no compromise in locoregional control if caution and appropriate knowledge are exercised in target determination and delineation.

Patient setup and verification for intensity-modulated radiation therapy (IMRT)

Intensity Modulated Radiation Therapy (IMRT) is now widely used in the radiation therapy community. The ability of IMRT to deliver complex dose distributions has allowed dose escalation to targets while sparing normal tissues. In IMRT the roles of the physicist, dosimetrist, and physician are changed. Inverse planning, which is inherent to IMRT, requires that the final dose solution be defined at the beginning of the planning process. The physician must define specific dose volume constraints for the target as well as normal tissues. The physicist and dosimetrist must evaluate the final plan and determine if it meets the goals of the treatment, even if it does not completely satisfy the initial constraints. Once a plan is decided upon, the ability of the clinic to safely and accurately deliver that plan to the patient must be confirmed. As with any new technology, IMRT has created a need for new quality assurance procedures. Here we describe our IMRT process from simulation through planning and treatment. By standardizing our simulations we have decreased setup times and decreased the threat of collisions. Comparison of pseudo-DRR's and multiple-exposure port films allows confirmation of patient positioning on the linac. Our treatment delivery quality assurance program using film and MOSFET detectors in a polystyrene phantom is also described. We provide insight on how to overcome some of the common problems encountered in treatment planning and delivery such as isocenter location, collision avoidance, table indexing, dose confirmation, and plan analysis.

Guidance document on delivery, treatment planning, and clinical implementation of IMRT: report of the IMRT Subcommittee of the AAPM Radiation Therapy Committee

Intensity-modulated radiation therapy (IMRT) represents one of the most significant technical advances in radiation therapy since the advent of the medical linear accelerator. It allows the clinical implementation of highly conformal nonconvex dose distributions. This complex but promising treatment modality is rapidly proliferating in both academic and community practice settings. However, these advances do not come without a risk. IMRT is not just an add-on to the current radiation therapy process; it represents a new paradigm that requires the knowledge of multimodality imaging, setup uncertainties and internal organ motion, tumor control probabilities, normal tissue complication probabilities, three-dimensional (3-D) dose calculation and optimization, and dynamic beam delivery of nonuniform beam intensities. Therefore, the purpose of this report is to guide and assist the clinical medical physicist in developing and implementing a viable and safe IMRT program. The scope of the IMRT program is quite broad, encompassing multileaf-collimator-based IMRT delivery systems, goal-based inverse treatment planning, and clinical implementation of IMRT with patient-specific quality assurance. This report, while not prescribing specific procedures, provides the framework and guidance to allow clinical radiation oncology physicists to make judicious decisions in implementing a safe and efficient IMRT program in their clinics.

Benchmarking beam alignment for a clinical helical tomotherapy device

A clinical helical tomotherapy treatment machine has been installed at the University of Wisconsin Comprehensive Cancer Center. Beam alignment has been finalized and accepted by UW staff. Helical tomotherapy will soon be clinically available to other sites. Clinical physicists who expect to work with this machine will need to be familiar with its unique dosimetric characteristics, and those related to the geometrical beam configuration and its verification are described here. A series of alignment tests and the results are presented. Helical tomotherapy utilizes an array of post-patient xenon-filled megavoltage radiation detectors. These detectors have proved capable of performing some alignment verification tests. That is particularly advantageous because those tests can then be automated and easily performed on an ongoing basis.

Comparison of ionization chambers of various volumes for IMRT absolute dose verification

