3D conformal intensity-modulated radiotherapy planning: interactive optimization by constrained matrix inversion

Fast IMRT by increasing the beam number and reducing the number of segments

PURPOSE

The purpose of this work is to develop fast deliverable step and shoot IMRT technique. A reduction in the number of segments should theoretically be possible, whilst simultaneously maintaining plan quality, provided that the reduction is accompanied by an increased number of gantry angles. A benefit of this method is that the segment shaping could be performed during gantry motion, thereby reducing the delivery time. The aim was to find classes of such solutions whose plan quality can compete with conventional IMRT.

MATERIALS/METHODS

A planning study was performed. Step and shoot IMRT plans were created using direct machine parameter optimization (DMPO) as a reference. DMPO plans were compared to an IMRT variant having only one segment per angle ("2-Step Fast"). 2-Step Fast is based on a geometrical analysis of the topology of the planning target volume (PTV) and the organs at risk (OAR). A prostate/rectum case, spine metastasis/spinal cord, breast/lung and an artificial PTV/OAR combination of the ESTRO-Quasimodo phantom were used for the study. The composite objective value (COV), a quality score, and plan delivery time were compared. The delivery time for the DMPO reference plan and the 2-Step Fast IMRT technique was measured and calculated for two different linacs, a twelve year old Siemens Primus™ ("old" linac) and two Elekta Synergy™ "S" linacs ("new" linacs).

RESULTS

2-Step Fast had comparable or better quality than the reference DMPO plan. The number of segments was smaller than for the reference plan, the number of gantry angles was between 23 and 34. For the modern linac the delivery time was always smaller than that for the reference plan. The calculated (measured) values showed a mean delivery time reduction of 21% (21%) for the new linac, and of 7% (3%) for the old linac compared to the respective DMPO reference plans. For the old linac, the data handling time per beam was the limiting factor for the treatment time reduction.

CONCLUSIONS

2-Step Fast plans are suited to reduce the delivery time, especially if the data handling time per beam is short. The plan quality can be retained or even increased for fewer segments provided more gantry angles are used.

2-Step IMAT and 2-Step IMRT in three dimensions

In two dimensions, 2-Step Intensity Modulated Arc Therapy (2-Step IMAT) and 2-Step Intensity Modulated Radiation Therapy (IMRT) were shown to be powerful methods for the optimization of plans with organs at risk (OAR) (partially) surrounded by a target volume (PTV). In three dimensions, some additional boundary conditions have to be considered to establish 2-Step IMAT as an optimization method. A further aim was to create rules for ad hoc adaptations of an IMRT plan to a daily changing PTV-OAR constellation. As a test model, a cylindrically symmetric PTV-OAR combination was used. The centrally placed OAR can adapt arbitrary diameters with different gap widths toward the PTV. Along the rotation axis the OAR diameter can vary, the OAR can even vanish at some axis positions, leaving a circular PTV. The width and weight of the second segment were the free parameters to optimize. The objective function f to minimize was the root of the integral of the squared difference of the dose in the target volume and a reference dose. For the problem, two local minima exist. Therefore, as a secondary criteria, the magnitude of hot and cold spots were taken into account. As a result, the solution with a larger segment width was recommended. From plane to plane for varying radii of PTV and OAR and for different gaps between them, different sets of weights and widths were optimal. Because only one weight for one segment shall be used for all planes (respectively leaf pairs), a strategy for complex three-dimensional (3-D) cases was established to choose a global weight. In a second step, a suitable segment width was chosen, minimizing f for this global weight. The concept was demonstrated in a planning study for a cylindrically symmetric example with a large range of different radii of an OAR along the patient axis. The method is discussed for some classes of tumor/organ at risk combinations. Noncylindrically symmetric cases were treated exemplarily. The product of width and weight of the additional segment as well as the integral across the segment profile was demonstrated to be an important value. This product was up to a factor of 3 larger than in the 2-D case. Even in three dimensions, the optimized 2-Step IMAT increased the homogeneity of the dose distribution in the PTV profoundly. Rules for adaptation to varying target-OAR combinations were deduced. It can be concluded that 2-Step IMAT and 2-Step IMRT are also applicable in three dimensions. In the majority of cases, weights between 0.5 and 2 will occur for the additional segment. The width-weight product of the second segment is always smaller than the normalized radius of the OAR. The width-weight product of the additional segment is strictly connected to the relevant diameter of the organ at risk and the target volume. The derived formulas can be helpful to adapt an IMRT plan to altering target shapes.

