Stereotactic radiotherapy of irregular targets: a comparison between static conformal beams and non-coplanar arcs

The effect of tumor shape irregularity on Gamma Knife treatment plan quality and treatment outcome: an analysis of 234 vestibular schwannomas

The primary aim of Gamma Knife (GK) radiosurgery is to deliver high-dose radiation precisely to a target while conforming to the target shape. In this study, the effects of tumor shape irregularity (TSI) on GK dose-plan quality and treatment outcomes were analyzed in 234 vestibular schwannomas. TSI was quantified using seven different metrics including volumetric index of sphericity (VioS). GK treatment plans were created on a single GK-Perfexion/ICON platform. The plan quality was measured using selectivity index (SI), gradient index (GI), Paddick's conformity index (PCI), and efficiency index (EI). Correlation and linear regression analyses were conducted between shape irregularity features and dose plan indices. Machine learning was employed to identify the shape feature that predicted dose plan quality most effectively. The treatment outcome analysis including tumor growth control and serviceable hearing preservation at 2 years, were conducted using Cox regression analyses. All TSI features correlated significantly with the dose plan indices (P < 0.0012). With increasing tumor volume, vestibular schwannomas became more spherical (P < 0.05) and the dose plan indices varied significantly between tumor volume subgroups (P < 0.001 and P < 0.01). VioS was the most effective predictor of GK indices (P < 0.001) and we obtained 89.36% accuracy (79.17% sensitivity and 100% specificity) for predicting PCI. Our results indicated that TSI had significant effects on the plan quality however did not adversely affect treatment outcomes.

A model of radiation-induced temporomandibular joint damage in mice

PURPOSE

A small animal radiation research platform (SARRP) equipped with a miniature beam system, an image-guided positioning system, and a dose planning system was used to develop and evaluate a mouse model of radiation-induced temporomandibular damage.

METHODS

Left jaw disks of adult male C57BL/6 mice and C3H mice were targeted using the SARRP for image-guided irradiation. The total radiation dose was 75 Gy. Experiment 1 (Scoping study): Mice in the C57BL/6 mouse test and control groups were sacrificed at 1, 3, 6, 9, 12, 15, and 18 weeks after irradiation, whereas mice in the C3H test and control groups were sacrificed at 1, 3, 6, 9, and 12 weeks after irradiation. Experiment 2 (Full -scale validation study): Mice in the C57BL/6 mouse test and control groups were sacrificed at 1, 3 and 6 weeks after irradiation. Histopathological analysis of the temporomandibular skeletal muscle in each group was performed using hematoxylin and eosin (H&E) and Masson staining; the temporal mandibular bone was examined through H&E staining.

RESULTS

SARRP delivered the rated dose to the temporomandibular joints of C57BL/6 and C3H mice. C3H and C57BL/6 mice in the test group showed different degrees of osteocytic necrosis and osteoporosis at different time points. H&E staining of skeletal muscle tissue showed slight fibrosis in the C57BL/6 test at 3 and 6 weeks time point.

CONCLUSION

We established a model of radiation-induced damage in the temporomandibular joint of C57BL/6 mice and demonstrated that the observed physiological and histological changes correspond to radiation damage observed in humans. Furthermore, the SARRP can deliver precise radiation doses.

Stereotactic radiotherapy for small and very small tumours (≤1 to ≤3 cc): evaluation of the influence of volumetric-modulated arc therapy in comparison to dynamic conformal arc therapy and 3D conformal radiotherapy as a function of flattened and unflattened beam models

Purpose:

The objective of this article is to evaluate the dosimetric efficacy of volumetric modulated arc therapy (VMAT) in comparison to dynamic conformal arc therapy (DCAT) and 3D conformal radiotherapy (3DCRT) for very small volume (≤1 cc) and small volume (≤3 cc) tumours for flattened (FF) and unflattened (FFF) 6 MV beams.

Materials and methods:

A total of 21 patients who were treated with single-fraction stereotactic radiosurgery, using either VMAT, DCAT or 3DCRT, were included in this study. The volume categorisation was seven patients each in <1, 1–2 and 2–3 cc volume. The treatment was planned with 6 MV FF and FFF beams using three different techniques: VMAT/Rapid Arc (RA) (RA_FF and RA_FFF), dynamic conformal arc therapy (DCA_FF and DCA_FFF) and 3DCRT (Static_FF and Static_FFF). Plans were evaluated for target coverage (V100%), conformity index, homogeneity index, dose gradient for 50% dose fall-off, total MU and MU/dose ratio [intensity-modulated radiotherapy (IMRT) factor], normal brain receiving >12 Gy dose, dose to the organ at risk (OAR), beam ON time and dose received by 12 cc of the brain.

Result:

The average target coverage for all plans, all tumour volumes (TVs) and delivery techniques is 96·4 ± 4·5 (range 95·7 ± 6·1–97·5 ± 2·9%). The conformity index averaged over all volume ranges <1, 2, 3 cc> varies between 0·55 ± 0·08 and 0·68 ± 0·04 with minimum and maximum being exhibited by DCA_FFF for 1 cc and Static_FFF/RA_FFF for 3 cc tumours, respectively. Mean IMRT factor averaged over all volume ranges for RA_FF, DCA_FF and Static_FF are 3·5 ± 0·8, 2·0 ± 0·2 and 2·0 ± 0·2, respectively; 50% dose fall-off gradient varies in the range of 0·33–0·42, 0·35–0·40 and 0·38–0·45 for 1, 2 and 3 cc tumours, respectively.

Conclusion:

This study establishes the equivalence between the FF and FFF beam models and different delivery techniques for stereotactic radiosurgery in small TVs in the range of ≤1 to ≤3 cc. Dose conformity, heterogeneity, dose fall-off characteristics and OAR doses show no or very little variation. FFF could offer only limited time advantage due to excess dose rate over an FF beam.

Efficacy of Stereotactic Conformal Radiotherapy vs Conventional Radiotherapy on Benign and Low-Grade Brain Tumors: A Randomized Clinical Trial

Importance

Evidence for application of stereotactic and other conformal radiotherapy techniques in treating brain tumors is largely based on data derived from dosimetric, retrospective, or small prospective studies. Therefore, we conducted a randomized clinical trial of stereotactic conformal radiotherapy (SCRT) compared with conventional radiotherapy (ConvRT) evaluating clinically meaningful end points.

Objective

To compare neurocognitive and endocrine functional outcomes and survival at 5 years in young patients with residual and/or progressive benign or low-grade brain tumors treated with SCRT and ConvRT techniques.

Design, Setting, and Participants

This phase 3 randomized clinical trial enrolled 200 young patients (ages 3-25 years) with residual or progressive benign or low-grade brain tumors at a single center between April 2001 to March 2012. Patients were randomly allocated (1:1) to either SCRT (n = 104) or ConvRT (n = 96) arms.

Interventions

Patients were randomly assigned to either high-precision SCRT or ConvRT to a dose of 54 Gy in 30 fractions over 6 weeks.

Main Outcomes and Measures

Detailed neuropsychological and neuroendocrine assessments were performed at preradiotherapy baseline, at 6 months, and annually thereafter until 5 years on longitudinal follow-up. Change in these functional parameters was compared between the 2 arms as the primary end point and overall survival (OS) as the secondary end point.

Results

In total, 200 young patients (median [interquartile range] age, 13 [9-17] years; 133 males and 67 females) were enrolled. Mean full-scale or global intelligence quotient (IQ) and performance IQ scores over a period of 5 years were significantly superior in patients treated with SCRT compared with those treated with ConvRT (difference in slope = 1.48; P = .04 vs difference in slope = 1.64; P = .046, respectively). Cumulative incidence of developing new neuroendocrine dysfunction at 5 years was significantly lower in patients treated with SCRT compared with ConvRT (31% vs 51%; P = .01) while developing a new neuroendocrine axis dysfunction in patients with preexisting dysfunction in at least 1 axis at baseline was also significantly lower in the SCRT arm compared with the ConvRT arm (29% vs 52%; P = .02). Five-year OS in SCRT and ConvRT arms was 86% and 91%, respectively (P = .54).

