Computer applications in radiation therapy treatment planning

A virtual revolution in computer capability has occurred in the last few years, based largely on rapidly decreasing cost and increasing reliability of digital memory and mass storage capability. These developments have now made it possible to consider the application of both computer and imaging technology to a much broader range of problems in radiation therapy including dose computation, therapy planning and treatment verification. In this paper, a review of the current status in the United States of dose computational algorithms for photon beam and electron beams and treatment planning display is presented.     

The use of intensity-modulated radiation therapy photon beams for improving the dose uniformity of electron beams shaped with MLC

Electrons are ideal for treating shallow tumors and sparing adjacent normal tissue. Conventionally, electron beams are collimated by cut-outs that are time-consuming to make and difficult to adapt to tumor shape throughout the course of treatment. We propose that electron cut-outs can be replaced using photon multileaf collimator (MLC). Two major problems of this approach are that the scattering of electrons causes penumbra widening because of a large air gap, and available commercial treatment planning systems (TPSs) do not support MLC-collimated electron beams. In this study, these difficulties were overcome by (1) modeling electron beams collimated by photon MLC for a commercial TPS, and (2) developing a technique to reduce electron beam penumbra by adding low-energy intensity-modulated radiation therapy (IMRT) photons (4 MV). We used blocks to simulate MLC shielding in the TPS. Inverse planning was used to optimize boost photon beams. This technique was applied to a parotid and a central nervous system (CNS) clinical case. Combined photon and electron plans were compared with conventional plans and verified using ion chamber, film, and a 2D diode array. Our studies showed that the beam penumbra for mixed beams with 90 cm source to surface distance (SSD) is comparable with electron applicators and cut-outs at 100 cm SSD. Our mixed-beam technique yielded more uniform dose to the planning target volume and lower doses to various organs at risk for both parotid and CNS clinical cases. The plans were verified with measurements, with more than 95% points passing the gamma criteria of 5% in dose difference and 5 mm for distance to agreement. In conclusion, the study has demonstrated the feasibility and potential advantage of using photon MLC to collimate electron beams with boost photon IMRT fields.     

Inverse radiotherapy planning for a concave-convex PTV in cervical and upper mediastinal regions

Aim

Three-dimensional inverse treatment planning with modulated beams was applied for dosimetric optimization of a lengthy (22 cm) and complex (concave-convex) shaped planning target volume (PTV) in the cervical and upper mediastinal regions.

Material and Method

The planning was done for 9 coplanar beams spaced evenly at 40° intervals. Properties of 15 MV photons from a linear accelerator were simulated. The optimization of the fluence modulation profiles for each beam was based on a definition of the desired/permitted relative dose levels in the PTV and organs at risk, and a definition of the strengths of the constraints to achieve these objectives.

Results

An adequate dose delivery to the PTV and protection of the spinal cord are completely achievable. The dose delivered to the lungs is clinically acceptable with respect to the risk of radiation-induced pneumonitis. For reasons of physics, no further decrease in the radiation burden on the lungs can be attained with X-rays without compromising the PTV coverage. The radiation burden on some critical part of normal tissues was effectively reduced by application of a dummy organ at risk.

Conclusion

The inverse planning is an effective method for conformal radiotherapy of large tumors as well. However, the power of the technique is insufficient when the tolerance dose of the neighbouring normal tissue is too low and its volume effect is high. Although requiring further operator interactions, introduction of dummy organs at risk may be of help in reducing the radiation burden on normal tissues.     

An overview of methods in stereotactic radiosurgery.

Stereotactic radiosurgery delivers a single high dose of ionizing radiation to a radiographically well-defined, small intracranial target without delivering a significant proportion of the prescribed dose to the surrounding brain tissue. Three methods of delivering the radiosurgical technique include the gamma knife, heavy charged particle beams and external high-energy photon beams from linear accelerators.     

