Impact of complexity and computer control on errors in radiation therapy

A number of recent publications in both the lay and scientific press have described major errors in patient radiation treatments, and this publicity has galvanised much work to address and mitigate potential safety issues throughout the radiation therapy planning and delivery process. The complexity of modern radiotherapy techniques and equipment, including computer-controlled treatment machines and treatment management systems, as well as sophisticated treatment techniques that involve intensity-modulated radiation therapy, image-guided radiation therapy, stereotactic body radiation therapy, volumetric modulated arc therapy, respiratory gating, and others, leads to concern about safety issues related to that complexity. This article illustrates the relationship between complexity and computer control, and various safety problems and errors that have been reported, and describes studies that address the issue of these modern techniques and whether their complexity does, in fact, result in more errors or safety-related problems. Clinical implications of these results are discussed, as are some of the ways in which the field should respond to the ongoing concerns about errors and complexity in radiation therapy.

Sensitivity analysis for lexicographic ordering in radiation therapy treatment planning

PURPOSE

To introduce a method to efficiently identify and calculate meaningful tradeoffs between criteria in an interactive IMRT treatment planning procedure. The method provides a systematic approach to developing high-quality radiation therapy treatment plans.

METHODS

Treatment planners consider numerous dosimetric criteria of varying importance that, when optimized simultaneously through multicriteria optimization, yield a Pareto frontier which represents the set of Pareto-optimal treatment plans. However, generating and navigating this frontier is a time-consuming, nontrivial process. A lexicographic ordering (LO) approach to IMRT uses a physician's criteria preferences to partition the treatment planning decisions into a multistage treatment planning model. Because the relative importance of criteria optimized in the different stages may not necessarily constitute a strict prioritization, the authors introduce an interactive process, sensitivity analysis in lexicographic ordering (SALO), to allow the treatment planner control over the relative sequential-stage tradeoffs. By allowing this flexibility within a structured process, SALO implicitly restricts attention to and allows exploration of a subset of the Pareto efficient frontier that the physicians have deemed most important.

RESULTS

Improvements to treatment plans over a LO approach were found by implementing the SALO procedure on a brain case and a prostate case. In each stage, a physician assessed the tradeoff between previous stage and current stage criteria. The SALO method provided critical tradeoff information through curves approximating the relationship between criteria, which allowed the physician to determine the most desirable treatment plan.

CONCLUSIONS

The SALO procedure provides treatment planners with a directed, systematic process to treatment plan selection. By following a physician's prioritization, the treatment planner can avoid wasting effort considering clinically inferior treatment plans. The planner is guided by criteria importance, but given the information necessary to accurately adjust the relative importance at each stage. Through these attributes, the SALO procedure delivers an approach well balanced between efficiency and flexibility.

Inverse plan optimization accounting for random geometric uncertainties with a multiple instance geometry approximation (MIGA)

Radiotherapy treatment plans that are optimized to be highly conformal based on a static patient geometry can be degraded by setup errors and/or intratreatment motion, particularly for IMRT plans. To achieve improved plans in the face of geometrical uncertainties, direct simulation of multiple instances of the patient anatomy (to account for setup and/or motion uncertainties) is used within the inverse planning process. This multiple instance geometry approximation (MIGA) method uses two or more instances of the patient anatomy and optimizes a single beam arrangement for all instances concurrently. Each anatomical instance can represent expected extremes or a weighted distribution of geometries. The current implementation supports mapping between instances that include distortions, but this report is limited to the use of rigid body translations/ rotations. For inverse planning, the method uses beamlet dose calculations for each instance, with the resulting doses combined using a weighted sum of the results for the multiple instances. Beamlet intensities are then optimized using the inverse planning system based on the cost for the composite dose distribution. MIGA can simulate various types of geometrical uncertainties, including random setup error and intratreatment motion. A limited number of instances are necessary to simulate Gaussian-distributed errors. IMRT plans optimized using MIGA show significantly less degradation in the face of geometrical errors, and are robust to the expected (simulated) motions. Results for a complex head/neck plan involving multiple target volumes and numerous normal structures are significantly improved when the MIGA method of inverse planning is used. Inverse planning using MIGA can lead to significant improvements over the use of simple PTV volume expansions for inclusion of geometrical uncertainties into inverse planning, since it can account for the correlated motions of the entire anatomical representation. The optimized plan results reflect the differing patient geometry situations which can be important near the surface or heterogeneities. For certain clinical situations, the MIGA optimization approach can correct for a significant part of the degradation of the plan caused by the setup uncertainties.

Is uniform target dose possible in IMRT plans in the head and neck?

PURPOSE

Various published reports involving intensity-modulated radiotherapy (IMRT) plans developed using automated optimization (inverse planning) have demonstrated highly conformal plans. These reported conformal IMRT plans involve significant target dose inhomogeneity, including both overdosage and underdosage within the target volume. In this study, we demonstrate the development of optimized beamlet IMRT plans that satisfy rigorous dose homogeneity requirements for all target volumes (e.g., +/-5%), while also sparing the parotids and other normal structures.

METHODS AND MATERIALS

The treatment plans of 15 patients with oropharyngeal cancer who were previously treated with forward-planned multisegmental IMRT were planned again using an automated optimization system developed in-house. The optimization system allows for variable sized beamlets computed using a three-dimensional convolution/superposition dose calculation and flexible cost functions derived from combinations of clinically relevant factors (costlets) that can include dose, dose-volume, and biologic model-based costlets. The current study compared optimized IMRT plans designed to treat the various planning target volumes to doses of 66, 60, and 54 Gy with varying target dose homogeneity while using a flexible optimization cost function to minimize the dose to the parotids, spinal cord, oral cavity, brainstem, submandibular nodes, and other structures.

RESULTS

In all cases, target dose uniformity was achieved through steeply varying dose-based costs. Differences in clinical plan evaluation metrics were evaluated for individual cases (eight different target homogeneity costlets), and for the entire cohort of plans. Highly conformal plans were achieved, with significant sparing of both the contralateral and ipsilateral parotid glands. As the homogeneity of the target dose distributions was allowed to decrease, increased sparing of the parotids (and other normal tissues) may be achieved. However, it was shown that relatively few patients would benefit from the use of increased target inhomogeneity, because the range of improvement in the parotid dose is relatively limited. Hot spots in the target volumes are shown to be unnecessary and do not assist in normal tissue sparing.

CONCLUSION

Sparing of both parotids in patients receiving bilateral neck radiation can be achieved without compromising strict target dose homogeneity criteria. The geometry of the normal tissue and target anatomy are shown to be the major factor necessary to predict the parotid sparing that will be possible for any particular case.

Calibration and quality assurance for rounded leaf-end MLC systems

Multileaf collimator (MLC) systems are available on most commercial linear accelerators, and many of these MLC systems utilize a design with rounded leaf ends and linear motion of the leaves. In this kind of system, the agreement between the digital MLC position readouts and the light field or radiation field edges must be achieved with software, since the leaves do not move in a focused motion like that used for most collimator jaw systems. In this work we address a number of the calibration and quality assurance issues associated with the acceptance, commissioning, and routine clinical use of this type of MLC system. These issues are particularly important for MLCs used for various types of intensity modulated radiation therapy (IMRT) and small, conformal fields. For rounded leaf end MLCs, it is generally not possible to make both the light and radiation field edges agree with the digital readout, so differences between the two kinds of calibrations are illustrated in this work using one vendor's MLC system. It is increasingly critical that the MLC leaf calibration be very consistent with the radiation field edges, so in this work a methodology for performing accurate radiation field size calibration is discussed. A system external to the vendor's MLC control system is used to correct or handle limitations in the MLC control system. When such a system of corrections is utilized, it is found that the MLC radiation field size can be defined with an accuracy of approximately 0.3 mm, much more accurate than most vendor's specifications for MLC accuracy. Quality assurance testing for such a calibration correction system is also demonstrated.