Conclusions and Relevance

In young patients with residual and/or progressive benign or low-grade brain tumors requiring radiotherapy for long-term tumor control, SCRT compared with ConvRT achieves superior neurocognitive and neuroendocrine functional outcomes over 5 years without compromising survival.

Trial Registration

clinicaltrials.gov Identifier: NCT00517959.

Linear accelerator radiosurgery for arteriovenous malformations: Updated literature review

Arteriovenous malformations (AVMs) are the leading causing of intra-cerebral haemorrhage. Stereotactic radiosurgery (SRS) is an established treatment for arteriovenous malformations (AVM) and commonly delivered using Gamma Knife within dedicated radiosurgery units. Linear accelerator (LINAC) SRS is increasingly available however debate remains over whether it offers an equivalent outcome. The aim of this project is to evaluate the outcomes using LINAC SRS for AVMs used within a UK neurosciences unit and review the literature to aid decision making across various SRS platforms. Results have shown comparability across platforms and strongly supports that an adapted LINAC based SRS facility within a dynamic regional neuro-oncology department delivers similar outcomes (in terms of obliteration and toxicity) to any other dedicated radio-surgical platform. Locally available facilities can facilitate discussion between options however throughput will inevitably be lower than centrally based dedicated national radiosurgery units.

Noncoplanar verification: a feasibility study using Philips' Pinnacle3 treatment planning system

Noncoplanar fields are normally used to improve the dose conformity of the target while sparing organs at risk. One of the methods to verify the dose distribution from the noncoplanar fields is by comparing their planar dose distributions from the treatment planning system (TPS) and the measured ones; for example, using film or electronic portal imaging devices (EPID). The planar dose distributions of the measurement tools, that are normally perpendicular to the central axis of the beam, can be calculated by creating special structures to mimic them in the TPS. With TPS commercially available today, however, it is not easy to create these special structures, especially in the noncoplanar configuration. For this work, we have written in‐house scripts in the Pinnacle³ TPS that can create the structures and define them as virtual planes. These virtual planes can be generated for any arbitrary gantry and couch angles, as well as source to planar distance, so that the planar dose maps at these planes can be computed. Two independent quality assurance (QA) tools were used to validate the planar dose distributions computed using the scripts for three open fields and one IMRT field at several different couch angles. The absolute planar dose patterns measured by the QA tools for all fields at all couch angles were found to be in good agreement, more than 95% (gamma criteria 3% delta dose and 3 mm distance to agreement), with the calculated ones by TPS. The results of this feasibility study can be valuable either for pretreatment dose verification or for in vivo dosimetry that directly implements the planar dose distributions from the TPS, particularly for the noncoplanar fields.

PACS numbers: 87.55.de, 87.55.Qr, 87.56.Fc

Target splitting non-coplanar RapidArc radiation therapy for a diffuse sebaceous carcinoma of the scalp: a novel delivery technique

Background and purpose

To compare conventional lateral photon-electron, fixed-beam intensity modulated radiation therapy (IMRT), coplanar and non-coplanar RapidArc for the treatment of a diffuse sebaceous gland carcinoma of the scalp.

Methods

Comprehensive dosimetry comparisons were performed among 3D-CRT, IMRT and various RapidArc plans. Target coverage, conformity index (CI), homogeneity index (HI) and doses to organs at risk (OAR) were calculated. Monitor unites (MUs) and delivery time of each treatment were also recorded to evaluate the execution efficiency. The influence of target splitting technique and non-coplanar planning on plan quality was discussed.

Results

IMRT was superior to 3D-CRT concerning targets’ coverage at the sacrifice of larger irradiated brain volumes to low doses. CIs and HIs were better in coplanar RapidArc and non-coplanar RapidArc plans than 3D-CRT and IMRT. Best dose coverage and sparing of OARs were achieved in non-coplanar plans using target splitting technique. Treatment delivery time was longest in the IMRT plan and shortest in the coplanar RapidArc plan without target splitting. The 3%/3 mm gamma test pass rates were above 95% for all the plans.

Conclusions

Target splitting technique and non-coplanar arcs are recommended for total scalp irradiation.

Comparison of stereotactic plans for brain tumors with two different multileaf collimating systems

Linac‐based stereotactic radiosurgery (SRS) has been widely used for treating small intracranial lesions. This technique allows conforming the dose distribution to the planning target volume (PTV), providing a steep dose gradient with the surrounding normal tissues. This is realized through dedicated collimation systems. The present study aims to compare SRS plans with two collimating systems: the beam modulator (BM) of the Elekta Synergy linac and the DirexGroup micromultileaf collimator (μMLC). Seventeen patients (25 PTVs) were planned both with BM and μMLC (mounted on an Elekta Precise linac) using the Odyssey (PerMedics) treatment planning system (TPS). Plans were compared in terms of dose‐volume histograms (DVH), minimum dose to the PTV, conformity index (CI), and homogeneity index (HI), as defined by the TPS, and doses to relevant organs at risk (OAR). The mean difference between the μMLC and the BM plans in minimum PTV dose was 5.7%±4.2% in favor of the μMLC plans. No statistically significant difference was found between the distributions of the CI values for the two planning modalities (p=0.54), while the difference between the distributions of the HI values was statistically significant (p=0.018). For both BM and μMLC plans, no differences were observed in CI and HI, depending on lesion size and shape. The PTV homogeneity achieved by BM plans was 15.1%±6.8% compared to 10.4%±6.6% with μMLC. Higher maximum and mean doses to OAR were observed in the BM plans; however, for both plans, dose constraints were respected. The comparison between the two collimating systems showed no substantial differences in terms of PTV coverage or OAR sparing. The improvements obtained by using μMLC are relatively small, and both systems turned out to be adequate for SRS treatments.

PACS numbers: 87.53.Ly, 87.55.dk, 87.56.nk

Long-term efficacy of fractionated radiotherapy for benign meningiomas

PURPOSE

To assess long term efficacy of fractionated stereotactic radiotherapy (fSRT) in the treatment of benign intracranial meningiomas.

MATERIALS AND METHODS

Retrospective study of 222 patients with histologically confirmed (58%) and unverified presumed (42%) grade I intracranial meningioma treated with fSRT in a single institution to doses of 50-55Gy in 30-33 fractions.

RESULTS

At a median follow-up of 43months (range 3-144) the 5 and 10years local control (LC) were 93% and 86%. Patients with tumors involving the optic nerve (42 patients) and patients with cavernous sinus/parasellar region meningiomas (78 patients) had 5 and 10years LC of 100%. The 5 and 10years survival probabilities were 93% and 84%. On multivariate analysis gender and tumor site were independent predictors of LC. Worsening of pre-existing cranial nerve deficit occurred in 8 (3.5%) and onset of new deficit in 1 (0.5%) patient. Two patients with optic nerve sheath meningioma (1%) developed radiation retinopathy. There were no cases of radiation necrosis or second brain tumors.

CONCLUSION

fSRT achieves excellent medium and long term tumor control with minimal morbidity particularly in patients with benign meningiomas involving the parasellar region and the optic nerves and questions the role of other treatment modalities for tumors at these locations.

Radiotherapy for pituitary adenomas

This article describes the technical aspects and the clinical results of conventional radiotherapy and modern stereotactic radiotherapy for pituitary adenomas. Systematic review of the published literature provides a factual basis for the comparison and the selection of appropriate radiation technique in patients who have secreting and nonfunctioning pituitary adenomas not cured with surgery and medical therapy.