A comparison of X-ray and proton beam low energy secondary electron track structures using the low energy models of Geant4

Abstract

Purpose: Lethal cell damage by ionising radiation is generally initiated by the formation of complex strand breaks, resulting from ionisation clusters in the DNA molecule. A better understanding of the effect of the distribution of ionisation clusters within the cell and particularly in regard to DNA segments could be beneficial to radiation therapy treatment planning. Low energy X-rays generate an abundance of low energy electrons similar to that associated with MeV protons. The study and comparison of the track structure of photon and proton beams could permit the substitution of photon microbeams for single cell ion irradiations at proton facilities used to predict the relative biological effectiveness (RBE) of charged particle fields. Materials and methods: The track structure of X-ray photons is compared with proton pencil beams in voxels of approximate DNA strand size (2 × 2 × 5 nm). The Very Low Energy extension models of the Monte Carlo simulation toolkit GEometry ANd Tracking 4 (Geant4) is used. Simulations were performed in a water phantom for an X-ray and proton beam of energies 100 keV and 20 MeV, respectively. Results: The track structure of the photon and proton beams are evaluated using the ionisation cluster size distribution as well as the radial dose deposition of the beam. Conclusions: A comparative analysis of the ionisation cluster distribution and radial dose deposition obtained is presented, which suggest that low energy X-rays could produce similar ionisation cluster distributions to MeV protons on the DNA scale of size at depths greater than ～10 μm and at distances greater than ～1 μm from the beam centre. Here the ionisation cluster size for each beam is less than ～100. The radial dose deposition is also approximately equal at large depths and at distances greater than 10 μm from the beam centre.     

Cyberknife stereotactic radiosurgery and radiation therapy treatment planning system

CyberKnife is an image-guided stereotactical dose delivery system designed for both focal irradiation and radiation therapy (SRT). Focal irradiation refers the use of many small beams to deliver highly focus dose to a small target region in a few fractions. The system consists of a 6-MV linac mounted to a robotic arm, coupled with a digital x-ray imaging system. The radiation dose is delivered using many beams oriented at a number of defined or nodal positions around the patients. The CyberKnife can be used for both intracranial and extracranial treaments unlike the Gamma Knife which is limited to intracranial cases. Multiplan (Accuray Inc., Sunnyvale, CA) is the treatment planning system developed to cooperate with this accurate and versatile SRS and SRT system, and exploit the full function of Cyberknife in high-precision radiosurgery and therapy. Optimized inverse treatment plan can be achieved by fine-tuning contours and planning parameters. Precision is the newest version of Cyberknife treatment planning system (TPS) and an upgrade to Multiplan. It offers several new features such as Monte Carlo for multileaf collimator (MLC) and retreatment for other modalities that added more support for the Cyberknife system. The Cybeknife TPS is an easy-to-use and versatile inverse planning platform, suitable for stereotactic radiosurgery and radiation therapy. The knowledge and experience of the planner in this TPS is essential to improve the quality of patient care.     

Head and neck tumors: dosimetric considerations of mixed-energy photon beam therapy

The goal of sophisticated treatment planning in radiation therapy is to maximize dose to the tumor or target volume, while the integral dose is minimized, to reduce normal tissue morbidity. In the head and neck regions, the anatomic irregularities of individual patients and the critical structures that limit the administration of dose within the irradiated volumes often complicate the optimization of dosimetry. The availability of dual-energy accelerators that deliver beams of low- and high-energy photons allows the convenient administration of mixed-energy photon irradiation and facilitates the development of optimal treatment strategies for selected lesions. Highly lateralized carcinomas of the upper aerodigestive tract, in which sparing of contralateral cervicofascial tissue is desirable, are particularly well suited for this technique. Treatment plans that make use of irradiation with conventional single-energy beams and mixed-energy photon therapy are compared for representative lesions of the head and neck.     

Intensity-modulated radiation therapy: not a dry eye in the house

Inverse planned intensity-modulated radiation therapy (IMRT) has been applied to patients in a conformal fashion in order to avoid the lacrimal gland. In the present study, we report a patient in which a potential planned dose of 63 Gy to the lacrimal gland for a conventional plan was reduced to 12 Gy to the lacrimal gland for the IMRT plan. Dose objective inverse planning was provided using a Pinnacle treatment planning computer and treatment was delivered using a Varian dynamic multileaf collimator (MLC) on a Varian linear accelerator. Because multiple MLC segments are used to deliver the modulated treatment, conventional dose checks by manual calculation are not practical. To aid in an alternative dosimetric verification process, the Pinnacle planning computer has two unique dose tools, which provide axial and beams eye view doses on user-specified check phantoms. The combined field axial dose tool matched our ion chamber dose checks within +/- 2.4% at the isocentre. The individual beams eye view dose tool matched film dose maps within +/- 3% in the umbra.     

Dosis-Volumen-Histogramme und 3D-Planung bei der brusterhaltenden Therapie des Mammakarzinoms

Aim

Improvement of the dose homogeneity in radiation treatment of the intact breast using 3D-planning and dose volume histograms.