An apparatus for applying strong longitudinal magnetic fields to clinical photon and electron beams

Monte Carlo studies have recently renewed interest in the use of the effect of strong transverse and longitudinal magnetic fields to manipulate the dose characteristics of clinical photon and electron beams. A 3.5 T superconducting solenoidal magnet was used to evaluate the effect of a longitudinal field on both photon and electron beams. This note describes the apparatus and demonstrates some of the effects on the beam trajectory and dose distributions for measurements in a homogeneous phantom. The effects were studied using film in air and in phantoms which fit in the magnet bore. The magnetic field focused and collimated the electron beams. The converging, non-uniform field confined the beam and caused it to converge with increasing depth in the phantom. Due to the field's collecting and focusing effect, the beam flux density increased, leading to increased dose deposition near the magnetic axis, especially near the surface of the phantom. This study illustrates some benefits and challenges associated with the use of non-uniform longitudinal magnetic fields in conjunction with clinical electron and photon beams.

Planning, computer optimization, and dosimetric verification of a segmented irradiation technique for prostate cancer

PURPOSE

To develop and verify a multisegment technique for prostate irradiation that results in better sparing of the rectal wall compared to a conventional three-field technique, for patients with a concave-shaped planning target volume (PTV) overlapping the rectal wall.

METHODS AND MATERIALS

Five patients have been selected with various degrees of overlap between PTV and rectal wall. The planned dose to the ICRU reference point is 78 Gy. The new technique consists of five beams, each having an open segment covering the entire PTV and several smaller segments in which the rectum is shielded. Segment weights are computer-optimized using an algorithm based on simulated annealing. The score function to be minimized consists of dose-volume constraints for PTV, rectal wall, and femoral heads. The resulting dose distribution is verified for each patient by using point measurements and line scans made with an ionization chamber in a water tank and by using film in a cylindrical polystyrene phantom.

RESULTS

The final number of segments in the five-field technique ranges from 7 to 9 after optimization. Compared to the standard three-field technique, the maximum dose to the rectal wall decreases by approximately 3 Gy for patients with a large overlap and 1 Gy for patients with no overlap, resulting in a reduction of the normal tissue complication probability (NTCP) by a factor of 1.3 and 1.2, respectively. The mean dose to the PTV is the same for the two techniques, but the dose distribution is slightly less homogeneous with the five-field technique (Average standard deviation of five patients is 1.1 Gy and 1.7 Gy for the three-field and five-field technique, respectively). Ionization chamber measurements show that in the PTV, the calculated dose is in general within 1% of the measured dose. Outside the PTV, systematic dose deviations of up to 3% exist. Film measurements show that for the complete treatment, the position of the isodose lines in sagittal and coronal planes is calculated fairly accurately, the maximum distance between measured and calculated isodoses being 4 mm.

CONCLUSIONS

We developed a relatively simple multisegment "step-and-shoot" technique that can be delivered within an acceptable time frame at the treatment machine (Extra time needed is approximately 3 minutes). The technique results in better sparing of the rectal wall compared to the conventional three-field technique. The technique can be planned and optimized relatively easily using automated procedures and a predefined score function. Dose calculation is accurate and can be verified for each patient individually.

Estimation of tumor control probability model parameters from 3-D dose distributions of non-small cell lung cancer patients

Tumor control probability (TCP) model calculations may be used in a relative manner to evaluate and optimize three-dimensional (3-D) treatment plans. Using a mathematical model which makes a number of simplistic assumptions, TCPs can be estimated from a 3-D dose distribution of the tumor given the dose required for a 50% probability of tumor control (D50) and the normalized slope (gamma) of the sigmoid-shaped dose-response curve at D50. The purpose of this work was to derive D50 and gamma from our clinical experience using 3-D treatment planning to treat non-small cell lung cancer (NSCLC) patients. Our results suggest that for NSCLC patients, the dose to achieve significant probability of tumor control may be large (on the order of 84 Gy) for longer (> 30 months) local progression-free survival.

Patterns of failure following high-dose 3-D conformal radiotherapy for high-grade astrocytomas: a quantitative dosimetric study

PURPOSE

To analyze the failure patterns for patients with high-grade astrocytomas treated with high-dose conformal radiotherapy (CRT) using a quantitative technique to calculate the dose received by the CT- or MR-defined recurrence volume and to assess whether the final target volume margin used in the present dose escalation study requires redefinition before further escalation.

METHODS AND MATERIALS

Between 4/89 and 10/95, 71 patients with high-grade supratentorial astrocytomas were entered in a phase I/II dose escalation study using 3-D treatment planning and conformal radiotherapy. All patients were treated to either 70 or 80 Gy in conventional daily fractions of 1.8-2.0 Gy. The clinical and planning target volumes (CTV, PTV) consisted of successively smaller volumes with the final PTV defined as the enhancing lesion plus 0.5 cm margin. As of 10/95, 47 patients have CT or MR evidence of disease recurrence/progression. Of the 47 patients, 36 scans obtained at the time of recurrence were entered into the 3-D radiation therapy treatment planning system. After definition of the recurrent tumor volumes, the recurrence scan dataset was registered with the pretreatment CT dataset so that the actual dose received by the recurrent tumor volumes during treatment could be accurately calculated and then analyzed dosimetrically using dose-volume histograms. Recurrences were divided into several categories: 1) "central," in which 95% or more of the recurrent tumor volume (Vrecur) was within D95, the region treated to high dose (95% of the prescription dose); 2) "in-field," in which 80% or more of Vrecur was within the D95 isodose surface; 3) "marginal," when between 20 and 80% of Vrecur was inside the D95 surface; 4) "outside," in which less than 20% of Vrecur was inside the D95 surface.

RESULTS

In 29 of 36 patients, a solitary lesion was seen on recurrence scans. Of the 29 solitary recurrences, 26 were central, 3 were marginal, and none were outside. Multiple recurrent lesions were seen in seven patients: three patients had multiple central and/or in-field lesions only, three patients had central and/or in-field lesions with additional small marginal or outside lesions, and one patent had 6 outside and one central lesion. Since total recurrence volume was used in the final analysis, 6 of the 7 patients with multiple recurrent lesions were classified into centra/in-field category.

CONCLUSION

Analysis of the 36 evaluable patients has shown that 32 of 36 patients (89%) failed with central or in-field recurrences, 3/36 (8%) had a significant marginal component to the recurrence, whereas only 1/36 (3%) could be clearly labeled as failing mainly outside the high-dose region. Seven patients had multiple recurrences, but only 1 of 7 had large-volume recurrences outside the high-dose region. This study shows that the great majority of patient recurrences that occur after high-dose (70 or 80 Gy) conformal irradiation are centrally located: only 1/36 patients (with 7 recurrent lesions) had more than 50% of the recurrence volume outside the region previously treated to high dose. Further dose escalation to 90 Gy (and beyond) thus seems reasonable, based on the same target volume definition criteria

Optimization and clinical use of multisegment intensity-modulated radiation therapy for high-dose conformal therapy

Intensity-modulated radiation therapy (IMRT) may be performed with many different treatment delivery techniques. This article summarizes the clinical use and optimization of multisegment IMRT plans that have been used to treat more than 350 patients with IMRT over the last 4.5 years. More than 475 separate clinical IMRT plans are reviewed, including treatments of brain, head and neck, thorax, breast and chest wall, abdomen, pelvis, prostate, and other sites. Clinical planning, plan optimization, and treatment delivery are summarized, including efforts to minimize the number of additional intensity-modulated segments needed for particular planning protocols. Interactive and automated optimization of segmental and full IMRT approaches are illustrated, and automation of the segmental IMRT planning process is discussed.

The impact of treatment complexity and computer-control delivery technology on treatment delivery errors

PURPOSE

To analyze treatment delivery errors for three-dimensional (3D) conformal therapy performed at various levels of treatment delivery automation and complexity, ranging from manual field setup to virtually complete computer-controlled treatment delivery using a computer-controlled conformal radiotherapy system (CCRS).

