Optimized intensity-modulated arc therapy for prostate cancer treatment

Patient-specific dosimetric endpoints based treatment plan quality control in radiotherapy

In intensity modulated radiotherapy (IMRT), the optimal plan for each patient is specific due to unique patient anatomy. To achieve such a plan, patient-specific dosimetric goals reflecting each patient's unique anatomy should be defined and adopted in the treatment planning procedure for plan quality control. This study is to develop such a personalized treatment plan quality control tool by predicting patient-specific dosimetric endpoints (DEs). The incorporation of patient specific DEs is realized by a multi-OAR geometry-dosimetry model, capable of predicting optimal DEs based on the individual patient's geometry. The overall quality of a treatment plan is then judged with a numerical treatment plan quality indicator and characterized as optimal or suboptimal. Taking advantage of clinically available prostate volumetric modulated arc therapy (VMAT) treatment plans, we built and evaluated our proposed plan quality control tool. Using our developed tool, six of twenty evaluated plans were identified as sub-optimal plans. After plan re-optimization, these suboptimal plans achieved better OAR dose sparing without sacrificing the PTV coverage, and the dosimetric endpoints of the re-optimized plans agreed well with the model predicted values, which validate the predictability of the proposed tool. In conclusion, the developed tool is able to accurately predict optimally achievable DEs of multiple OARs, identify suboptimal plans, and guide plan optimization. It is a useful tool for achieving patient-specific treatment plan quality control.

Three-dimensional radiochromic film dosimetry for volumetric modulated arc therapy using a spiral water phantom

We validated 3D radiochromic film dosimetry for volumetric modulated arc therapy (VMAT) using a newly developed spiral water phantom. The phantom consists of a main body and an insert box, each of which has an acrylic wall thickness of 3 mm and is filled with water. The insert box includes a spiral film box used for dose-distribution measurement, and a film holder for positioning a radiochromic film. The film holder has two parallel walls whose facing inner surfaces are equipped with spiral grooves in a mirrored configuration. The film is inserted into the spiral grooves by its side edges and runs along them to be positioned on a spiral plane. Dose calculation was performed by applying clinical VMAT plans to the spiral water phantom using a commercial Monte Carlo-based treatment-planning system, Monaco, whereas dose was measured by delivering the VMAT beams to the phantom. The calculated dose distributions were resampled on the spiral plane, and the dose distributions recorded on the film were scanned. Comparisons between the calculated and measured dose distributions yielded an average gamma-index pass rate of 87.0% (range, 91.2-84.6%) in nine prostate VMAT plans under 3 mm/3% criteria with a dose-calculation grid size of 2 mm. The pass rates were increased beyond 90% (average, 91.1%; range, 90.1-92.0%) when the dose-calculation grid size was decreased to 1 mm. We have confirmed that 3D radiochromic film dosimetry using the spiral water phantom is a simple and cost-effective approach to VMAT dose verification.

Evaluation of the sensitivity of two 3D diode array dosimetry systems to setup error for quality assurance (QA) of volumetric-modulated arc therapy (VMAT)

The purpose of this study is to evaluate the sensitivities of 3D diode arrays to setup error for patient‐specific quality assurance (QA) of volumetric‐modulated arc therapy (VMAT). Translational setup errors of ±1,±2, and ±3 mm in the RL, SI, and AP directions and rotational setup errors of ±1° and ±2° in the pitch, roll, and yaw directions were set up in two phantom systems, ArcCHECK and Delta4, with VMAT plans for 11 patients. Cone‐beam computed tomography (CBCT) followed by automatic correction using a HexaPOD 6D treatment couch ensured the position accuracy. Dose distributions of the two phantoms were compared in order to evaluate the agreement between calculated and measured values by using γ analysis with 3%/3 mm, 3%/2 mm, and 2%/2 mm criteria. To determine the impact on setup error for VMAT QA, we evaluated the sensitivity of results acquired by both 3D diode array systems to setup errors in translation and rotation. For the VMAT QA of all patients, the pass rate with the 3%/3 mm criteria exceeded 95% using either phantom. For setup errors of 3 mm and 2°, respectively, the pass rates with the 3%/3 mm criteria decreased by a maximum of 14.0% and 23.5% using ArcCHECK, and 14.4% and 5.0% using Delta4. Both systems are sensitive to setup error, and do not have mechanisms to account for setup errors in the software. The sensitivity of both VMAT QA systems was strongly dependent on the patient‐specific plan. The sensitivity of ArcCHECK to the rotational error was higher than that of Delta4. In order to achieve less than 3% mean pass rate reduction of VMAT plan QA with the 3%/3 mm criteria, a setup accuracy of 2 mm/1° and 2 mm/2° is required for ArcCheck and Delta4 devices, respectively. The cumulative effect of the combined 2 mm translational and 1° rotational errors caused 3.8% and 2.4% mean pass rates reduction with 3%/3 mm criteria, respectively, for ArcCHECK and Delta4 systems. For QA of VMAT plans for nasopharyngeal cancer (NPC) using the ArcCHECK system, the setup should be more accurate.