Simultaneous beam geometry and intensity map optimization in intensity-modulated radiation therapy

PURPOSE

In current intensity-modulated radiation therapy (IMRT) plan optimization, the focus is on either finding optimal beam angles (or other beam delivery parameters such as field segments, couch angles, gantry angles) or optimal beam intensities. In this article we offer a mixed integer programming (MIP) approach for simultaneously determining an optimal intensity map and optimal beam angles for IMRT delivery. Using this approach, we pursue an experimental study designed to (a) gauge differences in plan quality metrics with respect to different tumor sites and different MIP treatment planning models, and (b) test the concept of critical-normal-tissue-ring--a tissue ring of 5 mm thickness drawn around the planning target volume (PTV)--and its use for designing conformal plans.

METHODS AND MATERIALS

Our treatment planning models use two classes of decision variables to capture the beam configuration and intensities simultaneously. Binary (0/1) variables are used to capture "on" or "off" or "yes" or "no" decisions for each field, and nonnegative continuous variables are used to represent intensities of beamlets. Binary and continuous variables are also used for each voxel to capture dose level and dose deviation from target bounds. Treatment planning models were designed to explicitly incorporate the following planning constraints: (a) upper/lower/mean dose-based constraints, (b) dose-volume and equivalent-uniform-dose (EUD) constraints for critical structures, (c) homogeneity constraints (underdose/overdose) for PTV, (d) coverage constraints for PTV, and (e) maximum number of beams allowed. Within this constrained solution space, five optimization strategies involving clinical objectives were analyzed: optimize total intensity to PTV, optimize total intensity and then optimize conformity, optimize total intensity and then optimize homogeneity, minimize total dose to critical structures, minimize total dose to critical structures and optimize conformity simultaneously. We emphasize that the objectives that include optimizing conformity make use of the critical-normal-tissue-ring. Three tumor sites: head-and-neck, pediatric brain, and prostate are used for comparison.

RESULTS

The critical-normal-tissue-ring acts as a good device for enforcing conformity. Trends in the characteristics and quality of plans resulting from each model were observed. Attempts to reduce dose to critical structures tend to worsen PTV conformity (1.542 to 3.092) and homogeneity (1.223 to 1.984), depending on the relative size and spatial distance of the critical structures to the PTV. When the critical structures are relatively small compared with the PTV (as in the case for the pediatric brain tumor, where each is less than 15% in volume), dose reduction to critical structures is accompanied by much worse scores in conformity (2.482) and homogeneity (1.984). When the critical structures are larger, as in the case of head-and-neck (approximately 50%), the conformity and homogeneity deterioration is less significant (1.542 and 1.239, respectively). There is a clear tradeoff between homogeneity, conformity, and minimum dose to organs at risk (OARs). For head-and-neck and pediatric brain tumor, the model that minimizes total dose to critical structures and optimizes conformity simultaneously offers a compromise among these factors, resulting in reduced critical structure dose with conformal and homogeneous plans. In the prostate case, the tumor is smaller than the two large nearby critical structures, and all models provide very homogeneous PTV dose distribution. However, minimizing dose to critical structures worsens conformity, as it spreads the radiation to the area surrounding the PTV. The maximum dose to the critical structures also increases slightly. Compared with plans used in the clinic which generally have uniformly spaced beam angles, the optimal clinically acceptable plans obtained via the methods herein do not have equispaced beams. The optimal beam angles returned appear to be nonintuitive, and depend on PTV size and geometry and the spatial relationship between the tumor and critical structures.