METHODS AND MATERIALS

All treatment delivery errors which occurred in our department during a 15-month period were analyzed. Approximately 34,000 treatment sessions (114,000 individual treatment segments [ports]) on four treatment machines were studied. All treatment delivery errors logged by treatment therapists or quality assurance reviews (152 in all) were analyzed. Machines "M1" and "M2" were operated in a standard manual setup mode, with no record and verify system (R/V). MLC machines "M3" and "M4" treated patients under the control of the CCRS system, which (1) downloads the treatment delivery plan from the planning system; (2) performs some (or all) of the machine set up and treatment delivery for each field; (3) monitors treatment delivery; (4) records all treatment parameters; and (5) notes exceptions to the electronically-prescribed plan. Complete external computer control is not available on M3; therefore, it uses as many CCRS features as possible, while M4 operates completely under CCRS control and performs semi-automated and automated multi-segment intensity modulated treatments. Analysis of treatment complexity was based on numbers of fields, individual segments, nonaxial and noncoplanar plans, multisegment intensity modulation, and pseudoisocentric treatments studied for a 6-month period (505 patients) concurrent with the period in which the delivery errors were obtained. Treatment delivery time was obtained from the computerized scheduling system (for manual treatments) or from CCRS system logs. Treatment therapists rotate among the machines; therefore, this analysis does not depend on fixed therapist staff on particular machines.

RESULTS

The overall reported error rate (all treatments, machines) was 0.13% per segment, or 0.44% per treatment session. The rate (per machine) depended on automation and plan complexity. The error rates per segment for machines M1 through M4 were 0.16%, 0.27%, 0.12%, 0.05%, respectively, while plan complexity increased from M1 up to machine M4. Machine M4 (the most complex plans and automation) had the lowest error rate. The error rate decreased with increasing automation in spite of increasing plan complexity, while for the manual machines, the error rate increased with complexity. Note that the real error rates on the two manual machines are likely to be higher than shown here (due to unnoticed and/or unreported errors), while (particularly on M4) virtually all random treatment delivery errors were noted by the CCRS system and related QA checks (including routine checks of machine and table readouts for each treatment). Treatment delivery times averaged from 14 min to 23 min per plan, and depended on the number of segments/plan, although this analysis is complicated by other factors.

CONCLUSION

Use of a sophisticated computer-controlled delivery system for routine patient treatments with complex 3D conformal plans has led to a decrease in treatment delivery errors, while at the same time allowing delivery of increasingly complex and sophisticated conformal plans with little increase in treatment time. With renewed vigilance for the possibility of systematic problems, it is clear that use of complete and integrated computer-controlled delivery systems can provide improvements in treatment delivery, since more complex plans can be delivered with fewer errors, and without increasing treatment time.

A brain tumor dose escalation protocol based on effective dose equivalence to prior experience

PURPOSE

The current study describes the design of a dose escalation protocol for conformal irradiation of primary brain tumors that preserves the safe experience of a previous, sequential dose escalation scheme while enabling the delivery of substantially higher effective doses to a central target volume.

METHODS AND MATERIALS

Normalized isoeffective composite dose distributions were formed for 20 patients treated on the original protocol (which specified three progressively smaller planning target volumes [PTVs]) using the linear quadratic model (here corrected to equivalent 2 Gy fractions using alpha/beta=10 Gy). These distributions were investigated and a new protocol was designed to preserve a similar level of efficacy and lack of toxicity for the outer volumes, but allowing a higher dose to the inner PTV. Treatment plans were then investigated to determine if the objectives of the new protocol were achievable. In particular, plans that simultaneously achieved all biological treatment planning objectives (all fields treated each day) were investigated. Finally, the success of the protocol design was demonstrated by analysis of the effective dose distributions of 10 patients treated using the new protocol.

RESULTS

The composite normalized isoeffective minimum doses to the outer PTVs (PTV3 and PTV2) in the original protocol were close to 60 Gy and 75 Gy, respectively, and these values are specified as the minimum doses to those volumes for the new protocol. Homogeneity requirements to maintain equivalence for the outer target volume domains are: not more than 25% of [PTV3 exclusive of PTV2] >75 Gy; and not more than 50% of [PTV2 exclusive of PTV1] >85 Gy. Treatment plans using multiple noncoplanar arrangements of beams and static intensity modulation treat all volumes at each session. DVHs of the normalized isoeffective dose distributions reveal the equivalence of the new protocol plans to the sequential plans in the previous protocol as well as the ability to achieve a higher dose of 90 Gy to the isocenter of PTV1 (+/-5% homogeneity required).

CONCLUSION

The ability to incorporate past experience through use of the linear quadratic model in the design of a new dose escalation protocol is demonstrated.

Comprehensive irradiation of head and neck cancer using conformal multisegmental fields: assessment of target coverage and noninvolved tissue sparing

PURPOSE

Conformal treatment using static multisegmental intensity modulation was developed for patients requiring comprehensive irradiation for head and neck cancer. The major aim is sparing major salivary gland function while adequately treating the targets. To assess the adequacy of the conformal plans regarding target coverage and dose homogeneity, they were compared with standard irradiation plans.

METHODS AND MATERIALS

Fifteen patients with stage III/IV head and neck cancer requiring comprehensive, bilateral neck irradiation participated in this study. CT-based treatment plans included five to six nonopposed fields, each having two to four in-field segments. Fields and segments were devised using beam's eye views of the planning target volumes (PTVs), noninvolved organs, and isodose surfaces, to achieve homogeneous dose distribution that encompassed the targets and spared major salivary gland tissue. For comparison, standard three-field radiation plans were devised retrospectively for each patient, with the same CT-derived targets used for the clinical (conformal) plans. Saliva flow rates from each major salivary gland were measured before and periodically after treatment.

RESULTS

On average, the minimal dose to the primary PTVs in the conformal plans [95.2% of the prescribed dose, standard deviation (SD) 4%] was higher than in the standard plans (91%, SD 7%; p = 0.02), and target volumes receiving <95% or <90% of the prescribed dose were smaller in the conformal plans (p = 0.004 and 0.02, respectively). Similar advantages of the conformal plans compared to standard plans were found in ipsilateral jugular nodes PTV coverage. The reason for underdosing in the standard treatment plans was primarily failure of electron beams to fully encompass targets. No significant differences were found in contralateral jugular or posterior neck nodes coverage. The minimal dose to the retropharyngeal nodes was higher in the standard plans. However, all conformal plans achieved the planning goal of delivering 50 Gy to these nodes. In the conformal plans, the magnitude and volumes of high doses in noninvolved tissue were significantly reduced. The main reasons for hot spots in the standard plans (whose dose calculations included missing tissue compensators) were photon/electron match line inhomogeneities, which were avoided in the conformal plans. The mean doses to all the major salivary glands, notably the contralateral parotid (receiving on average 32% of the prescribed dose, SD 7%) were significantly lower in the conformal plans compared with standard radiation plans. The mean dose to the noninvolved oral cavity tended to be lower in the conformal plans (p = 0.07). One to 3 months after radiation, on average 60% (SD 49%) of the preradiation saliva flow rate was retained in the contralateral parotid glands and 10% (SD 16%) was retained in the submandibular/sublingual glands.

CONCLUSIONS

Planning and delivery of comprehensive irradiation for head and neck cancer using static, multisegmental intensity modulation are feasible. Target coverage has not been compromised and dose distributions in noninvolved tissue are favorable compared with standard radiation. Substantial major salivary gland function can be retained.

Spinal cord dose from standard head and neck irradiation: implications for three-dimensional treatment planning

BACKGROUND AND PURPOSE

Treatment with traditional standard field arrangements for patients with head and neck cancer rarely causes myelopathy. Often, initial treatment fields are reduced to avoid the spinal cord after 45 Gy has been delivered and the cord dose that is delivered by 'off-cord' fields is not calculated. To determine a conservative limit to set for the cord dose for conformally-planned field arrangements, the total spinal cord dose delivered with standard opposed lateral fields was evaluated.