Patients and Method

3D-planning, including the calculation of dose volume histograms of the planning target volume, was performed on 15 patients, who underwent radiation therapy with tangential photon beams. A standard plan and 2 modified or optimized plans were evaluated. Different dosimetric parameters like maximum dose, mean dose, standard deviation and the fractional volume which receives doses from 95 to 105% of the reference dose were compared and correlated with breast size.

Results

With increasing breast size standard planning leads to increased overdosage, both in magnitude and volume. Individual optimization by modifying weights and wedges gives no improvement in dose homogeneity, whereas a photon energy of 10 MV results in a more homogeneous dose distribution. The drawback of the higher energy is the increased underdosage of the skin.

Conclusion

Using the standard geomertry of tangential fields the dose homogeneity cannot be improved significantly by 3D-planning, compared to our standard technique.     

Image-assisted knowledge discovery and decision support in radiation therapy planning.

The need for quantified knowledge and decision-support tools to handle complex radiation therapy (RT) imaging and informatics data is becoming steadily apparent. Lessons can be learned from current CAD applications in radiology. This paper proposes a methodology to develop this quantified knowledge and decision-support tools to facilitate RT treatment planning. The methodology is applied to cancer patient cases treated by intensity modulated radiation therapy (IMRT). The use of the "inverse treatment planning" and imaging intensive nature of IMRT allows for the development of such image-assisted tools for supporting decision-making thus providing better workflow efficiency and more precise dose predictions.     

Leakage-Penumbra effect in intensity modulated radiation therapy step-and-shoot dose delivery

AIM: To study the leakage-penumbra (LP) effect with a proposed correction method for the step-and-shoot intensity modulated radiation therapy (IMRT). METHODS: Leakage-penumbra dose profiles from 10 randomly selected prostate IMRT plans were studied. The IMRT plans were delivered by a Varian 21 EX linear accelerator equipped with a 120-leaf multileaf collimator (MLC). For each treatment plan created by the Pinnacle³ treatment planning system, a 3-dimensional LP dose distribution generated by 5 coplanar photon beams, starting from 0^o with equal separation of 72^o, was investigated. For each photon beam used in the step-and-shoot IMRT plans, the first beam segment was set to have the largest area in the MLC leaf-sequencing, and was equal to the planning target volume (PTV). The overshoot effect (OSE) and the segment positional errors were measured using a solid water phantom with Kodak (TL and X-OMAT V) radiographic films. Film dosimetric analysis and calibration were carried out using a film scanner (Vidar VXR-16). The LP dose profiles were determined by eliminating the OSE and segment positional errors with specific individual irradiations. RESULTS: A non-uniformly distributed leaf LP dose ranging from 3% to 5% of the beam dose was measured in clinical IMRT beams. An overdose at the gap between neighboring segments, represented as dose peaks of up to 10% of the total BP, was measured. The LP effect increased the dose to the PTV and surrounding critical tissues. In addition, the effect depends on the number of beams and segments for each beam. Segment positional error was less than the maximum tolerance of 1 mm under a dose rate of 600 monitor units per minute in the treatment plans. The OSE varying with the dose rate was observed in all photon beams, and the effect increased from 1 to 1.3 Gy per treatment of the rectal intersection. As the dosimetric impacts from the LP effect and OSE may increase the rectal post-radiation effects, a correction of LP was proposed and demonstrated for the central beam profile for one of the planned beams. CONCLUSION: We concluded that the measured dosimetric impact of the LP dose inaccuracy from photon beam segment in step-and-shoot IMRT can be corrected.     

Why more needs to be known about RBE effects in modern radiotherapy

Radiation therapy remains a very effective tool in the clinical management and cure of cancer and new techniques of radiation delivery continue to be developed. Of particular note is the growing world-wide interest in particle beam therapy (PBT) using protons or light ions. Such beams (particularly light ions) are associated with an increased relative biological effectiveness (RBE) which, when viewed alongside the more favourable physical distributions of radiation dose available with all forms of particle beams, makes them especially attractive for treating tumours which are associated with disappointing outcomes following conventional X-ray therapy. Although the large body of clinical experience already gained with conventional X-ray therapy will be of paramount importance in guiding the development of treatment programmes using particle beams, understanding and quantification of the RBE effects which are unique to the latter will also be essential. This is because the magnitude of RBE effect is not fixed for any one radiation/tissue combination but is subject to a number of other radiobiological influences. Such relationships may be quantified within the linear–quadratic radiobiological model, within which the associated concept of biologically effective dose (BED) provides a way of inter-comparing the overall biological impact of existing and projected treatments. This paper summarises the main features of RBE and BED, discusses the main quantitative implications for PBT and highlights why clear understanding of RBE effects will be essential to make best use of PBT. It also summarises other clinical applications where knowledge of and allowance for RBE effects is important and suggests that more needs to be done to allow safer practical applications.     