CONCLUSIONS

The MIP model described allows simultaneous optimization over the space of beamlet fluence weights and beam and couch angles. Based on experiments with tumor data, this approach can return good plans that are clinically acceptable and practical. This work distinguishes itself from recent IMRT research in several ways. First, in previous methods beam angles are selected before intensity map optimization. Herein, we employ 0/1 variables to model the set of candidate beams, and thereby allow the optimization process itself to select optimal beams. Second, instead of incorporating dose-volume criteria within the objective function as in previous work, herein, a combination of discrete and continuous variables associated with each voxel provides a mechanism to strictly enforce dose-volume criteria within the constraints. Third, using the construct of critical-normal-tissue-ring within the objective function can enhance the achievement of conformal plans. Based on the three tumor sites considered, it appears that volume and spatial geometry with respect to the PTV are important factors to consider when selecting objectives to optimize, and in estimating how well suited a particular model is for achieving a specified goal.

2-Step IMAT and 2-Step IMRT: A geometrical approach

The purpose of this paper is to develop a method that reduces the number of segments for intensity modulated arc therapy (IMAT) and intensity modulated radiotherapy (IMRT) for concave target volumes (TV). The aim was to utilize no more than two intensity levels per organ at risk (OAR) and to derive both optimal segment widths and weights from geometric considerations. Brahme's model of an annular target surrounding a circular OAR was used as test model. Brahme's solution was substituted by a single segment added to a simple field blocking the OAR. Width and weight of the segment were the free parameters to optimize. The objective function to minimize was the root mean square (rms) error of the dose in the target volume. One boundary condition was--neglecting scatter--"zero-dose" to the OAR. The resulting rules for width and weight of the additive segment are referred to as "optimized 2-Step IMAT" and "2-Step IMRT." The recommendations were applied to some simplified plans representing clinical cases using a commercial planning system. Optimized 2-Step IMAT improved the rms by a factor of 4 with respect to techniques simply blocking the OAR. The additional segment reduced the rms below 3% for cases with gaps between OAR and TV larger than 8% of the TV diameter. The results for 2-Step IMAT are applicable to IMRT and aperture modulated arc therapy (AMAT). 2-Step IMAT can be utilized for noncylindrical cases and for more than one OAR. A geometrical and topological approach to IMAT and IMRT can be useful to understand fluence profiles. The results could be applied to ameliorate other topology-based procedures used in some planning systems. Basic mechanisms of 2-Step IMAT can assist with the creation of rules for adaptive IMRT to compensate for patient motion.

Clinical implementation of intensity-modulated arc therapy (IMAT) for rectal cancer

PURPOSE

In rectal cancer, combined radiotherapy and chemotherapy, either pre- or postoperatively, is an accepted treatment. Late small bowel (SB) toxicity is a feared side effect and limits radiation-dose escalation in a volume-dependent way. A planning strategy for intensity- modulated arc therapy (IMAT) was developed, and IMAT was clinically implemented with the aim to reduce the volume of SB irradiated at high doses and thus reduce SB toxicity. We report on the treatment plans of the first 7 patients, on the comparison of IMAT with conventional 3D planning (3D), and on the feasibility of IMAT delivery.

METHODS AND MATERIALS

Seven patients, who were referred to our department for preoperative (n = 4) or postoperative (n = 3) radiotherapy for rectal cancer, gave written consent for IMAT treatment. All patients had a planning CT in prone position. The delineation of the clinical target volume was done after fusion of CT and MRI, with the help of a radiologist. For the IMAT plan, arcs were generated using an anatomy-based segmentation tool. The optimization of the arcs was done by weight optimization (WO) and leaf position optimization (LPO), both of which were adapted for IMAT purposes. The 3D plans used one posterior and two lateral wedged beams, of which the outlines were shaped to the beam's-eye view projection of the planning target volume (PTV). Beam WO was done by constrained matrix inversion. For dose-volume histogram analysis, all plans were normalized to 45 Gy as median PTV dose. Polymer gel dosimetry (PGD) on a humanoid phantom was used for the validation of the total chain (planning to delivery). IMAT treatments were delivered by an Elekta SliPlus linear accelerator using prototype software with the same interlock class as in clinical mode.