MATERIALS AND METHODS

Two types of treatment plans were evaluated for 10 patients enrolled on a parotid-sparing protocol for bilateral head and neck treatment, i.e. (1) standard opposed lateral fields, including large initial fields treating nodal volumes to 45 Gy, off-cord fields for an additional 25 Gy and electron nodal boost fields for an additional 5 Gy and (2) complex 3-D treatment planned field geometries with conformal dose distributions (actual treatment fields). Treatment fields for the protocol conformal plans were arranged so that the maximum cord dose was not to exceed 50 Gy. Dose-volume histograms for both types of planned treatments were analyzed. The maximum and minimum dose to the 1 cm3 cord volume receiving the highest dose were reported.

RESULTS

The maximum dose to the cord from the standard composite plans was on average 52 Gy, with a range of 48.9-55.9 Gy. This consisted of an additional 6.3 Gy (average) from the scatter and block transmission dose from the off-cord lateral fields above the prescribed 45 Gy. For the conformal plans, the maximum dose was on average 49.4 Gy (which is protocol criteria).

DISCUSSION AND CONCLUSION

The maximum spinal cord dose of 50 Gy set as a dose constraint for 3-D treatment planning for conformal plans is a comparable dose to that given in standard opposed lateral head and neck treatments and has been determined to be a conservative spinal cord dose limit, which we have applied in our clinic.

Characteristics of scattered electron beams shaped with a multileaf collimator

Characteristics of dual-foil scattered electron beams shaped with a multileaf collimator (MLC) (instead of an applicator system) were studied. The electron beams, with energies between 10 and 25 MeV, were produced by a racetrack microtron using a dual-foil scattering system. For a range of field sizes, depth dose curves, profiles, penumbra width, angular spread in air, and effective and virtual source positions were compared. Measurements were made when the MLC alone provided collimation and when an applicator provided collimation. Identical penumbra widths were obtained at a source-to-surface distance of 85 cm for the MLC and 110 cm for the applicator. The MLC-shaped beams had characteristics similar to other machines which use trimmers or applicators to collimate scanned or scattered electron beams. Values of the effective source position and the angular spread parameter for the MLC beams were similar to those of the dual-foil scattered beams of the Varian Clinac 2100 CD and the scanned beams of the Sagittaire linear accelerators. A model, based on Fermi-Eyges multiple scattering theory, was adapted and applied successfully to predict penumbra width as a function of collimator-surface distance.

Volume and dose parameters for survival of non-small cell lung cancer patients

BACKGROUND AND PURPOSE

To determine the effect of tumor volume and dose factors derived from 3-D treatment planning dose distributions on survival outcome for non-small cell lung cancer patients.

MATERIALS AND METHODS

Seventy-six consecutive patients diagnosed with medically inoperable or locally advanced, unresectable non-small cell lung cancer planned with 3-D treatment planning between 1986 and 1992 were the subject of this retrospective study. Patient characteristics and dosimetric parameters were analyzed for influence on overall survival and local progression-free survival (LPFS) using univariate and multivariate analysis.

RESULTS

Nodal stage and stage were the most significant factors for overall survival and LPFS duration on both univariate and multivariate analysis. We found a wide range of primary tumor volume sizes for each stage. Patients with tumor volumes <200 cm3 had longer survival (P = 0.047). In an analysis stratifying patients into four groups by tumor volume (<200 cm3 versus >200 cm3) and nodes (negative versus positive), patients in the group with no nodal disease and <200 cm3 tumor volumes survived longer than patients in any other group (P = 0.046). No dose factors were statistically significant for longer survival. Longer LPFS was seen for (a) isocenter dose >70 Gy (P = 0.055) for the overall group of patients, (b) within a subgroup with no nodal disease and >73 Gy (P = 0.054), and (c) within a subgroup with no nodal disease and tumor volume <200 cm3 receiving >73 Gy (P = 0.086).

CONCLUSIONS

Several findings from the volume and dosimetric analysis in this study are noteworthy. Stage was found to be a poor predictor of primary tumor volume size. Also, tumor volume size (<200 cm3) in conjunction with nodal status (negative nodes) had an impact on survival though there was a mix of stage (I, IIIa, IIIb) in this group of patients. Finally, dose appears to influence local control (LPFS) for the overall group of patients and when tumor volumes are <200 cm3. Our data indicate that outcome following radiation may be better predicted by a staging system that takes into account tumor volume and nodal spread rather than a system that is largely based on anatomic location of disease. Dose prescription for lung cancer treatment might better be written based on tumor volume size.

Dose-volume complication analysis for visual pathway structures of patients with advanced paranasal sinus tumors

PURPOSE

The purpose of the present work was to relate dose and volume information to complication data for visual pathway structures in patients with advanced paranasal sinus tumors.

METHODS AND MATERIALS

Three-dimensional (3D) dose distributions for chiasm, optic nerve, and retina were calculated and analyzed for 20 patients with advanced paranasal sinus malignant tumors. 3D treatment planning with beam's eye view capability was used to design beam and block arrangements, striving to spare the contralateral orbit (to lessen the chance of unilateral blindness) and frequently the ipsilateral orbit (to help prevent bilateral blindness). Point doses, dose-volume histogram analysis, and normal tissue complication probability (NTCP) calculations were performed. Published tolerance doses that indicate significant risk of complications were used as guidelines for analysis of the 3D dose distributions.

RESULTS

Point doses, percent volume exceeding a specified published tolerance dose, and NTCP calculations are given in detail for patients with complications versus patients without complications. Two optic nerves receiving maximum doses below the published tolerance dose sustained damage (mild vision loss). Three patients (of 13) without optic nerve sparing and/or chiasm sparing had moderate or severe vision loss. Complication data, including individual patient analysis to estimate overall risk for loss of vision, are given.

CONCLUSION

3D treatment planning techniques were used successfully to provide bilateral sparing of the globe for most patients. It was more difficult to spare the optic nerves, especially on the ipsilateral side, when prescription dose exceeded the normal tissue tolerance doses. NTCP calculations may be useful in assessing complication risk better than point dose tolerance criteria for the chiasm, optic nerve, and retina. It is important to assess the overall risk of blindness for the patient in addition to the risk for individual visual pathway structures.

Electron dose calculations using the Method of Moments

The Method of Moments is generalized to predict the dose deposited by a prescribed source of electrons in a homogeneous medium. The essence of this method is (i) to determine, directly from the linear Boltzmann equation, the exact mean fluence, mean spatial displacements, and mean-squared spatial displacements, as functions of energy; and (ii) to represent the fluence and dose distributions accurately using this information. Unlike the Fermi-Eyges theory, the Method of Moments is not limited to small-angle scattering and small angle of flight, nor does it require that all electrons at any specified depth z have one specified energy E(z). The sole approximation in the present application is that for each electron energy E, the scalar fluence is represented as a spatial Gaussian, whose moments agree with those of the linear Boltzmann solution. Numerical comparisons with Monte Carlo calculations show that the Method of Moments yields expressions for the depth-dose curve, radial dose profiles, and fluence that are significantly more accurate than those provided by the Fermi-Eyges theory.

Aspects of enhanced three-dimensional radiotherapy treatment planning

Advances in computer technology have led to the availability of sophisticated three-dimensional treatment planning systems for use in many radiotherapy centers. However, additional complexity in both the planning and delivery of treatments has accompanied their use. Thus, even more computer-aided tools are beginning to appear to address these needs. Aspects of recent enhancements to 3-D treatment planning at the University of Michigan are presented.

A computer-controlled conformal radiotherapy system. IV: Electronic chart

PURPOSE

The design and implementation of a system for electronically tracking relevant plan, prescription, and treatment data for computer-controlled conformal radiation therapy is described.