PACS numbers: 87.55.ne, 87.55.Qr, 87.55.km

Assessment of the robustness of volumetric-modulated arc therapy for lung radiotherapy

Volumetric-modulated arc therapy (VMAT) is increasingly popular as a treatment method in radiotherapy owing to the speed with which treatments can be delivered. However, there has been little investigation into the effect of increased modulation in lung plans with regard to interfraction organ motion. This is most likely to occur where the planning target volume (PTV) lies within areas of low density. This paper aims to investigate the effect of modulation on the dose distribution using simulated patient movement and to propose a method that is less susceptible to such movement. Simulated interfraction motion is achieved by moving the plan isocentre in steps of 0.5 cm and 1.0 cm in six directions for five clinical VMAT patients. The proposed planning method involves optimisation using a density override of 1 g cm(-3), within the PTV in lung, to reduce segment boosting in the periphery of the PTV. This investigation shows that modulation can result in an increase in the maximum dose of >25%, an increase in PTV near-maximum dose of 17% and a reduction in near-minimum dose by 46%. Unacceptable organ at risk (OAR) doses are also seen. The proposed method reduces modulation, resulting in a maximum dose increase of 10%. Although safeguards are in place to prevent the increased dose to OARs from patient movement, there is nothing to prevent the increased dose as a result of modulation in lung. A simple planning method is proposed to safeguard against this effect. Investigation suggests that, where modulation exists in a plan, this method reduces it and is clinically viable.

Two-step intensity modulated arc therapy (2-step IMAT) with segment weight and width optimization

Background

2-step intensity modulated arc therapy (IMAT) is a simplified IMAT technique which delivers the treatment over typically two continuous gantry rotations. The aim of this work was to implement the technique into a computerized treatment planning system and to develop an approach to optimize the segment weights and widths.

Methods

2-step IMAT was implemented into the Prism treatment planning system. A graphical user interface was developed to generate the plan segments automatically based on the anatomy in the beam's-eye-view. The segment weights and widths of 2-step IMAT plans were subsequently determined in Matlab using a dose-volume based optimization process. The implementation was tested on a geometric phantom with a horseshoe shaped target volume and then applied to a clinical paraspinal tumour case.

Results

The phantom study verified the correctness of the implementation and showed a considerable improvement over a non-modulated arc. Further improvements in the target dose uniformity after the optimization of 2-step IMAT plans were observed for both the phantom and clinical cases. For the clinical case, optimizing the segment weights and widths reduced the maximum dose from 114% of the prescribed dose to 107% and increased the minimum dose from 87% to 97%. This resulted in an improvement in the homogeneity index of the target dose for the clinical case from 1.31 to 1.11. Additionally, the high dose volume V₁₀₅ was reduced from 57% to 7% while the maximum dose in the organ-at-risk was decreased by 2%.

Conclusions

The intuitive and automatic planning process implemented in this study increases the prospect of the practical use of 2-step IMAT. This work has shown that 2-step IMAT is a viable technique able to achieve highly conformal plans for concave target volumes with the optimization of the segment weights and widths. Future work will include planning comparisons of the 2-step IMAT implementation with fixed gantry intensity modulated radiotherapy (IMRT) and commercial IMAT implementations.

Volumetric-modulated arc therapy: effective and efficient end-to-end patient-specific quality assurance

PURPOSE

To explore an effective and efficient end-to-end patient-specific quality-assurance (QA) protocol for volumetric modulated arc radiotherapy (VMAT) and to evaluate the suitability of a stationary radiotherapy QA device (two-dimensional [2D] ion chamber array) for VMAT QA.