Killing two birds with one stone: a dosimetric study of dual target radiosurgery using a single isocenter

The treatment of hematogenous brain metastases is a frequent indication for stereotactic radiosurgery (SRS). It is common for more than one metastasis to be treated during the same SRS session. We retrospectively identified four cases where our m3 micro multileaf collimator (mMLC) was used to create two distinct apertures and treat adjacent lesions using a single isocenter. For these four cases, single isocenter plans with static conformal beams were dosimetrically compared to plans utilizing two isocenters with static conformal beams or conformal arcs. The effects on dose homogeneity, dose conformity, and the minimum isodose separating the two targets are minor and variable. On the other hand, the use of a single isocenter technique consistently halves delivery time and decreases the integral dose to normal tissue. For small adjacent metastases, which can simultaneously be encompassed within the high-resolution portion of the m3/Novalis mMLC collimator, the use of a single rather than a dual isocenter technique is feasible and generally advantageous.

Optimization of the primary collimator settings for fractionated IMRT stereotactic radiotherapy

Advances in field-shaping techniques for stereotactic radiosurgery/radiotherapy have allowed dynamic adjustment of field shape with gantry rotation (dynamic conformal arc) in an effort to minimize dose to critical structures. Recent work evaluated the potential for increased sparing of dose to normal tissues when the primary collimator setting is optimized to only the size necessary to cover the largest shape of the dynamic micro multi leaf field. Intensity-modulated radiotherapy (IMRT) is now a treatment option for patients receiving stereotactic radiotherapy treatments. This multisegmentation of the dose delivered through multiple fixed treatment fields provides for delivery of uniform dose to the tumor volume while allowing sparing of critical structures, particularly for patients whose tumor volumes are less suited for rotational treatment. For these segmented fields, the total number of monitor units (MUs) delivered may be much greater than the number of MUs required if dose delivery occurred through an unmodulated treatment field. As a result, undesired dose delivered, as leakage through the leaves to tissues outside the area of interest, will be proportionally increased. This work will evaluate the role of optimization of the primary collimator setting for these IMRT treatment fields, and compare these results to treatment fields where the primary collimator settings have not been optimized.

An integrated treatment delivery system for CSRS and CSRT and clinical applications

An integrated treatment delivery system for conformal stereotactic radiosurgery (CSRS) and radiotherapy (CSRT) has been developed through a collaboration involving Siemens Medical Systems, Inc., Tyco/Radionics, Inc., and The University of Texas M. D. Anderson Cancer Center. The system consists of a 6-MV linear accelerator (LINAC) equipped with a Tyco/Radionics miniature multileaf collimator (mMLC). For the conventional SRS treatment, the circular collimator housing can be attached to the opening window of the mMLC. The treatment delivery system is integrated with a radiotherapy treatment planning system and a record-and-verify system. The purpose of this study is to report the characteristics, performance, benefits, and the clinical applications of this delivery system. The technical specifications of the LINAC and mMLC were tested, and all the specifications were met. The 80% to 20% penumbral width for each mMLC leaf is approximately 3 mm and is nearly independent of the off-axis positions of a leaf. The maximum interleaf leakage is 1.4% (1.1% on average) and the maximum intra-leaf leakage is 1.0% (0.9% on average). The leaf position precision is better than 0.5 mm for all the leaves. The integration of the SRS/SRT treatment planning system, mMLC, and LINAC has been evaluated successfully for transferring the patient treatment data file through radiotherapy treatment planning system to the patient information and treatment record-and-verify server and the mMLC controller. Subsequently, the auto-sequential treatment delivery for SRS, CSRS/CSRT, and the step-and-shoot intensity-modulated radiotherapy has also been tested successfully. The accuracy of dose delivery was evaluated for a 2-cm spherical target in a Radiological Physics Center SRS head phantom with GAFChromic films and TLD. Five non-coplanar arcs, using a 2-cm diameter circular collimator, were used for this simulation treatment. The accuracy to aim the center of the spherical target was within 0.5 mm and the deviation of dose delivery to the isocenter of the target was within 2% of the calculated dose. For the irregularly shaped tumor, a tissue-equivalent head phantom was used to evaluate the accuracy of dose delivery for using either geometric conformal treatment or IMRT. The accuracy of dose delivery to the isocenter was within 2% and 3% of the calculated dose, respectively. From October 26, 1999 to September 30, 2002, we treated over 400 SRS patients and 70 SRT patients. Four representative cases are presented to illustrate the capabilities of this dedicated unit in performing conventional SRS, CSRS, and CSRT. For all the cases, the geometric conformal-plan dose distributions showed a high degree of conformity to the target shape. The degree of conformity can be evaluated using the target-volume-ratio (TVR). Our preferred TVR values for highly conformed dose distributions range from 1.6 to 2.0. The patient setup reproducibility for the Gill-Thomas-Cosman (GTC) noninvasive head frame ranges from 0.5 to 1 mm, and the head and neck noninvasive frame is within 2 mm. The integrated treatment delivery system offers excellent conformation for complicated planning target volumes with the stereotactic setup approach, ensuring that dose delivery can be achieved within the specified accuracy. In addition, the treatment time is comparable with that of single isocenter multiple-arc treatments.

Quality assurance devices for dynamic conformal radiotherapy

Two quality control devices, a light‐field device and a radiation‐field device, have been specially designed to assist the clinical implementation of conformal dynamic arc treatment (CDAT) and intensity‐modulated radiation therapy (IMRT). With these devices, the light field as well as the radiation field projected from the individual beams at any treatment position (i.e., arbitrary gantry angle) can be evaluated. The devices are attached at the front end of the couch and placed at the isocenter of the linear accelerator treatment system (LINAC). The devices are designed to be able to rotate parallel to the gantry head so that the light field and the radiation field projected from a direct beam can be assessed. The aim of this study was to evaluate the geometric precision of the beam placement and the dosimetric accuracy performed in CDAT and IMRT with the aid of these devices. The devices are placed separately from the LINAC during use and provide an independent check on the quality performance of the LINAC in three dimensions. The condition of gantry sagging and any mechanical displacement resulting in field shift can be observed and traced during gantry rotation. Mistakes that occur during the isocenter calibration can lead to significant displacement in the field projection, which would not be revealed with the conventional quality control setting (i.e., gantry 0°). This was demonstrated with the aid of the two quality control devices in the study. The influence of gravitational acceleration in the multi‐leaf collimator (MLC) leaf positioning error, which would consequently lead to inaccurate dose delivery, was investigated. The results of our study show that the existence of a gravitational influence is statistically significant, although the magnitude of the dose inaccuracy is small.

PACS numbers: 87.53.Kn, 87.56.Fc, 87.66.Cd

Comparison of a micro-multileaf collimator with a 5-mm-leaf-width collimator for intracranial stereotactic radiotherapy

PURPOSE

To dosimetrically compare a micro-multileaf collimator (minimum leaf width of 3 mm) with the 5-mm-leaf multileaf collimator (MLC) of a standard linear accelerator for stereotactic conformal radiotherapy treatment of intracranial lesions.

MATERIALS AND METHODS

Fourteen patients previously treated for a variety of irregularly shaped intracranial lesions using BrainLAB's micro-MLC were retrospectively replanned using the Varian Millennium MLC (5 mm leaf width). All planning was performed with the BrainSCAN v 5.1 software. The same fixed, noncoplanar beam arrangement was used for both plans, and identical target coverage was achieved by adjusting the MLC shape around the planning target volume (PTV). The isodose distributions and dose-volume histograms (DVH) were computed and plans were compared in terms of conformity of the prescription isodose to the PTV and dose received by surrounding normal tissue.