METHODS AND MATERIALS

The electronic charting system is implemented on a computer cluster coupled by high-speed networks to computer-controlled therapy machines. A methodical approach to the specification and design of an integrated solution has been used in developing the system. The electronic chart system is designed to allow identification and access of patient-specific data including treatment-planning data, treatment prescription information, and charting of doses. An in-house developed database system is used to provide an integrated approach to the database requirements of the design. A hierarchy of databases is used for both centralization and distribution of the treatment data for specific treatment machines.

RESULTS

The basic electronic database system has been implemented and has been in use since July 1993. The system has been used to download and manage treatment data on all patients treated on our first fully computer-controlled treatment machine. To date, electronic dose charting functions have not been fully implemented clinically, requiring the continued use of paper charting for dose tracking.

CONCLUSIONS

The routine clinical application of complex computer-controlled conformal treatment procedures requires the management of large quantities of information for describing and tracking treatments. An integrated and comprehensive approach to this problem has led to a full electronic chart for conformal radiation therapy treatments.

A computer-controlled conformal radiotherapy system. III: Graphical simulation and monitoring of treatment delivery

PURPOSE

Safe and efficient delivery of radiotherapy using computer-controlled machines requires new procedures to design and verify the actual delivery of these treatments. Graphical simulation and monitoring techniques for treatment delivery have been developed for this purpose.

METHODS AND MATERIALS

A graphics-based simulator of the treatment machine and a set of procedures for creating and manipulating treatment delivery scripts are used to simulate machine motions, detect collisions, and monitor machine positions during treatment. The treatment delivery simulator is composed of four components: a three-dimensional dynamic model of the treatment machine; a motion simulation and collision detection algorithm, user-interface widgets that mimic the treatment machine's control and readout devices; and an icon-based interface for creating and manipulating treatment delivery scripts. These components are used in a stand-alone fashion for interactive treatment delivery planning and integrated with a machine control system for treatment implementation and monitoring.

RESULTS

A graphics-based treatment delivery simulator and a set of procedures for planning and monitoring computer-controlled treatment delivery have been developed and implemented as part of a comprehensive computer-controlled conformal radiotherapy system. To date, these techniques have been used to design and help monitor computer-controlled treatments on a radiotherapy machine for more than 200 patients. Examples using these techniques for treatment delivery planning and on-line monitoring of machine motions during therapy are described.

CONCLUSION

A system that provides interactive graphics-based tools for defining the sequence of machine motions, simulating treatment delivery including collision detection, and presenting the therapists with continual visual feedback from the treatment machine has been successfully implemented for routine clinical use as part of an overall system for computer-controlled conformal radiotherapy treatment, and is considered a necessary part of the routine treatment methodology.

Advanced interactive planning techniques for conformal therapy: high level beam descriptions and volumetric mapping techniques

PURPOSE

To aid in design of conformal radiation therapy treatment plans involving many conformally shaped fields, this work investigates the use of two methodologies to enhance the ease of interactive treatment planning: high-level beam constructs and beam's-eye view volumetric mapping.

METHODS AND MATERIALS

High-performance computer graphics running on various workstations using a graphical visualization system (AVS) have been used in this work. Software specific to this application has been written in standard FORTRAN and C languages. A new methodology is introduced by defining radiation therapy "fields" to be composed of multiple beam "segments." Fields can then be defined as higher-level entities such as arcs, cones, and other shapes. A "segmental cone" field, for example, is defined by a symmetry axis and a cone angle, and can be used to rapidly place a series of beam segments that converge at the target volume, while reducing the degree of overlap elsewhere. A new beam's-eye view (BEV) volumetric mapping technique is presented to aid in selecting the placement of conformal radiation fields. With this technique, the relative average dose within an organ of interest is calculated for a sampling of isocentric, conformally shaped beams and displayed either as a "globe," which can be combined with the display of anatomical surfaces, or as a two-dimensionally mapped projection. The dose maps from multiple organs can be generated, stacked, or composited with relative weightings to aid in the placement of fields that minimize overlap with critical structures.

RESULTS

The use of these new methodologies is demonstrated for prostate and lung treatment sites and compared to conventional planning techniques.

DISCUSSION

The use of many beams for conformal treatment delivery is difficult with current interactive planning. The use of high-level beam constructs provides a means to quickly specify, place, and configure multiple beam arrangements. The BEV volumetrics aids in the placing of fields, which minimize involvement with critical normal tissues.

CONCLUSIONS

Early experience with the new methodologies suggest that the new methods help to enhance (or at least speed up) the ability of a treatment planner to create optimal radiation treatment field arrangements.

A computer-controlled conformal radiotherapy system. I: Overview

PURPOSE

Equipment developed for use with computer-controlled conformal radiotherapy (CCRT) treatment techniques, including multileaf collimators and/or computer-control systems for treatment machines, are now available. The purpose of this work is to develop a system that will allow the safe, efficient, and accurate delivery of CCRT treatments as routine clinical treatments, and permit modifications of the system so that the delivery process can be optimized.

METHODS AND MATERIALS

The needs and requirements for a system that can fully support modern computer-controlled treatment machines equipped with multileaf collimators and segmental or dynamic conformal therapy capabilities have been analyzed and evaluated. This analysis has been used to design and then implement a complete approach to the delivery of CCRT treatments.

RESULTS

The computer-controlled conformal radiotherapy system (CCRS) described here consists of a process for the delivery of CCRT treatments, and a complex software system that implements the treatment process. The CCRS system described here includes systems for plan transfer, treatment delivery planning, sequencing of the actual treatment delivery process, graphical simulation and verification tools, as well as an electronic chart that is an integral part of the system. The CCRS system has been implemented for use with a number of different treatment machines. The system has been used clinically for more than 2 years to perform CCRT treatments for more than 200 patients.

CONCLUSIONS

A comprehensive system for the implementation and delivery of computer-controlled conformal radiation therapy (CCRT) plans has been designed and implemented for routine clinical use with multisegment, computer-controlled, multileaf-collimated conformal therapy. The CCRS system has been successfully implemented to perform these complex treatments, and is considered quite important to the clinical use of modern computer-controlled treatment techniques.

A computer-controlled conformal radiotherapy system. II: Sequence processor

PURPOSE

A sequence processor (SP) is described as part of a larger computer-controlled conformal radiotherapy system (CCRS). The SP provides the means to accept and then translate highly sophisticated radiation therapy treatment plans into vendor specific instructions to control treatment delivery on a computer-controlled treatment machine.

METHODS AND MATERIALS

The sequence processor (SP) is a small workstation computer that interfaces to the control computer of computer-controlled treatment machines, and to other parts of the larger CCRS system. The system reported here has been interfaced to a computer-controlled racetrack microtron with two treatment gantries, and also to other linear accelerator treatment machines equipped with multileaf collimators. An extensive design process has been used in defining the role of the SP within the context of the larger CCRS project. Flexibility and integration with various components of the project, including databases, treatment planning system, graphical simulator, were key factors in the development. In conjunction with the planned set of treatment fields, a procedural scripting language is used to define the sequence of treatment events that are performed, including operator interactions, communications to other systems such as dosimetry and portal imaging devices, and database management.

RESULTS

A flexible system has been developed to allow investigation into procedural steps required for simulating and delivering complex radiation treatments. The system has been used to automate portions of the acceptance testing for the control system of the microtron, and is used for routine daily quality assurance testing. The sequence processor system described here has been used to deliver all clinical treatments performed on the microtron system in 2 years of clinical treatment (more than 200 patients treated to a variety of treatment sites).

CONCLUSIONS

The sequence processor system has enabled the delivery of complex treatment using computer-controlled treatment machines. The flexibility of the system allows integration with secondary devices and modification of procedural steps, making it possible to develop effective techniques for insuring safe and efficient computer-controlled conformal radiation therapy treatments.