METHODS AND MATERIALS

Three methods were used to analyze 39 VMAT treatment plans for brain, spine, and prostate: ion chamber (one-dimensional absolute, n = 39), film (2D relative, coronal/sagittal, n = 8), and 2D ion chamber array (ICA, 2D absolute, coronal/sagittal, n = 39) measurements. All measurements were compared with the treatment planning system dose calculation either via gamma analysis (3%, 3- to 4-mm distance-to-agreement criteria) or absolute point dose comparison. The film and ion chamber results were similarly compared with the ICA measurements.

RESULTS

Absolute point dose measurements agreed well with treatment planning system computed doses (ion chamber: median deviation, 1.2%, range, -0.6% to 3.3%; ICA: median deviation, 0.6%, range, -1.8% to 2.9%). The relative 2D dose measurements also showed good agreement with computed doses (>93% of pixels in all films passing gamma, >90% of pixels in all ICA measurements passing gamma). The ICA relative dose results were highly similar to those of film (>90% of pixels passing gamma). The coronal and sagittal ICA measurements were statistically indistinguishable by the paired t test with a hypothesized mean difference of 0.1%. The ion chamber and ICA absolute dose measurements showed a similar trend but had disparities of 2-3% in 18% of plans.

CONCLUSIONS

After validating the new VMAT implementation with ion chamber, film, and ICA, we were able to maintain an effective yet efficient patient-specific VMAT QA protocol by reducing from five (ion chamber, film, and ICA) to two measurements (ion chamber and single ICA) per plan. The ICA (Matrixx®, IBA Dosimetry) was validated for VMAT QA, but ion chamber measurements are recommended for absolute dose comparison until future developments correct the ICA angular dependence.

Intensity-modulated arc therapy: principles, technologies and clinical implementation

Intensity-modulated arc therapy (IMAT) was proposed by Yu (1995 Phys. Med. Biol. 40 1435-49) as an alternative to tomotherapy. Over more than a decade, much progress has been made. The advantages and limitations of the IMAT technique have also been better understood. In recent years, single-arc forms of IMAT have emerged and become commercially adopted. The leading example is the volumetric-modulated arc therapy (VMAT), a single-arc form of IMAT that delivers apertures of varying weights with a single-arc rotation that uses dose-rate variation of the treatment machine. With commercial implementation of VMAT, wide clinical adoption has quickly taken root. However, there remains a lack of general understanding for the planning of such arc treatments, as well as what delivery limitations and compromises are made. Commercial promotion and competition add further confusion for the end users. It is therefore necessary to provide a summary of this technology and some guidelines on its clinical implementation. The purpose of this review is to provide a summary of the works from the radiotherapy community that led to wide clinical adoption, and point out the issues that still remain, providing some perspective on its further developments. Because there has been vast experience in IMRT using multiple intensity-modulated fields, comparisons between IMAT and IMRT are also made in the review within the areas of planning, delivery and quality assurance.

Arc-modulated radiation therapy based on linear models

This paper reports an inverse arc-modulated radiation therapy planning technique based on linear models. It is implemented with a two-step procedure. First, fluence maps for 36 fixed-gantry beams are generated using a linear model-based intensity-modulated radiation therapy (IMRT) optimization algorithm. The 2D fluence maps are decomposed into 1D fluence profiles according to each leaf pair position. Second, a mixed integer linear model is used to construct the leaf motions of an arc delivery that reproduce the 1D fluence profile previously derived from the static gantry IMRT optimization. The multi-leaf collimator (MLC) sequence takes into account the starting and ending leaf positions in between the neighbouring apertures, such that the MLC segments of the entire treatment plan are deliverable in a continuous arc. Since both steps in the algorithm use linear models, implementation is simple and straightforward. Details of the algorithm are presented, and its conceptual correctness is verified with clinical cases representing prostate and head-and-neck treatments.

Comparison of Elekta VMAT with helical tomotherapy and fixed field IMRT: plan quality, delivery efficiency and accuracy

PURPOSE

Helical tomotherapy (HT) and volumetric modulated arc therapy (VMAT) are arc-based approaches to IMRT delivery. The objective of this study is to compare VMAT to both HT and fixed field IMRT in terms of plan quality, delivery efficiency, and accuracy.

METHODS

Eighteen cases including six prostate, six head-and-neck, and six lung cases were selected for this study. IMRT plans were developed using direct machine parameter optimization in the Pinnacle3 treatment planning system. HT plans were developed using a Hi-Art II planning station. VMAT plans were generated using both the Pinnacle3 SmartArc IMRT module and a home-grown arc sequencing algorithm. VMAT and HT plans were delivered using Elekta's PreciseBeam VMAT linac control system (Elekta AB, Stockholm, Sweden) and a TomoTherapy Hi-Art II system (TomoTherapy Inc., Madison, WI), respectively. Treatment plan quality assurance (QA) for VMAT was performed using the IBA MatriXX system while an ion chamber and films were used for HT plan QA.