Dose verification for patients undergoing IMRT.

At Emory Clinic intensity-modulated radiation therapy (IMRT) was started by using dynamic multileaf collimators (dMLC) as electronic tissue compensators in August 1998. Our IMRT program evolved with the inclusion of a commercially available inverse treatment planning system in September 1999. While the introduction of electronic tissue compensators into clinical use did not affect the customary radiation oncology practice, inverse treatment planning does alter our basic routines. Basic concepts of radiation therapy port designs for inverse treatment planning are different from conventional or 3D conformal treatments. With inverse treatment planning, clinicians are required to outline a gross tumor volume (GTV), a clinical target volume (CTV), critical normal structures, and to design a planning target volume (PTV). Clinicians do not designate the volume to be shielded. Because each IMRT radiation portal is composed of many beamlets with varying intensities, methods and practice used to verify delivered dose from IMRT portals are also different from conventional treatment portals. Often, the validity of measured data is in doubt. Therefore, checking treatment planning computer output with measurements are confusing and fruitless, at times. Commissioning an IMRT program and routine patient dose verification of IMRT require films and ionization chamber measurements in phantom. Additional specialized physics instrumentation is not required other than those available in a typical radiation oncology facility. At this time, we consider that routine quality assurance prior to patient treatments is necessary.     

Current status and future prospects of multi-dimensional image-guided particle therapy

In this review article we emphasize the importance of “imaging” and “image guidance” in advanced particle therapy from a clinical point of view. Although the image-guided radiotherapy (IGRT) technology used for photon and particle therapy is closely similar, the focus of its application in the two modalities differs. Here, we emphasize the challenges of IGRT for charged particle beams. Radiological Physics and Technology (RTPE) readers interested in the common technologies of IGRT for photons and protons are referred to the plentiful articles already published. Our present deeper insight into IGRT arose through the collaboration of two particle therapy centers, the National Institute of Radiological Sciences (NIRS) in Japan and the Paul Scherrer Institute (PSI) in Switzerland. We believe that international collaboration in rapidly developing fields such as IGRT provides a broad perspective over a wide range of the uses of such technology.     

3D reconstructions in radiotherapy planning

3D Reconstructions from tomographic images are used in the planning of radiation therapy to study important anatomical structures such as the body surface, target volumes, and organs at risk. The reconstructed anatomical models are used to define the geometry of the radiation beams. In addition, 3D voxel models are used for the calculation of the 3D dose distributions with an accuracy, previously impossible to achieve. Further uses of 3D reconstructions are in the display and evaluation of 3D therapy plans, and in the transfer of treatment planning parameters to the irradiation situation with the help of digitally reconstructed radiographs. 3D tomographic imaging with subsequent 3D reconstruction must be regarded as a completely new basis for the planning of radiation therapy, enabling tumor-tailored radiation therapy of localized target volumes with increased radiation doses and improved sparing of organs at risk. 3D treatment planning is currently being evaluated in clinical trials in connection with the new treatment techniques of conformation radiotherapy. Early experience with 3D treatment planning shows that its clinical importance in radiotherapy is growing, but will only become a standard radiotherapy tool when volumetric CT scanning, reliable and user-friendly treatment planning software, and faster and cheaper PACS-integrated medical work stations are accessible to radiotherapists.     

The physical basis of IMRT and inverse planning

Intensity-modulated radiation therapy (IMRT) can sculpt the high-dose volume around the site of disease with hitherto unachievable precision. Conformal avoidance of normal tissues goes hand in hand with this. Inhomogeneous dose painting is possible. The technique has become a clinical reality and is likely to be the dominant approach this decade for improving the clinical practice of photon therapy. This Series will explore all aspects of the "IMRT chain". Only 15 years ago just a handful of physicists were working on this subject. IMRT has developed so rapidly that its recent past is also its ancient history. This article will review the history of IMRT with just a glance at precursors. The physical basis of IMRT is then described including an attempt to introduce the concepts of convex and concave dose distributions, ill-conditioning, inverse-problem degeneracy, cost functions and complex solutions all with a minimum of technical jargon or mathematics. The many techniques for inverse planning are described and the review concludes with a look forward to the future of image-guided IMRT (IG-IMRT).