The development of conformal radiation therapy

The development and clinical use of conformal radiation therapy, and the application of modern computer-controlled radiation therapy treatment techniques to the delivery of conformal therapy, has been a major research area in the field of radiation oncology in recent years. The introduction of three-dimensional (3-D) patient imaging, 3-D treatment planning systems, computer-controlled treatment machines equipped with multileaf collimators, and the continuing increase in computer power and software sophistication has finally allowed the clinical implementation of the kinds of conformal treatment planning and delivery that were envisioned decades ago. The use of 3-D treatment planning in conjunction with conformal treatments and the analysis of clinical normal tissue complications has also begun to give the field the clinical complication (and eventually tumor control) probability data which can be used to truly optimize radiation treatments. At this centennial anniversary of the discovery of the x ray, it is appropriate to look back at the contributions which have led to the current developments in conformal therapy. Few if any of the "new developments" associated with modern computer-controlled conformal therapy (CCRT) are actually new. However, modern technology has finally progressed to the point that we can integrate all of the features that are necessary to really make the concepts work in the clinic. This paper traces some of the developments which have led to the current interest and progress in CCRT treatments. A brief summary of the current status of conformal therapy and the work still to be done is also included.

Mechanical and dosimetric quality control for computer controlled radiotherapy treatment equipment

Modern computer controlled radiotherapy treatment equipment offers the possibility of delivering complex, multiple field treatments with minimal operator intervention, thus making multiple field conformal therapy practical. Conventional quality control programs are inadequate for this new technology, so new quality control procedures are needed. A reasonably fast, sensitive, and complete daily quality control program has been developed in our clinic that includes nearly automated mechanical as well as dosimetric tests. Automated delivery of these quality control fields is performed by the control system of the MM50 racetrack microtron, directed by the CCRS sequence processor [D. L. McShan and B. A. Fraass, Proceedings of the XIth International Conference on the use of computers in Radiation Therapy, 20-24 March 1994, Manchester, U.K. (North Western Medical Physics Department, Manchester, U.K., 1994), pp. 210-211], which controls the treatment process. The mechanical tests involve multiple irradiations of a single film to check the accuracy and reproducibility of the computer controlled setup of gantry and collimator angles, table orientation, collimator jaws, and multileaf collimator shape. The dosimetric tests, which involve multiple irradiations of an array of ionization chambers in a commercial dose detector (Keithly model 90100 Tracker System) rigidly attached to the head of the treatment gantry, check the output and symmetry of the treatment unit as a function of gantry and collimator angle and other parameters. For each of the dosimetric tests, readings from the five ionization chambers are automatically read out, stored, and analyzed by the computer, along with the geometric parameters of the treatment unit for that beam.(ABSTRACT TRUNCATED AT 250 WORDS)

Expanding the use and effectiveness of dose-volume histograms for 3-D treatment planning. I: Integration of 3-D dose-display

PURPOSE

A technique is presented for overcoming a major deficiency of histogram analysis in three-dimensional (3-D) radiotherapy treatment planning; the lack of spatial information.

METHODS AND MATERIALS

In this technique, histogram data and anatomic images are displayed in a side-by-side fashion. The histogram curve is used as a guide to interactively probe the nature of the corresponding 3-D dose distribution. Regions of dose that contribute to a specific dose bin or range of bins are interactively highlighted on the anatomic display as a window-style cursor is positioned along the dose-axis of the histogram display. This dose range highlighting can be applied to two-dimensional (2-D) images and to 3-D views which contain anatomic surfaces, multimodality image data, and representations of radiation beams and beam modifiers. Additionally, as a range of histogram bins is specified, dose and volume statistics for the range are continually updated and displayed.

RESULTS

The implementation of these techniques is presented and their use illustrated for a nonaxial three field treatment of a hepatic tumor.

CONCLUSION

By integrating displays of 3-D doses and the corresponding histogram data, it is possible to recover the positional information inherently lost in the calculation of a histogram. Important questions such as the size and location of hot spots in normal tissues and cold spots within target volumes can be more easily uncovered, making the iterative improvement of treatment plans more efficient.

Dose-volume histogram and 3-D treatment planning evaluation of patients with pneumonitis

PURPOSE

Tolerance of normal lung to inhomogeneous irradiation of partial volumes is not well understood. This retrospective study analyzes three-dimensional (3-D) dose distributions and dose-volume histograms for 63 patients who have had normal lung irradiated in two types of treatment situations.

METHODS AND MATERIALS

3-D treatment plans were examined for 21 patients with Hodgkin's disease and 42 patients with nonsmall-cell lung cancer. All patients were treated with conventional fractionation, with a dose of 67 Gy (corrected) or higher for the lung cancer patients. A normal tissue complication probability description and a dose-volume histogram reduction scheme were used to assess the data. Mean dose to lung was also calculated.

RESULTS

Five Hodgkin's disease patients and nine lung cancer patients developed pneumonitis. Data were analyzed for each individual independent lung and for the total lung tissue (lung as a paired organ). Comparisons of averages of mean lung dose and normal tissue complication probabilities show a difference between patients with and without complications. Averages of calculated normal tissue complication probabilities for groups of patients show that empirical model parameters correlate with actual complication rates for the Hodgkin's patients, but not as well for the individual lungs of the lung cancer patients treated to larger volumes of normal lung and high doses.

CONCLUSION

This retrospective study of the 3-D dose distributions for normal lung for two types of treatment situations for patients with irradiated normal lung gives useful data for the characterization of the dose-volume relationship and the development of pneumonitis. These data can be used to help set up a dose escalation protocol for the treatment of nonsmall-cell lung cancer.

Dosimetric verification of a 3-D electron pencil beam dose calculation algorithm

A three-dimensional electron beam dose calculational algorithm implemented for use in a three-dimensional treatment planning system is described. The 3-D electron beam calculations have been in use clinically for more than seven years. The algorithm uses a pencil beam model based on small angle multiple Coulomb scattering theory. Our implementation allows volumetric CT-based inhomogeneity corrections and provides for irregular field shapes (fields shaped with cerrobend cutouts) and the use of bolus. As part of NCI-funded work evaluating the state of the art in electron beam treatment planning, extensive algorithm verification was undertaken and results of these tests for our implementation are presented.

Three-dimensional treatment planning of intracavitary gynecologic implants: analysis of ten cases and implications for dose specification

PURPOSE

Results of 3-dimensional treatment planning for ten intracavitary gynecologic implants and implications for dose specification are presented.

METHODS AND MATERIALS

Using a computed tomographic (CT) compatible intracavitary applicator we have performed CT scans during gynecologic brachytherapy in 10 cases. A CT-based treatment planning system with 3-dimensional capabilities was used to calculate and display dose in three dimensions. Conventional point doses including the estimated bladder and rectal maximum doses and dose to Point A were acquired from orthogonal simulation films. CT maximum bladder and rectal doses and minimum cervix doses were ascertained from isodose lines displayed on individual CT images. Dose volume histograms for the bladder, rectum and cervix were generated and used to obtain volume of the cervix target volume receiving less than the prescribed dose and the volume of bladder and rectum receiving more than the orthogonal maximum doses. The 5 cc volume of bladder and rectum receiving the highest dose were also calculated.

RESULTS

Average values of CT point doses and volumes are compared with the traditionally obtained doses. As demonstrated by others, much higher bladder and rectal doses are found using the CT information. The minimum dose to the cervix target volume is lower than the dose to Point A in each case. CT maximum bladder and rectum and minimum cervix target doses may not be the best index doses to correlate with outcome because of the small volumes receiving the dose.

CONCLUSION

We hypothesize that clinically useful bladder, rectal and cervix target volume doses will include volume information which is obtainable with dose volume histogram analysis.

A large screen digitizer system for radiation therapy treatment planning

PURPOSE

This paper describes a new technique for manually drawing contours of anatomy over image data for the purposes of radiation therapy treatment planning.

METHODS AND MATERIALS

A large area rear-projectible digitizer tablet is used together with a projection TV system to display computer graphics and image data. Large images of computed tomography or magnetic resonance cross-sections are displayed and the digitizer is used to directly trace outlines of important organs. Digitizer menus allow multiple functions for selecting images and structures, for changing the grayscale level and window, and for zooming and roaming the image.