RESULTS

The results demonstrate that both VMAT and HT are capable of providing more uniform target doses and improved normal tissue sparing as compared with fixed field IMRT. In terms of delivery efficiency, VMAT plan deliveries on average took 2.2 min for prostate and lung cases and 4.6 min for head-and-neck cases. These values increased to 4.7 and 7.0 min for HT plans.

CONCLUSIONS

Both VMAT and HT plans can be delivered accurately based on their own QA standards. Overall, VMAT was able to provide approximately a 40% reduction in treatment time while maintaining comparable plan quality to that of HT.

A generalized inverse planning tool for volumetric-modulated arc therapy

The recent development in linear accelerator control systems, named volumetric-modulated arc therapy (VMAT), has generated significant interest in arc-based intensity-modulated radiation therapy (IMRT). The VMAT delivery technique features simultaneous changes in dose rate, gantry angle and gantry rotation speed as well as multi-leaf collimator (MLC) leaf positions while radiation is on. In this paper, we describe a generalized VMAT planning tool that is designed to take full advantage of the capabilities of the new linac control systems. The algorithm incorporates all of the MLC delivery constraints such as restrictions on MLC leaf interdigitation and the MLC leaf velocity constraints. A key feature of the algorithm is that it is able to plan for both single- and multiple-arc deliveries. Compared to conventional step-and-shoot IMRT plans, our VMAT plans created using this tool can achieve similar or better plan quality with less MU and better delivery efficiency. The accuracy of the obtained VMAT plans is also demonstrated through plan verifications performed on an Elekta Synergy linear accelerator equipped with a conventional MLC of 1 cm leaf width using a PreciseBeam VMAT linac control system.

Development and evaluation of an efficient approach to volumetric arc therapy planning

An efficient method for volumetric intensity modulated arc therapy (VMAT) planning was developed, where a single arc (360 degrees or less) is delivered under continuous variation of multileaf collimator (MLC) segments, dose rate, and gantry speed. Plans can be generated for any current linear accelerator that supports these degrees of freedom. MLC segments are derived from fluence maps at relatively coarsely sampled angular positions. The beam segments, dose rate, and gantry speed are then optimized using direct machine parameter optimization based on dose volume objectives and leaf motion constraints to minimize arc delivery time. The method can vary both dose rate and gantry speed or alternatively determine the optimal plan at constant dose rate and gantry speed. The method was used to retrospectively generate variable dose rate VMAT plans to ten patients (head and neck, prostate, brain, lung, and tonsil). In comparison to step-and-shoot intensity modulated radiation therapy, dosimetric plan quality was comparable or improved, estimated delivery times ranged from 70 to 160 s, and monitor units were consistently reduced in nine out of the ten cases by an average of approximately 6%. Optimization and final dose calculation took between 5 and 35 min depending on plan complexity.

Dosimetric verification of RapidArc treatment delivery

PURPOSE

Recently, Varian Medical Systems have announced the introduction of a new treatment technique, in which dose is delivered over a single gantry rotation with variable MLC positions, dose rate and gantry speed. In February 2008, a preclinical installation of the RapidArc beam delivery approach was carried out on a Varian Clinac at Rigshospitalet in Copenhagen. The purpose of the installation was to perform measurements to verify the correctness of doses delivered with the RapidArc technique. In May 2008, the clinical release of the RapidArc application was installed at Rigshospitalet.

METHODS AND MATERIALS

Nine treatment plans were generated in the Eclipse version 8.5 including the RapidArc optimizer for H&N and prostate cases. The plans were delivered to the Scandidos Delta4 cylindrical diode array phantom. First, the measured dose distributions were compared with the calculated doses. All plans were then delivered several times to verify consistency of the delivery. Gamma analysis was used to verify the correspondence between dose distributions. The temporal resolution of the delivery was analysed by investigating the arc segments between control points separately.

RESULTS

Overall, good agreement was observed between measured and calculated doses in most cases with gamma values above 1 in >95% of measured points. The reproducibility of delivery was also very high. Gamma analysis between two consecutive runs of the same delivery plan generally showed gamma values above 1 in none of the measured points, and dose deviation less than 1%. Temporal analysis showed small discrepancies between doses delivered between control points (approximately 2 degrees of the rotation) in consecutive runs of a plan, however these were cancelled out in the accumulated dose.