RESULTS

The IMAT plan resulted in 3 to 6 arcs, with a mean delivery time of 6.3 min and a mean of 456 monitor units (MU) for a 180 cGy fraction. The minimal dose in the PTV was not significantly different between 3D and IMAT plans. Inhomogeneity was highest for the IMAT plans (14.1%) and lowest for the 3D plans (9.9%). Mean dose to the SB was significantly lower for the IMAT plans (12.4 Gy) than for the 3D plans (17.0 Gy). The volume of SB receiving less than any dose level was lower for the IMAT plans than for 3D plans. Integral dose was lower in the IMAT plans than for the 3D plans (respectively 244 J and 262 J to deliver 45 Gy). Differences between the PGD measured dose and the calculated dose were as small for IMAT as for 3D treatments.

CONCLUSION

IMAT plans are deliverable within a 5-10-minute time slot, and result in a lower dose to the SB than 3D plans, without creating significant underdosages in the PTV. PGD showed that IMAT delivery is as accurate as 3D delivery.

Therapeutic treatment plan optimization with probability density-based dose prescription

The dose optimization in inverse planning is realized under the guidance of an objective function. The prescription doses in a conventional approach are usually rigid values, defining in most instances an ill-conditioned optimization problem. In this work, we propose a more general dose optimization scheme based on a statistical formalism [Xing et al., Med. Phys. 21, 2348-2358 (1999)]. Instead of a rigid dose, the prescription to a structure is specified by a preference function, which describes the user's preference over other doses in case the most desired dose is not attainable. The variation range of the prescription dose and the shape of the preference function are predesigned by the user based on prior clinical experience. Consequently, during the iterative optimization process, the prescription dose is allowed to deviate, with a certain preference level, from the most desired dose. By not restricting the prescription dose to a fixed value, the optimization problem becomes less ill-defined. The conventional inverse planning algorithm represents a special case of the new formalism. An iterative dose optimization algorithm is used to optimize the system. The performance of the proposed technique is systematically studied using a hypothetical C-shaped tumor with an abutting circular critical structure and a prostate case. It is shown that the final dose distribution can be manipulated flexibly by tuning the shape of the preference function and that using a preference function can lead to optimized dose distributions in accordance with the planner's specification. The proposed framework offers an effective mechanism to formalize the planner's priorities over different possible clinical scenarios and incorporate them into dose optimization. The enhanced control over the final plan may greatly facilitate the IMRT treatment planning process.

Pre-clinical evaluation of an inverse planning module for segmental MLC based IMRT delivery

Phantom tests are performed for pre-clinical evaluation of a commercial inverse planning system (HELAX TMS, V 6.0) for segmented multileaf collimator (MLC) intensity modulated radiotherapy (IMRT) delivery. The optimization module has available two optimization algorithms: the target primary feasibility and the weighted feasibility algorithm, only the latter allows the user to specify weights for structures. In the first series, single beam tests are performed to evaluate the outcome of inverse planning in terms of plausibility for the following situations: oblique incidence, presence of inhomogeneities, multiple targets at different depths and multiple targets with different desired doses. Additionally, for these tests a manual plan is made for comparison. In the absence of organs at risk, both the optimization algorithms are found to assign the highest priority to low dose constraints for targets. In the second series, tests resembling clinical relevant configurations (simultaneous boost and concave target with critical organ) are performed with multiple beam arrangements in order to determine the impact of the system's configuration on inverse planning. It is found that the definition of certain segment number and segment size limitations does not largely compromise treatment plans when using multiple beams. On the other hand, these limitations are important for delivery efficiency and dosimetry. For the number of iterations and voxels per volume of interest, standard values in the system's configuration are considered to be sufficient. Additionally, it is demonstrated that precautions must be taken to precisely define treatment goals when using computerized treatment optimization. Similar phantom tests could be used for a direct dosimetric verification of all steps from inverse treatment planning to IMRT delivery.