RESULTS

This device has been in clinical operation for many years and has proven to greatly increase the speed of entering cross-sectional outlines defined for serial computed tomography images sets. A small timing study of clinical usage demonstrates up to a factor of ten improvement in the speed of contour entry.

CONCLUSION

For 3-dimensional radiation therapy, tumor, and target volumes, as well as important critical organs, must be delineated from serial sets of computed tomography or magnetic resonance images. Often 30 or more slices must be considered and the process of outlining structures on this number of slices can represent a significant fraction of the total treatment planning time. The device described in this paper greatly improve the ease and speed of manual contour entry for 3-dimensional radiation therapy planning.

Medical accelerator safety considerations: report of AAPM Radiation Therapy Committee Task Group No. 35

Ensuring safe operation for a medical accelerator is a difficult task. Users must assume more responsibility in using contemporary equipment. Additionally, users must work closely with manufacturers in promoting the safe and effective use of such complex equipment. Complex treatment techniques and treatment modality changeover procedures merit detailed, unambiguous written procedural instruction at the control console. A thorough "hands on" training period after receiving instructions, and before assuming treatment responsibilities, is essential for all technologists. Unambiguous written instructions must also be provided to guide technologists in safe response when equipment malfunctions or exhibits unexpected behavior or after any component has been changed or readjusted. Technologists should be given a written list of the appropriate individuals to consult when unexpected machine behavior occurs. They should be assisted in identifying aberrant behavior of equipment. Many centers already provide this instruction, but others may not. Practiced response and discussion with technologists should be a part of an ongoing quality assurance program. An important aspect of a safety program is the need for continuous vigilance. Table III gives a summary of a comprehensive safety program for medical accelerators. Table IV gives a list of summary recommendations as an example of how one might mitigate the consequences of an equipment failure and improve procedures and operator response in the context of the environment described. Most of these recommendations can be implemented almost immediately at any individual treatment center.

Use of an octree-like geometry for 3-D dose calculations

A representation used for 3-D graphical objects, the "octree," has been applied to the geometrical calculations needed to perform photon beam does calculations for radiotherapy treatment planning. This representation allows the algorithm to attempt to minimize the number of distinct geometrical calculations that are needed to perform dose calculations to a particular resolution. In this way, the calculational time can be minimized, since the geometrical part of the dose calculations is often the most time-intensive part of the calculation process. The octree-like system used here has sped up the photon dose calculations described here by up to a factor of 10.

A quantitative assessment of the addition of MRI to CT-based, 3-D treatment planning of brain tumors

Quantitative 3-D volumetric comparisons were made of composite CT-MRI macroscopic and microscopic tumor and target volumes to their independently defined constituents. Volumetric comparisons were also made between volumes derived from coronal and axial MRI data sets, and between CT and MRI volumes redefined at a repeat session in comparison to their original definitions. The degree of 3-D dose coverage obtained from use of CT data only or MRI data only in terms of coverage of composite CT-MRI volumes was also analyzed. On average, MRI defined larger volumes as well as a greater share of composite CT-MRI volumes. On average, increases in block margin on the order of 0.5 cm would have ensured coverage of volumes derived from use of both imaging modalities had only MRI data been used. However, the degree of inter-observer variation in volume definition is on the order of the magnitude of differences in volume definition seen between the modalities, and the question of which imaging modality best describes tumor volumes remains unanswered until detailed histologic studies are performed. Given that tumor volumes independently apparent on CT and MRI have equal validity, composite CT-MRI input should be considered for planning to ensure precise dose coverage for conformal treatments.

Verification data for electron beam dose algorithms

The Collaborative Working Group (CWG) of the National Cancer Institute (NCI) electron beam treatment planning contract has performed a set of 14 experiments that measured dose distributions for 28 unique beam-phantom configurations that simulated various patient anatomic structures and beam geometries. Multiple dose distributions were measured with film or diode detectors for each configuration, resulting in 78, 2-D planar dose distributions and one, 1-D depth-dose distribution. Measurements were made for 9- and 20-MeV electron beams, using primarily 6 x 6- and 15 x 15-cm applicators at several SSDs. Dose distributions were measured for shaped fields, irregular surfaces, and inhomogeneities (1-D, 2-D, and 3-D), which were designed to simulate many clinical electron treatments. The data were corrected for asymmetries, and normalized in an absolute manner. This set of measured data can be used for verification of electron beam dose algorithms and is available to others for that purpose.

Generation and use of measurement-based 3-D dose distributions for 3-D dose calculation verification

A 3-D radiation therapy treatment planning system calculates dose to an entire volume of points and therefore requires a 3-D distribution of measured dose values for quality assurance and dose calculation verification. To measure such a volumetric distribution with a scanning ion chamber is prohibitively time consuming. A method is presented for the generation of a 3-D grid of dose values based on beam's-eye-view (BEV) film dosimetry. For each field configuration of interest, a set of BEV films at different depths is obtained and digitized, and the optical densities are converted to dose. To reduce inaccuracies associated with film measurement of megavoltage photon depth doses, doses on the different planes are normalized using an ion-chamber measurement of the depth dose. A 3-D grid of dose values is created by interpolation between BEV planes along divergent beam rays. This matrix of measurement-based dose values can then be compared to calculations over the entire volume of interest. This method is demonstrated for three different field configurations. Accuracy of the film-measured dose values is determined by 1-D and 2-D comparisons with ion chamber measurements. Film and ion chamber measurements agree within 2% in the central field regions and within 2.0 mm in the penumbral regions.

The clinical utility of magnetic resonance imaging in 3-dimensional treatment planning of brain neoplasms

Results of the clinical experience gained since 1986 in the treatment planning of patients with brain neoplasms through integration of magnetic resonance imaging (MRI) into computerized tomography (CT)-based, three-dimensional treatment planning are presented. Data from MRI can now be fully registered with CT data using appropriate three-dimensional coordinate transformations allowing: (a) display of MRI defined structures on CT images; (b) treatment planning of composite CT-MRI volumes; (c) dose display on either CT or MRI images. Treatment planning with non-coplanar beam arrangements is also facilitated by MRI because of direct acquisition of information in multiple, orthogonal planes. The advantages of this integration of information are especially evident in certain situations, for example, low grade astrocytomas with indistinct CT margins, tumors with margins obscured by bone artifact on CT scan. Target definitions have repeatedly been altered based on MRI detected abnormalities not visualized on CT scans. Regions of gadolinium enhancement on MRI T1-weighted scans can be compared to the contrast-enhancing CT tumor volumes, while abnormalities detected on MRI T2-weighted scans are the counterpart of CT-defined edema. Generally, MRI markedly increased the apparent macroscopic tumor volume from that seen on contrast-CT alone. However, CT tumor information was also necessary as it defined abnormalities not always perceptible with MRI (on average, 19% of composite CT-MRI volume seen on CT only). In all, the integration of MRI data with CT information has been found to be practical, and often necessary, for the three-dimensional treatment of brain neoplasms.

Design of MRI scan protocols for use in 3-D, CT-based treatment planning

MRI has the potential of providing the radiation therapy treatment planner with new insights into the definition of target and normal tissue volumes to augment CT in 3-D treatment planning. The current speed of MR scan sequences is not sufficient to enable the acquisition of both T1 and T2 weighted images in all three orthogonal planes in a reasonable period of time. Therefore, compromises must be made in the design of protocols specifically for use in radiotherapy planning which: (1) provide enough information to readily enable image registration; (2) preserve the three-dimensionality provided by image acquisition directly in coronal and sagittal planes; (3) yield tissue contrast as well as tumor specificity (where available); but (4) can be completed in a short enough span of time (or with enough checks) that the patient position is not compromised. Protocols designed for use in planning treatment of the brain, head and neck, lung, prostate, cervix, and sarcomas are presented.