CONCLUSION

The delivery of RapidArc beam delivery has been verified to correspond well with calculated dose distributions for a number of different cases. The delivery was very reproducible, and was carried out with high stability of the accelerator performance.

RapidArc volumetric modulated therapy planning for prostate cancer patients

PURPOSE

Recently, Varian Medical Systems have announced the introduction of a new treatment technique, RapidArc, in which dose is delivered over a single gantry rotation with dynamically variable MLC positions, dose rate and gantry speed. At Rigshospitalet, the RapidArc technique was brought into clinical practice in May 2008 for treatment of prostate cancer patients. We report here our experiences with performing treatment planning using the Eclipse RapidArc optimization software for this patient group.

MATERIAL AND METHODS

A stand-alone installation of Eclipse 8.5 with RapidArc optimization capability was performed at Rigshospitalet. Patient data for 8 prostate cancer patients were imported, most of whom were previously treated at Rigshospitalet using IMRT. Three of the patients were treated at Rigshospitalet using the RapidArc technique. Treatment plans were optimized using objectives as given by standard guidelines for clinical treatment planning. RapidArc plans were compared to the IMRT plans by which the patients were actually treated or in the three cases treated with the RapidArc technique to IMRT plans achieved using standard guidelines. Comparison was done with respect to target coverage, doses to rectum and bladder, over-all maximum dose and number of monitor units.

RESULTS

Overall, the RapidArc treatment plans gave better or equal sparing of the organs at risk than the IMRT treatment plans. The number of monitor units was lower in most cases, by up to approximately 75%. However, the target dose homogeneity was not as high as for IMRT. The low-dose bath was larger than for IMRT.

CONCLUSION

RapidArc optimization is very promising, especially regarding the potential of reducing the number of monitor units, while providing good sparing of organs at risk. Some improvement is still warranted with respect to achieving high target dose homogeneity.

Quality Control of Portal Imaging with PTW EPID QC PHANTOM

PURPOSE

Quality assurance (QA) and quality control (QC) of different electronic portal imaging devices (EPID) and portal images with the PTW EPID QC PHANTOM.

MATERIAL AND METHODS

Characteristic properties of images of different file formats were measured on Siemens OptiVue500aSi, Siemens BeamView Plus, Elekta iView, and Varian PortalVision and analyzed with the epidSoft 2.0 program in four radiation therapy centers. The portal images were taken with Kodak X-OMAT V and the Kodak Portal Localisation ReadyPack films and evaluated with the same program.

RESULTS

The optimal exposition both for EPIDs and portal films of different kind was determined. For double exposition, the 2+1 MU values can be recommended in the case of Siemens OptiVue500aSi Elekta iView and Kodak Portal Localisation ReadyPack films, while for Siemens BeamView Plus, Varian PortalVision and Kodak X-OMAT V film 7+7 MU is recommended.

CONCLUSION

The PTW EPID QC PHANTOM can be used not only for amorphous silicon EPIDs but also for images taken with a video-based system or by using an ionization chamber matrix or for portal film. For analysis of QC tests, a standardized format (used at the acceptance test) should be applied, as the results are dependent on the file format used.

Comparison of Plan Quality Provided by Intensity-Modulated Arc Therapy and Helical Tomotherapy

PURPOSE

Intensity-modulated arc therapy (IMAT) is an arc-based approach to intensity-modulated radiotherapy (IMRT) that can be delivered on a conventional linear accelerator using a conventional multileaf collimator. In a previous work, we demonstrated that our arc-sequencing algorithm can produce highly conformal IMAT plans. Through plan comparisons, we explored the ability of IMAT to serve as an alternative to helical tomotherapy.

METHODS AND MATERIALS

The IMAT plans were created for 10 patients previously treated with helical tomotherapy. Treatment plan comparisons, according to the target dose coverage and critical structure sparing, were performed to determine whether similar plan quality could be achieved using IMAT.

RESULTS

In 8 of 10 patient cases, IMAT was able to provide plan quality comparable to that of helical tomotherapy. In 2 of these 8 cases, the use of non-axial coplanar or non-coplanar arcs in IMAT planning led to significant improvements in normal tissue sparing. The remaining 2 cases posed particular dosimetric challenges. In 1 case, the target was immediately adjacent to a spinal cord that had received previous irradiation. The second case involved multiple target volumes and multiple prescription levels. Both IMAT and tomotherapy were able to produce clinically acceptable plans. Tomotherapy, however, provided a more uniform target dose and improved critical structure sparing.