Intensity-modulated radiation therapy (IMRT): the radiation oncologist's perspective

Intensity-modulated radiation therapy (IMRT) is a new and evolving technological advance in high-precision radiation therapy. It is an extension of 3-dimensional conformal radiotherapy (3D-CRT) that allows the delivery of highly complex isodose profiles to the target while minimizing radiation exposure to surrounding normal tissues. Clinical data on IMRT are emerging and being collected, as more institutions are implementing or expanding the use of IMRT. However, the currently available IMRT and its applications are far from being well understood and established. In some circumstances, it remains impractical and too costly. This article discusses some practical issues from the radiation oncologist's perspective.

Node-positive left-sided breast cancer patients after breast-conserving surgery: potential outcomes of radiotherapy modalities and techniques

PURPOSE

To determine how much proton and intensity modulated photon radiotherapy (IMRT) can improve treatment results of node-positive left-sided breast cancer compared to conventional radiation qualities (X-rays and electrons) after breast-conserving surgery in terms of lower complication risks for cardiac mortality and radiation pneumonitis.

METHODS AND MATERIAL

For each of 11 patient studies, one proton plan, one IMRT, and two conventional (tangential and patched) plans were calculated using a three-dimensional treatment-planning system, Helax-TMS(). The evaluation of the different treatment plans was made by applying the normal tissue complication probability model (NTCP) proposed by Källman (also denoted the relative seriality model) on the dose distributions in terms of dose-volume histograms. The organs at risk are the spinal cord, the left lung, the heart, and the non-critical normal tissues (including the right breast).

RESULTS

The comparison demonstrated that the proton treatment plans provide significantly lower NTCP values for the heart and lung when compared to conventional radiation qualities including IMRT for all 11 patients. At a prescribed dose of 50 Gy in the PTV, the calculated mean NTCP value for the patients decreased, on the average, from 14.7 to 0.6% for the lung (radiation pneumonitis) for the proton plans compared with the best plan using conventional radiation qualities. The corresponding figures for the heart (cardiac mortality) were from 2.1 to 0.5%. The figures for cardiac mortality for IMRT, tangential technique and the patched technique were 2.2, 6.7, and 2.1%, respectively.

CONCLUSIONS

Protons appear to have major advantages in terms of lower complication risks when compared with treatments using conventional radiation qualities for treating node-positive left-sided breast cancer after breast-conserving surgery.

Examination of the effect of increasing the number of radiation beams on a radiation treatment plan

Within the confines of least-squares operations, it is possible to quantify the effect of the addition of treatment fields or beamlets to a treatment plan. Using linear algebra and eigenvalue perturbation theory, the effect of the increase in number of treatments is shown to be equivalent to adding a perturbation operator. The effect of adding additional fields will be negligible if the perturbation operator is small. The correspondence of this approach to an earlier work in beam-orientation optimization is also demonstrated. Results are presented for prostate, spinal and head and neck cases, and the connection to beam-orientation optimization is examined.

Intensity-modulated radiotherapy: current status and issues of interest

PURPOSE

To develop and disseminate a report aimed primarily at practicing radiation oncology physicians and medical physicists that describes the current state-of-the-art of intensity-modulated radiotherapy (IMRT). Those areas needing further research and development are identified by category and recommendations are given, which should also be of interest to IMRT equipment manufacturers and research funding agencies.

METHODS AND MATERIALS

The National Cancer Institute formed a Collaborative Working Group of experts in IMRT to develop consensus guidelines and recommendations for implementation of IMRT and for further research through a critical analysis of the published data supplemented by clinical experience. A glossary of the words and phrases currently used in IMRT is given in the. Recommendations for new terminology are given where clarification is needed.