IMRT plans are usually verified by phantom measurements: dose distributions are measured using film and the absolute dose using an ionization chamber. The measured and calculated doses are compared and planned MUs are modified if necessary. To achieve a conformal dose distribution, IMRT fields are composed of small subfields, or "beamlets." The size of beamlets is on the order of 1 x 1 cm2. Therefore, small chambers with sensitive volumes < or = 0.1 cm3 are generally used for absolute dose verification. A dosimetry system consisting of an electrometer, an ion chamber, and connecting cables may exhibit charge leakage. Since chamber sensitivity is proportional to volume, the effect of leakage on the measured charge is relatively greater for small chambers. Furthermore, the charge contribution from beamlets located at significant distances from the point of measurement may be below the small chambers threshold and hence not detected. On the other hand, large (0.6 cm3) chambers used for the dosimetry of conventional external fields are quite sensitive. Since these chambers are long, the electron fluence through them may not be uniform ("temporal" uniformity may not exist in the chamber volume). However, the cumulative, or "spatial" fluence distribution (as indicated by calculated IMRT dose distribution) may become uniform at the chamber location when the delivery of all IMRT fields is completed. Under the condition of "spatial" fluence uniformity, the charge collected by the large chamber may accurately represent the absolute dose delivered by IMRT to the point of measurement. In this work, 0.6, 0.125, and 0.009 cm3 chambers were used for the absolute dose verification for tomographic and step-and-shoot IMRT plans. With the largest, 0.6 cm3 chamber, the measured dose was equal to calculated within 0.5%, when no leakage corrections were made. Without leakage corrections, the error of measurement with a 0.125 cm3 chamber was 2.6% (tomographic IMRT) and 1.5% (step-and-shoot IMRT). When doses measured by a 0.125 cm3 chamber were corrected for leakage, the difference between the calculated and measured doses reduced to 0.5%. Leakage corrected doses obtained with the 0.009 cm3 chamber were within 1.5%-1.7% of calculated doses. Without leakage corrections, the measurement error was 16% (tomographic IMRT) and 7% (step-and-shoot IMRT).

Clinical implementation of intensity-modulated radiation therapy

The clinical implementation of intensity-modulated radiation therapy (IMRT) is a complex process because of the introduction of new treatment planning algorithms and beam delivery systems compared to conventional 3-dimensional conformal radiation therapy (3D-CRT) and the lack of established national performance protocols. IMRT uses an inverse-planning algorithm to create nonuniform fields that are only deliverable through a newly designed beam-modulating delivery system. The intent of this paper is to describe our experience and to elucidate the new clinical procedures that must be executed to have a successful IMRT program. Patients who undergo IMRT at our institution are immobilized and simulated before proceeding to computed tomography scan for patient data acquisition. Treatment planning involves the use of different prescription dose formats and different planning techniques compared to 3D-CRT. The desired dose goals for the target and sensitive structures must be specified before initiating the planning process, which is computer intensive. After the plan is completed, the delivery instructions are transferred to the delivery system via either a floppy disk for MIMiC-based IMRT or through the network for MLC-based IMRT. Target localizations are carried out using orthogonal radiographs. Ultrasound imaging system (BAT) is used to localize the prostate. Dose validation is performed using films, ion chambers or dose-calculation-based techniques.

[Upper aerodigestive tract cancers: clinical benefits of conformal radiotherapy and intensity modulation]

The conformal radiotherapy approach, three-dimensional conformal radiotherapy (3DCRT) or intensity-modulated radiotherapy (IMRT), is based on modern imaging modalities, efficient 3D treatment planning systems, sophisticated immobilization systems and rigorous quality assurance and treatment verification. The central objective of conformal radiotherapy is to ensure a high dose distribution tailored to the limits of the target volume while reducing exposure of normal tissues. These techniques would then allow further tumor dose escalation. Head-and-neck tumors are some of the most attractive localizations to test conformal radiotherapy. They combine ballistic difficulties due to particularly complex shapes (nasopharynx, ethmoid) and problems due to the number and low tolerance of neighbouring organs like parotids, eyes, brainstem and spinal cord. The therapeutic irradiation of head-and-neck tumors thus remains a challenge for the radiation oncologist. Conformal radiotherapy does have a significant potential for improving local control and reducing toxicity when compared to standard radiotherapy. However, in the absence of prospective randomized trials, it is somewhat difficult at present to evaluate the real benefits drawn from 3DCRT and IMRT. The published clinical reports on the use of conformal radiotherapy are essentially dealing with dosimetric comparisons on relatively small numbers of patients. Recently, a few publications have emphasized the clinical experience of several precursor teams with a suitable follow-up. This paper describes the current state-of-the-art of 3DCRT and IMRT in order to evaluate the impact of these techniques on head-and-neck cancers irradiation.