CONCLUSIONS

For most cases, IMAT can provide plan qualities comparable to that of helical tomotherapy. For some intracranial tumors, IMAT's ability to deliver non-coplanar arcs led to significant dosimetric improvements. Helical tomotherapy, however, can provide improved dosimetric results in the most complex cases.

Radiotherapy treatment of early-stage prostate cancer with IMRT and protons: a treatment planning comparison

PURPOSE

To compare intensity-modulated photon radiotherapy (IMRT) with three-dimensional conformal proton therapy (3D-CPT) for early-stage prostate cancer, and explore the potential utility of intensity-modulated proton therapy (IMPT).

METHODS AND MATERIALS

Ten patients were planned with both 3D-CPT (two parallel-opposed lateral fields) and IMRT (seven equally spaced coplanar fields). Prescribed dose was 79.2 Gy (or cobalt Gray-equivalent, [CGE] for protons) to the prostate gland. Dose-volume histograms, dose conformity, and equivalent uniform dose (EUD) were compared. Additionally, plans were optimized for 3D-CPT with nonstandard beam configuration, and for IMPT assuming delivery with beam scanning.

RESULTS

At least 98% of the planning target volume received the prescription dose. IMRT plans yielded better dose conformity to the target, whereas proton plans achieved higher dose homogeneity and better sparing of rectum and bladder in the range below 30 Gy/CGE. Bladder volumes receiving more than 70 Gy/CGE (V70) were reduced, on average, by 34% with IMRT vs. 3D-CPT, whereas rectal V70 were equivalent. EUD from 3D-CPT and IMRT plans were indistinguishable within uncertainties for both bladder and rectum. With the use of small-angle lateral-oblique fields in 3D-CPT and IMPT, the rectal V70 was reduced by up to 35% compared with the standard lateral configuration, whereas the bladder V70 increased by less than 10%.

CONCLUSIONS

In the range higher than 60 Gy/CGE, IMRT achieved significantly better sparing of the bladder, whereas rectal sparing was similar with 3D-CPT and IMRT. Dose to healthy tissues in the range lower than 50% of the target prescription was substantially lower with proton therapy.

An arc-sequencing algorithm for intensity modulated arc therapy

Intensity modulated arc therapy (IMAT) is an intensity modulated radiation therapy delivery technique originally proposed as an alternative to tomotherapy. IMAT uses a series of overlapping arcs to deliver optimized intensity patterns from each beam direction. The full potential of IMAT has gone largely unrealized due in part to a lack of robust and commercially available inverse planning tools. To address this, we have implemented an IMAT arc-sequencing algorithm that translates optimized intensity maps into deliverable IMAT plans. The sequencing algorithm uses simulated annealing to simultaneously optimize the aperture shapes and weights throughout each arc. The sequencer enforces the delivery constraints while minimizing the discrepancies between the optimized and sequenced intensity maps. The performance of the algorithm has been tested for ten patient cases (3 prostate, 3 brain, 2 head-and-neck, 1 lung, and 1 pancreas). Seven coplanar IMAT plans were created using an average of 4.6 arcs and 685 monitor units. Additionally, three noncoplanar plans were created using an average of 16 arcs and 498 monitor units. The results demonstrate that the arc sequencer can provide efficient and highly conformal IMAT plans. An average sequencing time of approximately 20 min was observed.

Study on surface dose generated in prostate intensity-modulated radiation therapy treatment