RESULTS

IMRT, an advanced form of external beam irradiation, is a type of three-dimensional conformal radiotherapy (3D-CRT). It represents one of the most important technical advances in RT since the advent of the medical linear accelerator. 3D-CRT/IMRT is not just an add-on to the current radiation oncology process; it represents a radical change in practice, particularly for the radiation oncologist. For example, 3D-CRT/IMRT requires the use of 3D treatment planning capabilities, such as defining target volumes and organs at risk in three dimensions by drawing contours on cross-sectional images (i.e., CT, MRI) on a slice-by-slice basis as opposed to drawing beam portals on a simulator radiograph. In addition, IMRT requires that the physician clearly and quantitatively define the treatment objectives. Currently, most IMRT approaches will increase the time and effort required by physicians, medical physicists, dosimetrists, and radiation therapists, because IMRT planning and delivery systems are not yet robust enough to provide totally automated solutions for all disease sites. Considerable research is needed to model the clinical outcomes to allow truly automated solutions. Current IMRT delivery systems are essentially first-generation systems, and no single method stands out as the ultimate technique. The instrumentation and methods used for IMRT quality assurance procedures and testing are not yet well established. In addition, many fundamental questions regarding IMRT are still unanswered. For example, the radiobiologic consequences of altered time-dose fractionation are not completely understood. Also, because there may be a much greater ability to trade off dose heterogeneity in the target vs. avoidance of normal critical structures with IMRT compared with traditional RT techniques, conventional radiation oncology planning principles are challenged. All in all, this new process of planning and treatment delivery has significant potential for improving the therapeutic ratio and reducing toxicity. Also, although inefficient currently, it is expected that IMRT, when fully developed, will improve the overall efficiency with which external beam RT can be planned and delivered, and thus will potentially lower costs.

CONCLUSION

Recommendations in the areas pertinent to IMRT, including dose-calculation algorithms, acceptance testing, commissioning and quality assurance, facility planning and radiation safety, and target volume and dose specification, are presented. Several of the areas in which future research and development are needed are also indicated. These broad recommendations are intended to be both technical and advisory in nature, but the ultimate responsibility for clinical decisions pertaining to the implementation and use of IMRT rests with the radiation oncologist and radiation oncology physicist. This is an evolving field, and modifications of these recommendations are expected as new technology and data become available.

An anatomy-based beam segmentation tool for intensity-modulated radiation therapy and its application to head-and-neck cancer

PURPOSE

In segmental intensity-modulated radiation therapy (IMRT), the beam fluences result from superposition of unmodulated beamlets (segments). In the inverse planning approach, segments are a result of ''clipping'' intensity maps. At Ghent University Hospital, segments are created by an anatomy-based segmentation tool (ABST). The objective of this report is to describe ABST.

METHODS AND MATERIALS

For each beam direction, ABST generates segments by a multistep procedure. During the initial steps, beam's eye view (BEV) projections of the planning target volumes (PTVs) and organs at risk (OARs) are generated. These projections are used to make a segmentation grid with negative values across the expanded OAR projections and positive values elsewhere inside the expanded PTV projections. Outside these regions, grid values are set to zero. Subsequent steps transform the positive values of the segmentation grid to increase with decreasing distance to the OAR projections and to increase with longer pathlengths measured along rays from their entrance point through the skin contours to their respective grid point. The final steps involve selection of iso-value lines of the segmentation grid as segment outlines which are transformed to leaf and jaw positions of a multileaf collimator (MLC). Segment shape approximations, if imposed by MLC constraints, are done in a way that minimizes overlap between the expanded OAR projections and the segment aperture.

RESULTS

The ABST procedure takes about 3 s/segment on a Compaq Alpha XP900 workstation. In IMRT planning problems with little complexity, such as laryngeal (example shown) or thyroid cancer, plans that are in accordance with the clinical protocol can be generated by weighting the segments generated by ABST without further optimization of their shapes. For complex IMRT plans such as paranasal sinus cancer (not shown), ABST generates a start assembly of segments from which the shapes and weights are further optimized.

CONCLUSIONS

ABST is a fast procedure to generate a set of segments for IMRT planning. The plan is finalized by assigning weights to the segments or by direct optimization of segment shapes and weights. ABST allows us to avoid the step of translating optimized intensity maps to sequences of segments.