Independent monitor unit calculation for intensity modulated radiotherapy using the MIMiC multileaf collimator

A self-consistent monitor unit (MU) and isocenter point-dose calculation method has been developed that provides an independent verification of the MU for intensity modulated radiotherapy (IMRT) using the MIMiC (Nomos Corporation) multileaf collimator. The method takes into account two unique features of IMRT using the MIMiC: namely the gantry-dynamic arc delivery of intensity modulated photon beams and the slice-by-slice dose delivery for large tumor volumes. The method converts the nonuniform beam intensity planned at discrete gantry angles of 5 degrees or 10 degrees into conventional nonmodulated beam intensity apertures of elemental arc segments of 1 degree. This approach more closely simulates the actual gantry-dynamic arc delivery by MIMiC. Because each elemental arc segment is of uniform intensity, the MU calculation for an IMRT arc is made equivalent to a conventional arc with gantry-angle dependent beam apertures. The dose to the isocenter from each 1 degree elemental arc segment is calculated by using the Clarkson scatter summation technique based on measured tissue-maximum-ratio and output factors, independent of the dose calculation model used in the IMRT planning system. For treatments requiring multiple treatment slices, the MU for the arc at each treatment slice takes into account the MU, leakage and scatter doses from other slices. This is achieved by solving a set of coupled linear equations for the MUs of all involved treatment slices. All input dosimetry data for the independent MU/isocenter point-dose calculation are measured directly. Comparison of the MU and isocenter point dose calculated by the independent program to those calculated by the Corvus planning system and to direct measurements has shown good agreement with relative difference less than +/-3%. The program can be used as an independent initial MU verification for IMRT plans using the MIMiC multileaf collimators.

[IMCO(++)--a Monte Carlo based IMRT system]

The application of intensity modulated radiotherapy (IMRT) to dose escalation in the target volume sets particular demands in terms of accuracy of dose calculation. Dose calculation errors due to approximations are compensated by the optimization algorithm, a procedure that ultimately leads to incorrect fluence modulation. Such inaccuracies affect particularly the dose distribution in areas with secondary electron disequilibrium. In case tissues heterogeneity predominates, conventional dose calculation methods (such as Pencil Beam) can produce relative errors up to more than 10%. The accuracy can be significantly improved by the application of a Monte-Carlo (MC) algorithm. This paper describes a MC-based inverse treatment planning algorithm (IMCO++), based on a non-iterative approach with a feedback-controlling process. The convergence behavior of IMCO++ was investigated and the used MC dose-calculation codes MMms and XVMC were compared by means of a heterogeneous phantom. IMCO++ plans were optimized in various phantoms. All plans showed conformity in terms of dose distribution of the target volume and dose reduction in risk organs (according to the requirements of the target parameter), as well as a very fast convergence of the algorithm (in less than 10 optimization steps).

Determination and delineation of nodal target volumes for head-and-neck cancer based on patterns of failure in patients receiving definitive and postoperative IMRT

PURPOSE

We present the guidelines for target volume determination and delineation of head-and-neck lymph nodes based on the analysis of the patterns of nodal failure in patients treated with intensity-modulated radiotherapy (IMRT).

METHODS AND MATERIALS

Data pertaining to the natural course of nodal metastasis for each head-and-neck cancer subsite were reviewed. A system was established to provide guidance for nodal target volume determination and delineation. Following these guidelines, 126 patients (52 definitive, 74 postoperative) were treated between February 1997 and December 2000 with IMRT for head-and-neck cancer. The median follow-up was 26 months (range 12-55), and the patterns of nodal failure were analyzed.

RESULTS

These guidelines define the nodal target volume based on the location of the primary tumor and the probability of microscopic metastasis to the ipsilateral and contralateral (Level I-V) nodal regions. Following these guidelines, persistent or recurrent nodal disease was found in 6 (12%) of 52 patients receiving definitive IMRT, and 7 (9%) of 74 patients receiving postoperative IMRT had failure in the nodal region.