The surface doses of 6- and 15-MV prostate intensity-modulated radiation therapy (IMRT) irradiations were measured and compared to those from a 15-MV prostate 4-beam box (FBB). IMRT plans (step-and-shoot technique) using 5, 7, and 9 beams with 6- and 15-MV photon beams were generated from a Pinnacle treatment planning system (version 6) using computed tomography (CT) scans from a Rando Phantom (ICRU Report 48). Metal oxide semiconductor field effect transistor detectors were used and placed on a transverse contour line along the Phantom surface at the central beam axis in the measurement. Our objectives were to investigate: (1) the contribution of the dynamic multileaf collimator (MLC) to the surface dose during the IMRT irradiation; (2) the effects of photon beam energy and number of beams used in the IMRT plan on the surface dose. The results showed that with the same number of beams used in the IMRT plan, the 6-MV irradiation gave more surface dose than that of 15 MV to the phantom. However, when the number of beams in the plan was increased, the surface dose difference between the above 2 photon energies became less. The average surface dose of the 15-MV IMRT irradiation increased with the number of beams in the plan, from 0.86% to 1.19%. Conversely, for 6 MV, the surface dose decreased from 1.33% to 1.24% as the beam number increased from 7 to 9. Comparing the 15-MV FBB and 6-MV IMRT plans with 2 Gy/fraction, the IMRT irradiations gave generally more surface dose, from 15% to 30%, depending on the number of beams in the plan. It was found that the increase in surface dose for the IMRT technique compared to the FBB plan was predominantly due to the number of beams and the calculated monitor units required to deliver the same dose at the isocenter in the plans. The head variation due to the dynamic MLC movement changing the surface dose distribution on the patient was reflected by the IMRT dose-intensity map. Although prostate IMRT in this study had an average higher surface dose than that of FBB, the more even distribution of relatively lower surface dose in IMRT field could avoid the big dose peaks at the surface positions directly under the FBB fields. Such an even and low surface dose distribution surrounding the patient in IMRT is believed to give less skin complication than that of FBB with the same prescribed dose.

Treatments of exceptionally large prostate cancer patients with low-energy intensity-modulated photons

An inverse planning technique using 6‐MV intensity‐modulated photon beams was developed for treating large‐size patients with prostate cancer. Comparisons of treatment plans using 6‐MV and 18‐MV intensity‐modulated beams were carried out for a cohort of 10 patient cases. For these cases, we analyzed the dependence of plan quality on the beam energies. We found that 6‐MV beams resulted in plans equivalent to those for 18‐MV beams both for targets and for critical structures such as the rectum and bladder. The differences between the plans in the integral dose and the mean dose to the normal tissue surrounding the target were found to be small, in contrast to those for 3D conformal plans. Our findings showed that the low entrance dose of the high‐energy photon beams is mostly compensated by the high exit dose for even exceptionally large patients. In conclusion, 6‐MV intensity‐modulated beams are a feasible choice for treating large‐size patients with prostate cancer, provided that proper inverse planning techniques are adopted.

PACS number: 87.50.Gi, 87.53.Tf

Sweeping-window arc therapy: an implementation of rotational IMRT with automatic beam-weight calculation

Sweeping-window arc therapy (SWAT) is a variation of intensity-modulated radiation therapy (IMRT) with direct aperture optimization (DAO) that is initialized with a leaf sequence of sweeping windows that move back and forth periodically across the target as the gantry rotates. This initial sequence induces modulation in the dose and is assumed to be near enough to a minimum to allow successful optimization, done with simulated annealing, without requiring excessive leaf speeds. Optimal beam weights are calculated analytically, with easy extension to allow for variable beam weights. In this paper SWAT is tested on a phantom model and clinical prostate case. For the phantom, constant and variable beam weights are used. Although further work (in particular, improving the dose model) is required, the results show SWAT to be a feasible approach to generating deliverable dynamic arc treatments that are optimized.

Simplified intensity-modulated arc therapy for dose escalated prostate cancer radiotherapy

Simplified intensity-modulated arc therapy (SIMAT) employs forward planned, conformal, and avoidance arc combinations with dynamic multileaf collimation (MLC) as a simpler alternative to other forms of intensity-modulated radiotherapy (IMRT). In this work, we compare SIMAT with 4-field (4F) and 6-field (6F) 3D conformal radiation therapy (CRT) for prostate cancer treatment. Prostate, seminal vesicle, bladder, and rectum were contoured on the CT images of 10 patients being planned for radiotherapy. Two planning target volumes (PTV) were defined: PTV1 (prostate + seminal vesicles + 1.0-cm margin) and PTV2 (prostate + 1.0-cm margin). SIMAT, 4F, and 6F plans were generated with a prescription dose of 78 Gy to prostate and 54 Gy to the seminal vesicles. Differences in the 3 techniques in terms of target and rectal coverage were compared. In addition, dose distributions of the SIMAT plans were verified with measurements in a phantom. Mean dose to PTV2 (4F, 76 Gy; 6F, 78 Gy; SIMAT, 76 Gy) and the dose delivered to 95% of the target volume (D(95)) were similar between the 3-techniques. Target conformity was better with SIMAT. Mean dose and calculated NTCP for the rectum were lower for SIMAT than those for 4F and 6F plans (4F 55.6 Gy vs. 6F 49.0 Gy vs. SIMAT 42.7 Gy). Mean dose to femoral heads was lower for the 4F technique vs. 6F and SIMAT techniques (4F 44.5 Gy vs. 6F 48.9 Gy vs. SIMAT 49.5 Gy). In-phantom measurement demonstrated good agreement between the plans and SIMAT treatments delivered in phantom. We concluded that SIMAT demonstrates advantages over 4F and 6F in terms of target conformity mean rectal dose and NTCP with good reproducibility in phantom. On the basis of this analysis, we have commenced a clinical pilot study of SIMAT for prostate cancer radiotherapy.