An implementation strategy for IMRT of ethmoid sinus cancer with bilateral sparing of the optic pathways

PURPOSE

To develop a protocol for the irradiation of ethmoid sinus cancer, with the aim of sparing binocular vision; of developing a strategy of intensity-modulated radiation therapy (IMRT) planning that produces dose distributions that (1) are consistent with the protocol prescriptions and (2) are deliverable by static segmental IMRT techniques within a 15-minute time slot; of fine tuning the implementation strategy to a class solution approach that is sufficiently automated and efficient, allowing routine clinical application; of reporting on the early clinical implementation involving 11 patients between February 1999 and July 2000. patients and methods: Eleven consecutive T1-4N0M0 ethmoid sinus cancer patients were enrolled in the study. For Patients 1-8, a first protocol was implemented, defining a planning target volume prescription dose of 60 to 66 Gy in 30-33 fractions and a maximum dose (Dmax) of 50 Gy to optic pathway structures and spinal cord and limit of 60 Gy to brainstem. For Patients 9-11, an adapted (now considered mature) protocol was implemented, defining a (planning target volume) prescription dose of 70 Gy in 35 fractions and a Dmax to optic pathway structures and brainstem of 60 Gy and to spinal cord of 50 Gy.

RESULTS

The class solution-directed strategy developed during this study reduced the protocol translation process from a few days to about 2 hours of planner time. The mature class solution involved the use of 7 beam incidences (20-37 segments), which could be delivered within a 15-minute time slot. Acute side effects were limited and mild. None of the patients developed dry eye syndrome or other visual disturbances. The follow-up period is too short for detection of retinopathy or optic nerve and chiasm toxicity.

CONCLUSION

Conventional radiotherapy of ethmoid sinus tumors is associated with serious morbidity, including blindness. We hypothesize that IMRT has the potential to save binocular vision. The dose to the optic pathway structures can be reduced selectively by IMRT. Further enrollment of patients and longer follow-up will show whether the level of reduction tested by the clinical protocol is sufficient to save binocular vision. An adaptive strategy of IMRT planning was too inefficient for routine clinical practice. A class solution-directed strategy improved efficiency by eliminating human trial and error during the IMRT planning process.

Radiation techniques for head and neck tumors

PURPOSE

A technique that combines some advantages of conforming techniques for advanced oro- and hypopharyngeal carcinomas is proposed. The aim is to increase the dose homogeneity in the target volume relative to lateral opposed fields.

METHODS AND MATERIALS

This publication compares conforming radiation techniques based on standard equipment, standard linear accelerator setup and commercially available planning software with lateral opposed fields. More advanced conformal techniques reported in the literature are taken into account in a semi-quantitative manner. Our standard method uses an arc rotation, sparing the spinal cord. In contrast to earlier methods of this type, the resulting dose deficit in the vicinity of the spine is compensated by static lateral wedged fields. Dose distributions for 25 consecutive patients were planned.

RESULTS

The conforming techniques were found to produce more homogeneous dose distributions than lateral opposed fields. In the planning target volume (PTV) (mean: 940 cm(3)) a standard deviation of dose of 4.6% was achieved. Ninety-five percent of the PTV were enclosed by the 90% isodose. The maximal spinal cord dose was limited to 45 Gy. The dose distributions of these techniques could compete with literature data on advanced techniques (the published dose-volume histogram (DVHs) of PTVs were evaluated). At the linear accelerator time for realization took 14 min on average. The planning time is 1-4 h (mean: less than 2 h).

CONCLUSION

A rotational technique applicable with standard equipment is presented. Dose coverage of target volumes is improved, while the spinal cord is spared.

Radiotherapy: the last 25 years

Some of the major changes in radiotherapy over the last years are reviewed in this paper. Radiotherapy has played a role in the changes in oncological practice including an increase in organ-sparing treatment and achieving good local control and improving survival. About half of all breast cancer patients are now treated with breast conserving therapy. Organ preservation, usually with multimodality therapy, has also been further developed in the treatment of cancers in the head and neck, anus, bladder and soft tissue sarcomas. Developments in radiobiology have led to the development of new fractionation schedules. Hyperfractionation allows an increase in the tumour dose whilst sparing normal tissues and accelerated fractionation combats accelerated tumour proliferation during treatment. Advances in accelerator technology and computerized treatment planning have enabled the development of three-dimensional conformal radiotherapy. This gives the oportunity to spare normal tissues and escalate the dose to the tumour. Quality control and standardization of dosimetry and treatment delivery at departmental and international level has also improved treatment results.