CONCLUSION

On the basis of our clinical experience in implementing inverse-planning IMRT for head-and-neck cancer, we present guidelines using a simplified, but clinically relevant, method for nodal target volume determination and delineation. The intention was to provide a foundation that enables different institutions to exchange clinical experiences in head-and-neck IMRT. These guidelines will be subject to future refinement when the clinical experience in head-and-neck IMRT advances.

Quality assurance of intensity-modulated radiotherapy

Intensity-modulated radiation therapy (IMRT) requires the use of inverse treatment planning and nonuniform fluence beams delivered by a series of complex radiation portals. The quality assurance procedures for conventional three-dimensional conformal radiation therapy (3D-CRT) have been developed and are in worldwide clinical use, but the more complex nature of IMRT limits the application of much of the quality assurance (QA) procedures developed for IMRT. Although consensus has not yet been reached regarding which procedures will eventually become recommended by official organizations, the field is rapidly coming to agreement on a basic set of procedures. This manuscript describes some of the novel techniques recently developed for IMRT QA, both for the validation of the calculated dose distribution and for assuring that the dose distribution reaches its intended targets.

IMRT in the treatment of head and neck cancer: is the present already the future?

Disease outcome in locally advanced head and neck cancer patients is far from satisfactory. The main causes of failure remain linked to locoregional recurrences, which are due to incomplete eradication of clonogenic cells. Conventional radiation therapy or 3-dimensional conformal radiation therapy are currently carried out at their extreme possibilities due to their intrinsic limitation--namely the impossibility to generate concave dose distributions without compromising tumor irradiation. Approximately a third of patients treated with radiotherapy and most head and neck cancer cases present concave shapes of the target volumes. With the advent of intensity modulated radiation therapy--clinically available for only few years--head and neck patients can now benefit from strategies based on highly conformal techniques. It is possible to exploit efficiently dose-escalation protocols to increase probabilities to eradicate clonogens, to reduce overall treatment time, to control repopulation problems and to keep as low as reasonably necessary the irradiation of healthy tissues minimizing acute and late complications. Today, both planning and clinical studies demonstrate these advantages but larger controlled trials are necessary to assess the true potentialities of techniques based on intensity modulation for head and neck cancers. In a speculative view, proton therapy, possibly with intensity modulation, or light ion therapy should be considered for selected cases or for reirradiation due to their higher biological efficacy and their degree of dose-conformation to target volumes.

Toward automated quality assurance for intensity- modulated radiation therapy

PURPOSE

To investigate whether high-quality, relatively inexpensive, document and transparency scanners used as densitometers are sufficiently quantitative for routine quality assurance (QA).

METHODS AND MATERIALS

The scanner we investigated used a linear amplifier, digitizing gray-scale images to 12-bit resolution with a user-selected spatial resolution of 0.170 mm(2) pixels. To reduce Newton's rings artifacts, the standard glass platen was replaced by glass with an antireflective coating. Conversion of reading to transmission was conducted by permanently placing a calibrated photographic step tablet on the scanner platen. After conversion to light transmission, a zero-phase two-dimensional Wiener filter was used to reduce pixel-to-pixel signal variation. Light-scatter artifacts were removed by deconvolution of a measured light-spread kernel. The light-spread kernel artifacts were significant along the scanner's detector axis, but were insignificant along the scanning axis.

RESULTS

Pixel-to-pixel noise was better than 2% for optical densities, ranging from 0.4 to 2.0 and 0 to 2.7 for the unfiltered and filtered images, respectively. The document scanning system response was compared against a confocal scanning laser densitometer. A series of IMRT dose distribution and dose calibration film sets were scanned using the two scanners, and the measured dose was compared. The maximum mean and standard deviation of the measured dose difference between the document scanner and confocal scanner was 1.48% and 1.06%, respectively.

CONCLUSION

While the document scanners are not as flexible as dedicated film densitometers, these results indicate that, using the intensity and scatter corrections, the system provides accurate and precise measurements up to an optical density of 2.0, sufficient for routine IMRT film QA. For some film types, this requires the reduction in monitor units to limit the dose delivered to the film. The user must be cautious that the delivered IMRT dose is scaled appropriately. This inexpensive and accurate system is being integrated into an automated QA program.