Whole abdominopelvic radiotherapy (WAPRT) using intensity-modulated arc therapy (IMAT): first clinical experience

PURPOSE

Whole abdominopelvic radiation therapy (WAPRT) is a treatment option in the palliation of patients with relapsed ovarian cancer. With conventional techniques, kidneys and liver are the dose- and homogeneity-limiting organs. We developed a planning strategy for intensity-modulated arc therapy (IMAT) and report on the treatment plans of the first 5 treated patients.

METHODS AND MATERIALS

Five consecutive patients with histologically proven relapsed ovarian cancer were sent to our department for WAPRT. The target volumes and organs at risk (OAR) were delineated on 0.5-cm-thick CT slices. The clinical target volume (CTV) was defined as the total peritoneal cavity. CTV and kidneys were expanded with 0.5 cm. In a preset range of 8 degrees interspaced gantry angles, machine states were generated with an anatomy-based segmentation tool. Machine states of the same class were stratified in arcs. The optimization of IMAT was done in several steps, using a biophysical objective function. These steps included weight optimization of machine states, leaf position optimization adapted to meet the maximal leaf speed constraint, and planner-interactive optimization of the start and stop angles. The final control points (machine states plus associated cumulative monitor unit counts) were calculated using a collapsed cone convolution/superposition algorithm. For comparison, two conventional plans (CONV) were made, one with two fields (CONV2), and one with four fields (CONV4). In these CONV plans, dose to the kidneys was limited by cerrobend blocks. The IMAT and the CONV plans were normalized to a median dose of 33 Gy to the planning target volume (PTV). Monomer/polymer gel dosimetry was used to assess the dosimetric accuracy of the IMAT planning and delivery method.

RESULTS

The median volume of the PTV was 8306 cc. The mean treatment delivery time over 4 patients was 13.8 min. A mean of 444 monitor units was needed for a fraction dose of 150 cGy. The fraction of the PTV volume receiving more than 90% of the prescribed dose (V(90)) was 9% higher for the IMAT plan than for the CONV4 plan (89.9% vs. 82.5%). Outside a build-up region of 0.8 cm and 1 cm away from both kidneys, the inhomogeneity in the PTV was 15.1% for the IMAT plans and 24.9% for the CONV4 plans (for CONV2 plans, this was 34.9%). The median dose to the kidneys in the IMAT plans was lower for all patients. The 95th percentile dose for the kidneys was significantly higher for the IMAT plans than for the CONV4 and CONV2 plans (28.2 Gy vs. 22.2 Gy and 22.6 Gy for left kidney, respectively). No relevant differences were found for liver. The gel-measured dose was within clinical planning constraints.

CONCLUSION

IMAT was shown to be deliverable in an acceptable time slot and to produce dose distributions that are more homogeneous than those obtained with a CONV plan, with at least equal sparing of the OARs.

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.

Inverse planning for intensity-modulated arc therapy using direct aperture optimization

Intensity-modulated arc therapy (IMAT) is a radiation therapy delivery technique that combines gantry rotation with dynamic multi-leaf collimation (MLC). With IMAT, the benefits of rotational IMRT can be realized using a conventional linear accelerator and a conventional MLC. Thus far, the advantages of IMAT have gone largely unrealized due to the lack of robust automated planning tools capable of producing efficient IMAT treatment plans. This work describes an inverse treatment planning algorithm, called 'direct aperture optimization' (DAO) that can be used to generate inverse treatment plans for IMAT. In contrast to traditional inverse planning techniques where the relative weights of a series of pencil beams are optimized, DAO optimizes the leaf positions and weights of the apertures in the plan. This technique allows any delivery constraints to be enforced during the optimization, eliminating the need for a leaf-sequencing step. It is this feature that enables DAO to easily create inverse plans for IMAT. To illustrate the feasibility of DAO applied to IMAT, several cases are presented, including a cylindrical phantom, a head and neck patient and a prostate patient.