RESULTS

Equivalent PTV coverage was achieved using the 5-mm collimator by adjusting the MLC shape around the target in every case. There was a statistically significant increase in the conformity index for the Varian MLC compared with the micro-MLC (p < 0.001), indicating a worse conformity of the prescription isodose to the PTV, but this parameter was within our (and Radiation Therapy Oncology Group) clinical criterion in all cases. There was no statistically significant difference in the maximum dose to critical structures, but DVH curves demonstrated an increased volume of normal tissue irradiated to the lower isodose levels. The mean increase in the volume of critical structure enclosed within the 50% and 70% isodose surfaces was 5.7% and 4.9%, respectively.

CONCLUSIONS

The micro-MLC consistently improves both PTV conformity and surrounding tissue sparing when compared to that of a standard linear accelerator. However, when viewed quantitatively, the improvements are small enough that individual centers may question their choice of equipment when outfitting a stereotactic radiotherapy service.

Dosimetric Comparison of CyberKnife with Other Radiosurgical Modalities for an Ellipsoidal Target

OBJECTIVE

To compare treatment plans obtained with the CyberKnife (CK) (Accuray, Inc., Sunnyvale, CA) with those of other commonly used radiosurgical modalities, such as the gamma knife (GK), linear accelerator multiple arcs, conformally shaped static fields, and intensity-modulated radiotherapy (IMRT).

METHODS

An ellipsoidal simulated target was chosen centrally located in a three-dimensional model of a patient's head acquired with magnetic resonance or computed tomographic imaging. It was 25 mm in diameter and 35 mm long. The aims of treatment plans were 100% target volume coverage with an appropriate isodose line, minimum radiation dose to normal tissue, and clinically acceptable delivery. These plans were evaluated by use of a dose-volume histogram and other commonly used radiosurgical parameters such as target coverage, homogeneity index, and conformity index.

RESULTS

All selected treatment modalities were equivalent in providing full target coverage. For dose homogeneity, all modalities except for multiple isocenter plans for GK (homogeneity index, 2.0) were similar (homogeneity index, ≅1.25). Dose conformity was essentially equivalent for all treatment plans except for IMRT, which had a slightly higher value (conformity index, ≅1.27). There was a substantial variation in the radiation dose to normal tissue between the studied modalities, particularly at the lower dose levels.

CONCLUSION

CK plans seemed to be more flexible for a given target size and shape. For a target of limited volume and essentially of any shape, one could obtain similarly good conformal dosimetry with CK and GK. For a regular-shaped but other than spherical target, homogeneous dose distribution could be obtained with all selected modalities except for multiple isocenters, linear accelerator multiple arcs, or GK. Both IMRT and conformally shaped static fields offered good alternative treatment modalities to CK, GK, or linear accelerator multiple arc radiosurgery, with slightly inferior dosimetry in conformity (IMRT).

Intensity-modulated stereotactic radiotherapy vs. stereotactic conformal radiotherapy for the treatment of meningioma located predominantly in the skull base

PURPOSE

This study evaluates a possible advantage of intensity-modulated stereotactic radiotherapy (IMSRT) over stereotactic conformal radiotherapy (SCRT) in the treatment of lesions in the base of the skull.

METHODS AND MATERIALS

Ten patients (7 with a skull base meningioma) planned for routine SCRT were replanned for IMSRT. The criteria for comparison were the same for both methods: optimal dose to the planning target volume (PTV) and optimal sparing of the organs at risk (OAR). For SCRT, sparing of OAR was achieved by conformal avoidance using 5-6 fields. The IMSRT inverse planning process used optimized OAR sparing through user-defined dose constraints. Dose to the PTV and OAR were assessed by dose-volume histograms, maximum dose, 2 conformity indices, and volumes of relevant isodoses.

RESULTS

The conformity index is consistently higher for IMSRT, the largest improvement being for the multifocal and irregular cases. Volumes of the 90% and 80% isodoses were smaller for IMSRT, whereas the volume of the 30% isodose was larger for IMSRT in 6 cases. The maximum dose was consistently higher for IMSRT (mean values 102% and 108% for SCRT and IMSRT, respectively). Sparing of OAR was better with IMSRT, especially for those OARs situated in or near a concave PTV.

CONCLUSIONS

In terms of PTV coverage, there is an advantage in using IMSRT for all target shapes, but especially for irregular and concave targets. The dose to OAR is lower with IMSRT, although the volume of normal tissue receiving a low dose can be larger than for SCRT.

Treatment planning for stereotactic radiosurgery with photon beams

Stereotactic Radiosurgery (SRS) has evolved as a unique discipline that combines aspects of both surgery and radiation oncology. Technological developments in the past few decades have provided a wide array of treatment techniques, including (i) the Gamma Knife; (ii) Linac-based stereotactic techniques using circular collimators or using micro multileaf collimators (mMLCs); (iii) the Cyber Knife, using an x-band linac mounted on a robotic arm; and (iv) serial and spiral tomotherapy. This paper provides a review of the treatment planning methods for stereotactic radiosurgery. Because of the differences in planning strategies used for each SRS technique, this paper will provide both a general review of the pre-requisites and common features of SRS treatment planning and the planning techniques specific to each of the SRS techniques.

Gentlemen (and ladies), choose your weapons: Gamma knife vs. linear accelerator radiosurgery

This article compares and contrasts Gamma Knife radiosurgery with linear accelerator-based radiosurgery; where appropriate, Cyberknife technology is discussed. Topics covered are: positioning of the head (invasive versus non-invasive positioning systems); collimator construction; beam properties; beam arrangements; treatment planning; and issues regarding manpower (including a discussion of patient repositioning during treatment), machine availability, and financial considerations.

Impact of a micromultileaf collimator on stereotactic radiotherapy of uveal melanoma

PURPOSE

To evaluate the impact of a micro multileaf collimator (mMLC) on Linac-based stereotactic radiotherapy (SRT) of uveal melanoma by comparing circular arc with static conformal, dynamic arc, and intensity-modulated SRT.

MATERIALS AND METHODS

Forty uveal melanoma patients were selected from approximately 100 patients treated with SRT since 1996. For each patient, four treatment plans (BrainSCAN XL, V5.0) were made: conventional arc, static conformal, dynamic arc plan, and intensity-modulated radiotherapy (IMRT). The goal of treatment planning was to fully encompass the planning target volume (PTV) by the 80% isodose while minimizing doses to the optic nerve and lens. The following parameters were evaluated: target conformity; target homogeneity; ratio of the target volume and 50% isodose volume; normal tissue receiving doses >/=80%, >/=50%, and >/=20%; central nervous system volume irradiated to >/=20%; optical nerve volume irradiated >/=50%, D(max) of the lens; lens volume receiving >/=20%; and monitor units.

RESULTS

PTVs ranged from 0.68 to 4.90 cm(3) (mean 1.97 +/- 0.97 cm(3)). The average reduction of the prescription isodose volume was 1-1.5 cm(3) for conformal (range 2.6-0.3 cm(3)), dynamic arc (range 2.5-0.3 cm(3)), and IMRT plans (range 3.9-0.1 cm(3)), compared with conventional arc therapy. Central nervous system volumes irradiated to doses >/=20% were smallest for conventional or dynamic arc treatments. Average target dose homogeneity values were 1.74 +/- 0.50 for arc, 1.27 +/- 0.02 for static mMLC, 1.26 +/- 0.01 for dynamic arc, and 1.15 +/- 0.03 for IMRT plans. IMRT helped to reduce doses to the lens but did not provide an advantage for optical nerve sparing. When applying IMRT, the monitor units increased by approximately one-third compared with static mMLC-based SRT.

CONCLUSIONS

Conformal mMLC and dynamic arc SRT are the treatment options of choice for Linac-based SRT of uveal melanoma. They present dosimetric advantages, while being highly efficient in treatment planning and delivery.