[Intensity modulated radiotherapy with dynamic multileaf collimator. Technique and clinical experience]

Since early 1997, dynamic multileaf collimators (DMLCs) have been used in our division for intensity modulated radiotherapy (IMRT). We have used IMRT to: irradiate concave targets (head and neck, paraspinal tumors); combine beams with shallow hinge angles (mediastinum, lung tumors); deliver intentionally inhomogeneous dose distributions (prostate, paranasal sinuses, brain tumors). IMRT is now our standard treatment for locoregional relapse (after high-dose radiotherapy) for head and neck cancer and for radical treatment of localized prostate cancer. For a variety of other tumors, conventional 3D-plans are compared with IMRT-plans, the latter being clinically implemented if superior. We developed a geometry based IMRT planning strategy to create assemblies of static intensity modulated (IM)-beams which consist of uniform (unmodulated) segments. By a translator program, segments are combined in a single prescription which allows delivery under computer control. Cost-containment is further improved by automation of the planning. After manual or semi-automated contouring of PTV and the organs at risk, prostate IMRT plans, based on a class solution, are generated and optimized by a computer. IMRT for pharyngeal relapses and most other tumor sites is planned semi-automatically. IMRT replaces gradually conventional treatments in our division. Interesting dose distributions generated by IMRT allow a better sparing of normal tissues with decreased acute and late toxicity, and offer a window for further dose escalation.

Quality of life in patients cured from a carcinoma of the head and neck by radiotherapy: the importance of the target volume

PURPOSE

To assess the health-related quality of life (QOL) of long-term survivors of carcinomas of different subsites of the head and neck following curative radiotherapy (RT).

PATIENTS AND METHODS

Patients continuously free from recurrence or second primary tumors treated 1988-1994 were contacted 5.1 to 5.9 years after RT and asked to fill in the EORTC QLQ-C30 core questionnaire and the H&N cancer module. RT had been restricted to the glottis (group A; carcinomas of the vocal cord T1-2 N0), or had included bilateral neck nodes and the primary tumor outside the nasopharynx (group B; AJC Stage II to IV) or within the nasopharynx, respectively (group C; Stage II to IV). Response rate was 97% (group A; n = 41), 69% (group B; n = 26) and 71% (group C; n = 12), respectively. The groups were different with respect to age (older in group A), alcohol consumption (absent in group C) and proportion of females (more in group C).

RESULTS

Patients with nasopharyngeal cancer reported the highest morbidity on the H&N module (dry mouth, sticky saliva, trismus, problems with teeth, trouble eating). However, these symptoms did not have a high impact on global QOL or function scores on the QLQ-C30 core questionnaire. Patients in group B reported a lower global QOL but less severe symptoms on the module.

CONCLUSION

The high morbidity of patients treated for a nasopharyngeal cancer may be explained by the location of the target volume which included the bilateral temporo-mandibular joints and the salivary glands. These patients require appropriate care during follow-up and will probably profit most from new RT techniques with sparing of normal tissues.

An objective function for radiation treatment optimization based on local biological measures

The implementation of biological optimization of radiation treatment plans is impeded by both computational and modelling problems. We derive an objective function from basic model assumptions which includes the normal tissue constraints as interior penalty functions. For organs that are composed of parallel subunits, a mean response model is proposed which leads to constraints similar to dose-volume constraints. This objective function is convex in the case when no parallel organs lie in the treatment volume. Otherwise, an argument is given to show that a number of local minima may exist which are near degenerate to the global minimum. Thus, together with the measure quality of the objective function, highly efficient gradient algorithms can be used. The number of essential biological model parameters could be reduced to a minimum. However, if the optimization constraints are given as TCP/NTCP values, Lagrange multiplier updates have to be performed by invoking comprehensive biological models.