Temperature increases associated with polymerization of irradiated PAG dosimeters

Polyacrylamide gel (PAG) dosimeters show considerable promise as three-dimensional dosimeters for the verification of complex dose distributions associated with conformal therapy. However, the potential of PAG dosimeters has not yet been borne out in clinical practice and it is apparent that basic investigations of these dosimeters are still required. The polymerization reactions in PAG dosimeters are exothermic and the heat given off by the reactions may influence polymerization reaction kinetics. We report the results of in situ measurements of local temperature increases in irradiated PAG resulting from heat generated by the radiation-induced exothermic polymerization reactions. Temperature changes proportional to the absorbed dose were observed in the irradiated gels, reaching a maximum of 12 degrees C under high-dose conditions, depending on the thermal boundary conditions. This has practical implications, for example, using small vials of PAG to calibrate large phantoms may not be appropriate since temperature differences during irradiation between the calibration vials and phantom may alter the morphology and quantity of the polymer formed, even when irradiated to the same dose. The inhibition of radiation-induced polymerization associated with low-level oxygen contamination is manifested by a delay in the onset of temperature rise during irradiation. The observed temperature changes are used to estimate the percentage conversion of double bonds from the bis/acrylamide monomers by polymerization reactions.

Considerations on treatment efficiency of different conformal radiation therapy techniques for prostate cancer

BACKGROUND AND PURPOSE

To evaluate the treatment efficiency of different conformal radiation therapy techniques in prostate cancer.

MATERIALS AND METHODS

Three major classes of intensity-modulated radiation therapy (IMRT) delivery as well as a conformal rotation technique have been evaluated: sequential tomotherapy, dynamic multileaf collimation (DMLC) with conventional MLC, DMLC with miniMLC and dynamic field shaping arc. Treatment planning for the IMRT techniques has been performed with inverse planning. Forward planning was used for the dynamic arc technique. The four techniques have been compared to treat two different prostate cases with a conservative target dose of 70 Gy: a convex shaped target volume and one containing concavities formed by the bladder and rectum. Cumulative dose volume histograms, tumor control probability and normal tissue complication probability, conformity index and dose heterogeneity, and finally efficiency of treatment delivery have been evaluated.

RESULTS

For the convex shaped target, all treatment modalities met the desired treatment goals, although the conventional MLC delivered more dose to the bladder. Compared to the dynamic arc modality, both tomotherapy and the conventional MLC technique needed a tenfold higher number of monitor units per target dose, and the miniMLC a twofold higher number. The same trend has been observed for the concave target, yet the dynamic arc did not meet the desired dose reduction for the rectum. The miniMLC configuration represented the best compromise for both targets with respect to treatment goals and delivery efficiency. Sequential tomotherapy performed adequately with respect to conformity at the cost of efficiency.

CONCLUSIONS

Together with conformity and delivery efficiency the shape of the target should be considered as an important parameter in the selection of the treatment modality.

Assessment of the acceptability of the Elekta multileaf collimator (MLC) within the Corvus planning system for static and dynamic delivery of intensity modulated beams (IMBs)

The sliding window technique used for static and dynamic segmentation of intensity modulated beams is evaluated. Dynamic delivery is preferred since the resulting distributions correspond better with the calculated distributions, the treatment beam is used more efficiently and the delivery is less sensitive to small variations in the accuracy of the multileaf collimator (MLC).

Applicator-guided intensity-modulated radiation therapy

PURPOSE

We are introducing a novel method for delivering highly conformal dose distributions to cervical cancer tumors using external beam intensity-modulated radiation therapy. The method, termed applicator-guided intensity-modulated radiation therapy (AGIMRT), will use an applicator substitute placed in the vagina and uterus to provide spatial registration and immobilization of the gynecologic organs. The main reason for the applicator substitute will be to localize the fornices, cervix, and uterus with the expectation that the other nearby organs will also be reproducibly positioned with respect to the applicator substitute. Intensity-modulated radiation therapy (IMRT) dose distributions will be used as a substitute for high-dose-rate intracavitary brachytherapy procedures. The flexibility of IMRT will enable customized dose distributions that have the potential to reduce complications and improve local control, especially for locally advanced disease.