Conformal radiotherapy optimization with micromultileaf collimators: comparison with radiosurgery techniques

PURPOSE

Conformal radiotherapy (CRT) consists of irradiating the target volume while avoiding the healthy peripheral tissues and organs at risk as far as possible. One technique used to treat intracranial tumors consists of using micromultileaf collimators (MMLCs). Given the dose constraints involved, it is of interest to optimize MMLC irradiation parameters and compare the results of this technique with those of conventional radiosurgery (RT) techniques (Gamma Knife and linear accelerator stereotactic RT).

METHODS AND MATERIALS

MMLC protocols are optimized in two stages. The orientation of the fields, delimited by a beam's eye view technique, is determined using a genetic algorithm method. The weighting of the fields and subfields when using intensity modulation and the position of the leaves are optimized using a simulated annealing method. We compared the results obtained for 8 clinical cases using 5 intensity-modulated fields with those obtained using the two radiosurgery techniques. The comparison indexes are those defined by the Radiation Therapy Oncology Group (RTOG).

RESULTS

The results of this study demonstrated the advantages of using intensity modulation and the improvement obtained for the RTOG indexes in the case of CRT with MMLC, although the healthy peripheral tissues were less exposed to radiation with the radiosurgery techniques. The results also highlight the difficulty encountered with radiosurgery techniques in obtaining satisfactory dose homogeneity when the protocol is defined with numerous iosocenters.

CONCLUSION

In CRT with MMLC, intensity modulation makes it possible to reduce the number of fields used. It is especially useful to optimize the orientations in the case of target volumes of complex shape or when volumes at risk are in the vicinity of the target. If used correctly, MMLC can be a valuable alternative to conventional radiosurgery techniques.

Stereotactically guided conformal radiotherapy for progressive low-grade gliomas of childhood

PURPOSE

To describe the rationale, technique, and early results of stereotactically guided conformal radiotherapy (SCRT) in the treatment of progressive or inoperable low-grade gliomas (LGGs) of childhood.

METHODS AND MATERIALS

Between September 1994 and May 1999, 14 children (median age 6 years, range 5-16) with LGG were treated with SCRT at the Royal Marsden NHS Trust. Tumors were located at the optic chiasm (n = 9), third ventricle (n = 2), hypothalamus, craniocervical junction, and pineal region (each n = 1). Four patients received chemotherapy before SCRT. Immobilization was in a Gill-Thomas-Cosman frame (n = 12) and subsequently in a specially designed pediatric version of the frame (n = 2). Stereotactic coordinates and the tumor were defined by CT scanning with a fiducial system and MRI fusion. The median tumor volume was 19.5 cm(3) (range 7.5-180). The planning target volume was defined as the area of enhancing tumor plus a 5-10-mm margin. The treatment technique consisted of 4 isocentric, noncoplanar, conformal, fixed fields. Treatment was delivered in 30-33 daily fractions to a total dose of 50-55 Gy.

RESULTS

SCRT was well tolerated, with transient hair loss the only acute toxicity. The median follow-up was 33 months (range 2-53). At 6 months after SCRT, 4 of 12 children with neurologic deficits improved and 5 remained stable. Twelve children were available for MRI evaluation. Two had a complete response, 6 a partial response, and 4 stable disease. One child with optic chiasm glioma had local progression at 25 months, and 1 developed diffuse leptomeningeal disease without local progression at 27 months. The 3-year local progression-free survival and overall survival rate after SCRT was 87% and 100%, respectively, compared with 89% and 98% for an historic control treated with conventional RT. New endocrine deficiencies were noted in 2 children after a follow-up of 20 and 23 months.

CONCLUSION

SCRT is a feasible, high-precision technique of RT for children with LGGs for whom RT is considered appropriate. The local control and acute toxicity of SCRT are comparable to a historic control of patients with conventionally delivered RT. The frequency of delayed hypothalamic-pituitary axis dysfunction reflects tumor location adjacent to the hypothalamus and pituitary. Additional follow-up is required to demonstrate that SCRT contributes to a reduction in treatment-related late toxicity, while maintaining the local control achieved with conventionally delivered RT in children with progressive LGGs.

High precision focused irradiation in the form of fractionated stereotactic conformal radiotherapy (SCRT) for benign meningiomas predominantly in the skull base location

OBJECTIVE

To present early clinical results of stereotactic conformal radiotherapy (SCRT) in patients with benign predominantly skull base meningiomas.

MATERIAL AND METHODS

Between August 1994 and August 1999, 41 patients with benign residual or recurrent meningiomas were treated with SCRT. Thirty-three were histologically verified. All patients were immobilized in a GTC stereotactic relocatable frame, and underwent a post-contrast CT localization scan with additional MRI for fusion in 15 patients. Treatment was delivered on a 6 MV linear accelerator using three (12 patients), or 4 (29 patients) non-coplanar conformal fixed fields to doses of 50-55 Gy in 30-33 daily fractions. Tumours were relatively large with a median gross tumour volume (GTV) of 17.9 cm3 (range 2.5-183 cm3).

RESULTS

At a median follow-up of 21 months (range 6-62 months) none of 41 patients have recurred. The current imaging tumour control rate is 100% at 1 and 3 years. The actuarial survival at 2 years is 100% and 91% at 3 and 5 years. Following SCRT tumour decreased in size in 9 patients. SCRT was well tolerated. Five patients had improvement in vision, and six patients improvement in cranial nerve function. Two patients whose planning target volume (PTV) included the sella developed hypopituitarism during and at 18 months after SCRT. One patient with pre-existing hydrocephalus due to pineal region meningioma developed cognitive impairment 7 months after treatment. One patient with involvement of the optic nerve had visual deterioration at 18 months.

CONCLUSIONS

SCRT is a feasible high precision irradiation technique for residual and recurrent skull base meningiomas including both small and larger tumours with excellent early tumour control and low toxicity. Longer follow-up is necessary to demonstrate sustained tumour control and low morbidity of such high precision localized method of fractionated irradiation.

Analysis of treatment parameters for conformal shaped field stereotactic irradiation: comparison with non-coplanar arcs

The change in configuration from circular convergent arcs to shaped static fields for stereotactic radiosurgery raises questions regarding comparability of dose distributions between the techniques. This study aims to quantify the optimization of planning parameters to achieve dose distributions minimizing dose to healthy tissue. Dose volume histograms were calculated and averaged from several patient treatments to measure dose homogeneity and healthy tissue irradiation inherent in variable PTV margins, the effect of increasing numbers of static shaped fields, the dose fall-off outside the PTV and of field placement. Our results show that adding a 2 mm margin around the target volume when defining field shapes maximizes dose coverage and homogeneity without substantially increasing the volume of healthy tissue irradiated to high dose levels. We demonstrate that 5-6 static fields may be optimal for typical lesions and that placement of these fields may not always play a major role in healthy tissue sparing. This work illustrates a systematic approach to conformal static field treatment plan optimization which relies on the prior determination of parameters such as optimum margin width to account for field penumbra. Complex irregularly shaped lesions still require careful patient-specific assessment of healthy tissue irradiation.

[Gross tumor volume (GTV) and clinical target volume (CTV) in radiotherapy of benign skull base tumors]

Skull base tumours represent about 35 to 40% of all intracranial tumours. There are now many reports in the literature confirming the fact that about 80 to 90% of such tumours are controlled with fractionated radiotherapy. Stereotactic and 3-dimensional treatment planning techniques increase local control and central nervous system tolerance. Definition of the gross tumor volume (GTV) is generally easy with currently available medical imaging systems and computers for 3-dimensional dosimetry. The definition of the clinical target volume (CTV) is more difficult to appreciate; it is defined from the CTV plus a margin, which depends on the histology and anterior therapeutic history of the tumour. It is important to take into account the visible tumour and its possible extension pathways (adjacent bone, holes at the base of skull) and/or an anatomic region (sella turcica + adjacent cavernous sinus). It is necessary to evaluate these volumes with CT Scan and MRI to appreciate tumor extension in a 3-dimentional approach, in order to reduce the risk of marginal recurrences. The aim of this paper is to discuss volume definition as a function of tumour site and tumour type to be irradiated.