Technical considerations in the use of 3-D beam arrangements in the abdomen

The practical utilization of 3-D treatment planning to reduce doses to normal tissues in the abdomen is illustrated for irradiation of hepatic masses using fields with central axes rotated out of the transverse plane. The beams were arranged to go through the minimum amount of normal liver tissue, while exiting above or below a kidney. Although these beam arrangements were not coplanar with standard transverse body sections, they were designed for dose delivery through use of standard Megavoltage equipment. The planning process for these techniques illustrates the need for and use of several tools usually associated with 3-D treatment planning systems. Beam's eye-view planning with perspective display of the relevant anatomy in the projective beam geometry is required for designing the placement of focused blocks for these oblique fields. Three-dimensional volumetric dose calculations are required to evaluate dose distributions. Additionally, port-film-type radiographs, digitally reconstructed from the CT dataset, are found to be useful in understanding the correctness of simulation and verification films. The reduction in dose to normal tissues over that achievable using standard plans with beams entering the patient at right angles to the central axis of the body is illustrated using dose-volume histograms. These techniques have allowed the initiation of a radiation dose escalation protocol for tumors involving the liver and porta hepatis.

Three-dimensional treatment planning of astrocytomas: a dosimetric study of cerebral irradiation

To demonstrate that 3-dimensional planning is both practical and applicable to the treatment of high-grade astrocytomas, 50 patients over a 2-year period have received cerebral irradiation delivered in focussed, non-axial techniques employing from 2 to 5 beams. Astrocytomas have been planned using rapid, practical incorporation of CT data to define appropriate tumor volumes. Tumor + 3.0 cm and tumor + 1.5 cm volumes have been treated to conventional doses of 4500 cGy and 5940 cGy, respectively, using beam orientations that maximally spared normal remaining parenchyma. Analyses of 3-dimensionally calculated plans have been performed using integral dose-volume histograms (DVH) to help select treatment techniques. Using identical CT-based volumetric data as input for generation of Beam's Eye View (BEV) designed blocks, DVH curves demonstrate dosimetric advantages of non-axial techniques over conventional parallel-opposed orientations. Assessment of the non-axial techniques in selected cases indicates that uniform target volume coverage could be maintained with a typical reduction of 30% in the total amount of brain tissue treated to high dose (95% isodose line).

Improving precision and safety in the use of beam modifying devices in radiation therapy

Reliable and safe implementation of beam modifying devices such as wedges and block trays requires careful design and construction. Inappropriate design may pose problems ranging from user-hostile operation to hard-to-track, but significant variations in actual position in a beam. This may cause variation in actual wedge output factors, or variation in the position of a block tray. In case of simple mechanical failure or personnel mistake, design related mechanical conditions may result in injury to either a patient or a staff member. This paper is based on experience with linear accelerators from one manufacturer, but similar conditions are likely to exist with other radiation machines. A simple technical modification is offered which improves both accuracy and reproducibility in the placement of wedge-type filters. For our machines the solution also provides improved safety in the use of both wedge trays and block trays.

Full integration of the beam's eye view concept into computerized treatment planning

A complete set of beam's eye view (BEV) and beam portal design features have been integrated into a computerized 3-dimensional radiotherapy treatment planning system. Among the features implemented is the ability to mix BEV graphics with gray-scale images such as simulator and verification radiographs, and digital reconstructed radiographs. Image processing techniques have been developed to both enhance verification images and to detect radiation field boundaries. These portal simulation and presentation techniques are being used clinically to design and verify radiation fields with manual or automatically-designed field shaping blocks. The ability to perform computer dose calculations for planes which are parallel or perpendicular to a specified beam's central axis is available and this feature has also proven useful for treatment plan evaluation and optimization. Finally, direct comparison of computer-generated portal images with actual simulation and verification radiographs is also possible. These techniques allow the direct integration of "CT-directed treatment planning" with block design, simulator films and port films, and other Beam's Eye View-type displays.

A CT-compatible version of the Fletcher system intracavitary applicator: clinical application and 3-dimensional treatment planning

A new acrylic version of the familiar Fletcher intracavitary applicator, the Ann Arbor (AA) applicator, has been developed. This new device eliminates the problem of "streak" artifacts on CT images, but unlike other plastic applicators the ability to shield portions of the bladder and rectum is retained through the use of tungsten alloy shields which are afterloaded with the radioactive sources. To minimize changes in placement geometry and to take advantage of the wide clinical experience with the Fletcher system, the new applicator nearly duplicates the physical dimensions of the Fletcher applicator. With the Ann Arbor applicator in place, dummy sources are easier to locate on standard radiographic simulations. CT scans are free of artifact and provide clear, detailed visualizations of cross-sectional anatomy. The new applicator thus allows CT images to be used to their potential in evaluating crucial anatomic relationships and in performing 3-D dosimetry with dose volume analysis. Using a treatment planning system with 3-D capabilities, solid surface graphic display of applicator, cervix, rectum, bladder, and treatment isodose volume has been performed. In addition, dose volume histograms can be generated to obtain precise measurements of the volume of cervix, rectum, or bladder receiving specified doses.

Comparison of automated and manual shielding block fabrication

This work reports the results of a study comparing computer controlled and manual shielding block cutting. The general problems inherent in automated block cutting have been identified and minimized. A system whose accuracy is sufficient for clinical applications has been developed. The relative accuracy of our automated system versus experienced technician controlled cutting was investigated. In general, it is found that automated cutting is somewhat faster and more accurate than manual cutting for very large fields, but that the reverse is true for most smaller fields. The relative cost effectiveness of automated cutting is dependent on the percentage of computer designed blocks which are generated in the clinical setting. At the present time, the traditional manual method is still favored.

Boost treatment of the prostate using shaped, fixed fields

Using a CT-based, 3-D treatment planning system and Beam's Eye-View (BEV) displays, shaped fixed-field techniques have been developed for external beam boost treatment of Stage C carcinoma of the prostate. The basic technique comprises three sets of opposing beams (laterals and +/- 45 degrees with respect to the lateral) into a 6-field arrangement. Target volumes together with bladder and rectal wall volumes are outlined on axial CT slices and combined to form 3-D volumes. For each field, an interactive BEV display is produced showing the target volume in its correct 3-D geometrical perspective and an auto-block routine is used to design focused blocks which conform to that volume. Full 3-D volume calculations computed for those plans on 17 patients were analyzed along with similar calculations for more traditional unblocked 4-field box and bilateral arc techniques. Compared to the 95% isodose volume for the 6-field conformational technique, traditional open beam full target coverage techniques typically produce high dose volumes which cover up to five times as much uninvolved tissue. Dose volume histograms illustrate that typically half as much bladder and rectal tissue is treated to high dose using the conformational boost techniques. From the dosimetric perspective of sparing normal tissues, shaped fixed-field boost techniques are shown to be clearly superior to traditional full coverage bilateral arc techniques. Smaller 8 cm X 8 cm arc techniques are shown to be quantitatively unacceptable for treatment of this advanced stage disease, as they typically misses 20-35% of the target volume.

From manual to 3-D computerized treatment planning for 125-I stereotactic brain implants

Aspects of planning for the treatment of high grade primary or recurrent brain tumors with stereotactically placed catheters afterloaded with high activity 125-I seeds are discussed. At our institution, planning has evolved from a simple manual process, which assumed geometric symmetry, through a more advanced manual process, that took advantage of certain mechanical properties of the stereotactic frame used, into a sophisticated, computerized planning approach that includes optimization of the source distribution and 3-D displays. Use of the simple manual method is limited to the rare situations where target volumes are quite regular in shape. The advanced manual method provides some customization for irregularly shaped volumes, but is slow and tedious to implement. The interactive, computerized approach permits identification of target volumes directly on CT slices, reconstructions in arbitrary planes, and optimization of catheter placement, source separation along each catheter, and selection of source strengths from an available inventory. A multi-format display feature which includes a probe's eye view perspective is provided to aid in planning. Integral dose-volume histograms for the target volume point out the advantages in using sophisticated, 3-D, computerized planning systems for these implants.