METHODS AND MATERIALS

To test the advantages of IMRT over intracavitary brachytherapy, volumetric scans of three cervical cancer patients were obtained with implanted CT-compatible applicators. IMRT dose distribution simulations using tomotherapy, were compared against intracavitary brachytherapy using cesium tubes to investigate the dosimetric differences of the two modalities. Because these tumor volumes do not image well on CT, the target volumes were defined as the isodose surface containing the traditional point A, defined as 2 cm superior to the vaginal fornices and 2 cm lateral to the intrauterine canal. One patient had a uterus that wrapped superior and anterior to the bladder. For this case, the cervix and uterus were selected as the target volume. To determine the potential for using an applicator substitute to localize internal organs, the posterior bladder and anterior rectal surfaces were localized relative to the colpostats. Comparisons of the colpostat-localized surfaces were conducted for two scan studies for 3 patients.

RESULTS

The IMRT distributions covered the point-A isodose surfaces while reducing doses to the bladder and rectum. Brachytherapy showed extensive underdose regions in the target volume for the wrapped-around target. Spatial positioning was better than 0.7 and 1.3 cm in the rectum and bladder, respectively, indicating the potential that an applicator substitute may be able to localize these structures.

CONCLUSIONS

AGIMRT has the potential for improving critical structure avoidance while maintaining highly reproducible and accurate internal organ registration found with brachytherapy.

Dosimetric verification of inverse planned step and shoot multileaf collimator fields from a commercial treatment planning system

An inverse treatment planning (ITP) module on a commercial treatment planning system (TPS) (Helax AB, Uppsala, Sweden) is being used for an in-house clinical trial for treatment of nasopharyngeal cancer with contralateral parotid sparing. Intensity modulated radiation therapy (IMRT) fields are delivered by step and shoot multileaf collimator (MLC) with a DMLC enabled Varian 2300 CD (Varian Associates, Palo Alto, CA). A series of testing procedures have been devised to quantify the modeling and delivery accuracy of routine clinical inverse planned IMRT using Helax TMS and the Varian step and shoot MLC delivery option. Testing was done on specific aspects of the TPS modeling germane to DMLC. Measured relative dose factors (head scatter plus phantom scatter) for small MLC fields, normalized to a 10x10 cm2 non-MLC field, were found to differ by 2-3% from the TPS values for the smallest of the fields tested. Relative distributions for small off axis fields were found to be in good agreement. A process for the routine clinical verification of IMRT fields has been implemented. Each IMRT field in an inverse plan is imported into a flat water tank plan and a "beam's eye view" (BEV) dose distribution is generated. This is compared to the corresponding measured BEV dose distribution. The IMRT verification process has also been performed using an anthropomorphic phantom. Large clinical fields (i.e., greater than 14.5 cm in the leaf direction) caused difficulties due to a vendor specific machine restriction, and several techniques for dealing with these were examined. These techniques were (i) the use of static stepping of closed junctions, (ii) the use of two separate IMRT fields for a given gantry angle, and (iii) restricting the overall maximum field size used. The overall process has allowed implementation of an in-house protocol for IMRT use on an initial clinical site. Results of the verification measurements for the first ten patients treated at this center reveal an average maximum dose per IMRT field delivered of 71.0 cGy, with a mean local deviation from the planned dose of -1.2 cGy, and a standard deviation of 2.4 cGy.

Clinical experience of head-and-neck cancer IMRT with serial tomotherapy

New radiotherapy planning and delivery techniques are undergoing rapid progress and change due to computer hardware and software technologies that have led to the development of sophisticated 3-dimensional (3D) radiation treatment planning and computer-controlled radiation therapy delivery systems. Intensity-modulated radiation therapy (IMRT) is the most recent and advanced form of external beam radiation therapy often used to perform 3D conformal radiotherapy. It represents one of the most important technical advances in radiotherapy. IMRT has the potential to achieve a much higher degree of target conformity and normal tissue sparing than most other treatment techniques, especially for target volumes at risk with complex shapes and concave regions such as head-and-neck cancer. In this review, we summarize our own IMRT treatment techniques with serial tomotherapy and our clinical experience with 126 patients with head-and-neck IMRT.