Design and implementation of a system for treating paediatric patients with stereotactically-guided conformal radiotherapy

BACKGROUND AND PURPOSE

Stereotactically-guided conformal radiotherapy (SCRT) allows the delivery of highly conformal dose distributions to localised brain tumours. This is of particular importance for children, whose often excellent long-term prognosis should be accompanied by low toxicity. The commercial immobilisation system in use at our hospital for adults was felt to be too heavy for children, and precluded the use of anaesthesia, which is sometimes required for paediatric patients. This paper therefore describes the design and implementation of a system for treating children with SCRT. This system needed to be well tolerated by patients, with good access for treating typical childhood malignancies.

MATERIALS AND METHODS

A lightweight frame was developed for immobilisation, with a shell-based alternative for patients requiring general anaesthetic. Procedures were set up to introduce the patients to the frame system in order to maximise patient co-operation and comfort. Film measurements were made to assess the impact of the frame on transmission and surface dose. The reproducibility of the systems was assessed using electronic portal images.

RESULTS

Both frame and shell systems are in clinical use. The frame weighs 0.6 kg and is well tolerated. It has a transmission of 92-96%, and fields which pass through it deliver surface doses of 58-82% of the dose at d(max), compared to 18% when no frame is present. However, the frame is constructed to maximise the availability of unobstructed beam directions. Reproducibility measurements for the frame showed a mean random error of 1.0+/-0.2mm in two dimensions (2D) and 1.4+/-0.7 mm in 3D. The mean systematic error in 3D was 2.2mm, and 90% of all overall 3D errors were less than 3.4mm. For the shell system, the mean 2D random error was 1.5+/-0.2mm.

CONCLUSIONS

Two well-tolerated immobilisation devices have been developed for fractionated SCRT treatment of paediatric patients. A lightweight frame system gives a wide range of possible unobstructed beam directions, although beams that intersect the frame are not precluded, provided that output corrections are applied. A shell system allows the use of general anaesthesia. Both systems give reproducible immobilisation to complement the high-precision treatment delivery.

Intensity-modulated stereotactic radiosurgery using dynamic micro-multileaf collimation

PURPOSE

The implementation of dynamic leaf motion on a micro-multileaf collimator system provides the capability for intensity-modulated stereotactic radiosurgery (IMSRS), and the consequent potential for improved dose distributions for irregularly shaped tumor volumes adjacent to critical organs. This study explores the use of IMSRS to provide improved tumor coverage and normal tissue sparing for small cranial tumors relative to plans based on multiple fixed uniform-intensity beams or traditional circular collimator arc-based stereotactic techniques.

METHODS AND MATERIALS

Four patient cases involving small brain lesions are presented and analyzed. The cases were chosen to include a representative selection of target shapes, number of targets, and adjacent critical areas. Patient plans generated for these comparisons include standard arcs with multiple circular collimators, and fixed noncoplanar static fields with uniform-intensity beams and IMSRS. Parameters used for evaluation of the plans include the percentage of irradiated volume to tumor volume (PITV), normal tissue dose-volume histograms, and dose-homogeneity ratios. All IMSRS plans were computed using previously established IMRT techniques adapted for use with the BrainLAB M3 micro-multileaf collimator. The algorithms comprising the IMRT system for optimization of intensity distributions and conversion into leaf trajectories of the BrainLab M3 were developed at our institution. The ADAC Pinnacle(3) radiation treatment-planning system was used for dose calculations and for input of contours for target volumes and normal critical structures.

RESULTS

For all cases, the IMSRS plans showed a high degree of conformity of the dose distribution with the target shape. The IMSRS plans provided either (1) a smaller volume of normal tissue irradiated to significant dose levels, generally taken as doses greater than 50% of the prescription, or (2) a lower dose to an important adjacent critical organ. The reduction in volume of normal tissue irradiated in the IMSRS plans ranged from 10% to 50% relative to the other arc and uniform fixed-field plans.

CONCLUSION

The case studies presented for IMSRS demonstrate significant dosimetric improvements for small, irregularly shaped lesions of the brain when compared to treatments using multiple static fields or standard SRS arc techniques with circular collimators. For all cases, the IMSRS plan yielded a smaller volume of normal tissue irradiated, and/or a reduction in the volume of an adjacent critical organ (i.e., brainstem) irradiated to significant dose levels.

Optimization of primary jaw settings for stereotactic radiosurgery/radiotherapy

The primary goal of stereotactic radiosurgery/radiotherapy is to provide a technique by which the dose to a target volume can be maximized while minimizing the dose to uninvolved structures. Initially, circular apertures were applied through the use of multiple arcs and one or more isocenters in an effort to achieve these goals. Advances in field-shaping techniques, such as more elaborate cerrobend shaping and micromultileaf collimators, have allowed for improved target conformality with further reductions in dose to normal tissues. The shape of these secondary collimation devices is usually set at the precise size and shape necessary to encompass only the volume of interest with a small margin. Often, however, the primary collimators are set at a default setting that may be much larger than required to encompass the treatment area. This results in unnecessary transmission through the secondary collimators and added dose to the uninvolved tissues. This paper compares the dose delivered to normal tissues surrounding the target volume when a "standard" collimator setting is used to dose delivered when the primary collimator setting is optimized to only that necessary to encompass the treatment volume.

A comparison of dose distributions of proton and photon beams in stereotactic conformal radiotherapy of brain lesions

PURPOSE

Micromultileaf collimators (mMLC) have recently been introduced to conform photon beams in stereotactic irradiation of brain lesions. Proton beams and stereotactic conformal radiotherapy (SCRT) can be used to tailor the dose to nonspherical targets, as most tumors of the brain are irregularly shaped. Comparative planning of brain lesions using either proton or stereotactically guided photon beams was done to assess the institution's clinically available modality for three-dimensional conformal radiotherapy.

METHODS AND MATERIALS

For the photon treatment, multiple stereotactically guided uniform intensity beams from a linear accelerator were used, each conformed to a projection of the planning target volume (PTV) by a mMLC. Proton beams were delivered from an isocentrically mounted gantry, using the spot-scanning technique and energy modulation. Seven patients were scanned in a stereotactic frame; target volumes and organs at risk (OAR) were delineated with the help of MR images. Four different lesions were selected: (1) concave, (2) ellipsoid isolated, (3) superficial and close to an organ at risk, and (4) irregular complex. Dose distributions in the PTV and critical structures were calculated using three-dimensional treatment-planning systems, followed by both a quantitative (by dose--volume histogram and conformity index) and qualitative (visual inspection) assessment of the plans.

RESULTS

A high degree of conformation was achieved with a mMLC and stereotactic uniform intensity beams with comparable conformity indices to protons for 5 out of 7 plans, especially for superficial or spherical lesions. In the cases studied, the conformity index was better for protons than for photons for complex or concave lesions, or when the PTV was in the neighborhood of critical structures.

CONCLUSION

The results for the cases studied, show that for simple geometries or for superficial lesions, there is no advantage in using protons. However, for complex PTV shapes, or when the PTV is in the vicinity of critical structures, protons seem to be potentially better than the fixed-field photon technique.

Dynamic arc radiosurgery field shaping: a comparison with static field conformal and noncoplanar circular arcs

PURPOSE

Recent advances in field-shaping technology and linac multileaf collimator (MLC) integration have resulted in new approaches to performing stereotactic radiosurgery. We present a modeling study comparing the absolute dose distributions from three radiosurgery delivery techniques: a conventional approach utilizing noncoplanar circular arcs, a static field conformal approach, and a dynamic arc field-shaping approach. In the latter, the MLC leaves more in a continuous fashion, conforming to the beam's-eye-view projection of the target at every increment along the path of an arc.

METHODS AND MATERIALS

For the analysis, we devised a simulated target consisting of three overlapping spheres. This was chosen because it offered a straightforward planning approach for all three techniques, primarily the multiple isocenter approach. In addition, three representative cases were selected from the prior radiosurgery experience. These range in increasing size, from 0.50 to 9.79 cm(3), and in complexity, requiring from 3 isocenters to 16 in the case of circular arcs. In each situation, the goals were twofold: (1) to cover the entire volume with as high an appropriate isodose level (90% in the case of the conformal and dynamic arc techniques, 50% in the case of circular collimators) while (2) minimizing the dose to normal brain and where applicable, any adjacent radiation-sensitive structures. Because of the latter requirement, a single isocenter circular arc approach was ruled out for the analysis.

RESULTS

In the case of large or irregularly shaped lesions, the circular arc technique requires multiple isocenters, producing a high level of dose heterogeneity within the target volume. Both the static field and dynamic arc conformal techniques, as with all single isocenter approaches, produce a highly homogeneous dose throughout the target region. For a given large dose, peripheral dose is decreased as additional beams or arc degrees are added with either of the conformal approaches. Dose--volume histogram analysis evaluating the peripheral dose shows that, in many cases, dose to surrounding structures can be reduced through the use of a conformal static or dynamic arc approach over the conventional multiple isocenter, circular arc techniques.

CONCLUSIONS

Dynamic arc shaping is an efficient and effective method for accurately delivering a homogeneous target dose while simultaneously minimizing peripheral dose in radiosurgery applications.

Application of enhanced dynamic wedge to stereotactic radiotherapy

Stereotactic radiotherapy has developed into a useful treatment technique in which conformal dose distributions can be delivered with precision and accuracy. In some cases, the position of the target volume relative to surrounding critical structures demands careful evaluation of fixed beam paths so that dose to these critical structures can be minimized. Micromultileaf collimators aid in conforming dose to the target volume but may not allow adjustment of an individual beam's intensity (intensity modulation) in an effort to achieve dose uniformity throughout the treatment volume. Enhanced dynamic wedge (EDW) is demonstrated to be a valuable tool in improving the dose distribution in stereotactic radiotherapy treatments in which these fixed, conformal fields must be used due to constraints in beam trajectories. Four cases are presented which show the potential for gain in dose uniformity with the addition of EDW. These cases represent typical applications of EDW to conformal stereotactic radiotherapy.

Comparison of interpolated vs. calculated micromultileaf settings in dynamic conformal arc treatment

Stereotactic radiosurgery has developed into a technique where patient positioning and treatment delivery can be performed with submillimeter precision. Achievement of this level of precision has allowed margins to be significantly reduced, and in some cases, removed altogether. Joined with these reductions in treatment margin has come a desire to shape the radiation beam, further limiting dose to normal tissues. Initial applications of shaped radiosurgery fields utilized circular blocking apertures in an attempt to shape the beam to these small volumes. The resultant dose distributions conformed well to spherical treatment volumes but were inadequate for situations where the volume of interest was irregular in shape. Other techniques, such as applying these circular apertures through multiple isocenter positions to a single volume, have been investigated as possible ways to better conform dose distributions to these irregularly-shaped volumes. Recent technological advances allow the use of micromultileaf collimators which dynamically shape the beam by adjustment of individual leaves as the gantry rotates through the are. With margins potentially so tight, accurate evaluation of these dynamically adjusting treatment parameters becomes critical. Our current treatment planning software evaluates adjustments of the leaf positions in increments of 10 degrees and then does a linear interpolation between increments. Treatment delivery, however, is performed with adjustment in leaf position more consistent with a 1 degree increment. This paper compares the individual position of each leaf as determined for the 10 degrees interpolation to required changes in leaf position when the calculation is performed at increments of less than 10 degrees. Our data suggest that there are instances where improvements can be seen when corrections in leaf positions are made at these smaller increments.

Stereotactic conformal radiotherapy for pituitary adenomas: technique and preliminary experience

OBJECTIVE

Stereotactic conformal radiotherapy (SCRT) is a high precision technique of fractionated radiotherapy which ensures accurate delivery of radiation with reduction in the volume of normal tissue irradiated as compared to conventional external beam radiotherapy. We describe the technique and preliminary experience of SCRT in patients with residual and recurrent pituitary adenomas.

PATIENTS AND METHODS

Between February 1995 and March 1999, 22 patients (mean age: 45.3, range: 20-67 years) with residual or recurrent pituitary adenomas (13 nonfunctioning, nine secretory) were treated with SCRT. All were immobilized in a relocatable Gill-Thomas-Cosman (GTC) frame and tumour was localized on a postcontrast planning computerized tomography (CT) and MRI scan. The gross tumour volume (GTV) and the critical structures were outlined on contiguous 2-3 mm separated slices. A margin of 5 mm (12 patients) to 10 mm (10 patients) was grown around GTV in three-dimensions (3-D) to generate the planning target volume (PTV). The treatment was delivered by three (five patients) and four (17 patients) maximally separated conformal fixed fields with each field conformed to the shape of the tumour using customized lead alloy blocks (19 patients) or multileaf collimator (three patients). The patients were treated on a 6-MV linear accelerator to a dose of 45 Gy in 25 fractions (18 patients) and 50 Gy in 30 fractions (four patients).

RESULTS

The technique of SCRT has become a part of the routine work of the radiotherapy department. The treatment was well tolerated with minimal acute toxicity. One patient developed transient quadrantanopia 2 weeks after treatment with full recovery after a short course of corticosteroids. One patient had a transient visual deterioration 7 months after treatment due to cystic degeneration of the tumour which fully recovered following surgical decompression. Nine of the 15 patients presenting with visual impairment had improvement after treatment and the visual status remained stable in all others. One patient with acromegaly and one with a prolactinoma achieved normalization of elevated hormonal abnormality four and 10 months after SCRT, respectively. The remaining seven patients with a secretory adenoma had declining hormone levels at last follow-up. Newly initiated hormone replacement therapy was required in five patients. At a median follow-up of 9 months (range 1-44 months), the 1 and 2 year actuarial progression free and overall survival were 100%.

CONCLUSION

Stereotactic conformal radiotherapy is a high precision technique suitable for the treatment of pituitary adenomas requiring radiotherapy. Preliminary results suggest effective tumour control and low toxicity within the range expected for conventional external beam radiotherapy. While the technique is of potential benefit in reducing the volume of normal brain irradiated, the advantages in terms of sustained tumour control and reduced toxicity over conventional radiotherapy need to be demonstrated in long-term prospective studies.

Intensity modulated radiation therapy: a clinical review

Intensity modulated radiotherapy represents a significant advance in conformal radiotherapy. In particular, it allows the delivery of dose distributions with concave isodose profiles such that radiosensitive normal tissue close to, or even within a concavity of, a tumour may be spared from radiation injury. This article reviews the clinical application of this technique to date, and discusses the practical issues of treatment planning and delivery from the clinician's perspective.

Dosimetric comparison of three photon radiosurgery techniques for an elongated ellipsoid target

PURPOSE

To examine the dosimetric differences among three radiosurgery techniques: gamma knife, linac multiple arcs, and conformally-shaped static fields.

METHODS AND MATERIALS

A simulated target was taken to be a prolate ellipsoid, 25 mm in diameter, 35 mm in length, centrally located in a three-dimensional (3D) model of a patient head taken from MR images. Single isocenter linac treatment plans were developed, 9 portals for the static shaped field technique, and a 7-arc plan for the multiple arc method. A total of 13 isocenters with 3 different collimators were used in the gamma knife plan.

RESULTS

At dose levels from 25% to 50% of the reference dose, multiple arc and shaped-field plans treated a greater volume than the gamma knife plan. The linac plans, however, delivered the dose more homogeneously across the target volume as compared to the gamma knife plan. For the dose levels between 50-100%, the shaped fields and gamma knife plan have a similar dose distribution, and treated slightly less volume than the multiple arc plan.

CONCLUSION

For a target of limited volume and essentially any shape, one can obtain closely conformal dosimetry with the gamma knife. For a regular-shaped target, the single isocenter multiple arc technique gives a more homogenous dose distribution within the target. Static shaped fields offer an alternative radiosurgery technique, with dosimetry similar to the multiple arc method, applicable to targets of any shape.

Optimization of stereotactically-guided conformal treatment planning of sellar and parasellar tumors, based on normal brain dose volume histograms

PURPOSE

To investigate the optimal treatment plan for stereotactically-guided conformal radiotherapy (SCRT) of sellar and parasellar lesions, with respect to sparing normal brain tissue, in the context of routine treatment delivery, based on dose volume histogram analysis.

METHODS AND MATERIALS

Computed tomography (CT) data sets for 8 patients with sellar- and parasellar-based tumors (6 pituitary adenomas and 2 meningiomas) have been used in this study. Treatment plans were prepared for 3-coplanar and 3-, 4-, 6-, and 30-noncoplanar-field arrangements to obtain 95% isodose coverage of the planning target volume (PTV) for each plan. Conformal shaping was achieved by customized blocks generated with the beams eye view (BEV) facility. Dose volume histograms (DVH) were calculated for the normal brain (excluding the PTV), and comparisons made for normal tissue sparing for all treatment plans at > or =80%, > or =60%, and > or =40% of the prescribed dose.

RESULTS

The mean volume of normal brain receiving > or =80% and > or =60% of the prescribed dose decreased by 22.3% (range 14.8-35.1%, standard deviation sigma = 7.5%) and 47.6% (range 25.8-69.1%, sigma = 13.2%), respectively, with a 4-field noncoplanar technique when compared with a conventional 3-field coplanar technique. Adding 2 further fields, from 4-noncoplanar to 6-noncoplanar fields reduced the mean normal brain volume receiving > or =80% of the prescribed dose by a further 4.1% (range -6.5-11.8%, sigma = 6.4%), and the volume receiving > or =60% by 3.3% (range -5.5-12.2%, sigma = 5.4%), neither of which were statistically significant. Each case must be considered individually however, as a wide range is seen in the volume spared when increasing the number of fields from 4 to 6. Comparing the 4- and 6-field noncoplanar techniques to a 30-field conformal field approach (simulating a dynamic arc plan) revealed near-equivalent normal tissue sparing.

CONCLUSION

Four to six widely spaced, fixed-conformal fields provide the optimum class solution for the treatment of sellar and parasellar lesions, both in terms of normal brain tissue sparing and providing a relatively straightforward patient setup. Increasing the number of fields did not result in further significant sparing, with no clear benefit from techniques approaching dynamic conformal radiotherapy in the cases examined.

[Fractionated irradiation of cerebellopontine angle neurinoma: 12 years' experience of the Bordeaux University Hospital Center]

PURPOSE

To evaluate retrospectively the long-term results of fractionated radiation therapy (RT) in cerebello-pontine angle neurinomas (CPA).

METHODS AND MATERIAL

From January 1986 to October 1995, 29 patients with stage III and IV neurinomas were treated with external fractionated RT. One patient was irradiated on both sides and indications for RT were as follows: (1) general contraindications for surgery (16 patients); (2) hearing preservation in bilateral neurinomas after controlateral tumor exeresis (six patients); (3) partial tumor removal (five patients); and, (4) non-surgical recurrence (three patients). A three to four fields technique with coplanar static beams and conformal cerobend blocks was used; doses were calculated on a 95 to 98% isodoses and were given five days a week for a median total dose of 51 Gy (1.8 Gy/fraction). Most patients were irradiated with 6 to 10 MV photons).

RESULTS

Median follow-up was 66 months (seven to 120 months). Seven patients died, two with progressive disease, five from non-tumoral causes. Tumor shrinkage was observed in 13 patients (43.3%), stable disease in 14 (46.6%), and tumor progression in three. Two patients underwent total tumor removal after RT (one stable and one growing tumor). Hearing was preserved in four out of six patients. No patient experienced facial or trigeminal neuropathy.

CONCLUSION

Fractionated RT is a well tolerated and efficacious treatment of large non-surgical CPA neurinomas.

Comparison of a multi-leaf collimator with conformal blocks for the delivery of stereotactically guided conformal radiotherapy

Stereotactically-guided conformal radiotherapy is a practical technique for irradiating irregular lesions in the brain. The shaping of the conformal fields may be achieved using lead alloy blocks, a conventional multi-leaf collimator (MLC) or a mini/micro-MLC. Although the former gives more precise shaping, it is labour intensive. The latter methods are more practical as both mould room and treatment room times are reduced, but the shaping is limited by the finite leaf-width. This study compares treatment plans, in terms of normal tissue doses and tumour coverage, for fields shaped using conformal blocks and a conventional MLC in two series of geometrical shapes and nine patient tumours. For the range of tumour sizes considered (volumes 14-264 cm3, minimum dimension 30 mm, maximum 102 mm), the MLC treats, on average, 14% (range 3-34%) and 17% (range 0-36%) more normal brain tissue than conformal blocks to >50% and >80% of the prescription dose, respectively. The large variability is due to strong dependence on tumour shape and the presence of partial leaf-widths in the MLC fit. It is therefore important to consider both of these effects when deciding whether the MLC is appropriate for a particular target volume.

Radiotherapy in the treatment of benign meningioma of the skull base

OBJECT

This study was undertaken to assess the long-term efficacy and toxicity of conventional fractionated external-beam radiation in the treatment of benign skull base meningioma.

METHODS

This is a retrospective study of 82 patients with histologically verified benign skull base meningioma treated by surgery followed by fractionated external-beam radiation at the Royal Marsden Hospital between 1962 and 1992. The 5- and 10-year progression-free survival (PFS) rates were 92% and 83%, respectively, with the site of disease being the only independent prognostic factor for tumor control according to multivariate analysis. The 10-year PFS rate for patients with sphenoid ridge meningiomas was 69% compared with 90% for those with tumors in the parasellar region. The overall 10-year survival rate was 71%, with performance status and patient age found to be significant independent prognostic factors. Six patients had worsening vision, which was due to cataract in five cases and retinopathy in one. There were no recorded cases of cranial nerve neuropathy.

CONCLUSIONS

The excellent long-term tumor control and length of survival with minimal toxicity associated with conventional external-beam radiation should serve as a baseline for evaluation of new treatment strategies such as radiosurgery and skull base surgery.

Commissioning of a micro multi-leaf collimator and planning system for stereotactic radiosurgery

PURPOSE

A computer controlled micro multi-leaf collimator, m3 mMLC, has been commissioned for conformal, fixed-field radiosurgery applications. Measurements were made to characterise the basic dosimetric properties of the m3, such as leaf transmission, leakage and beam penumbra. In addition, the geometric and dosimetric accuracy of the m3 was verified when used in conjunction with a BrainSCAN v3.5 stereotactic planning system.

MATERIALS AND METHODS

The m3 was detachably mounted to a Varian Clinac 2100C accelerator delivering 6 MV X-rays. Leaf transmission, leakage, penumbra and multiple, conformal fixed field dose distributions were measured using calibrated film in solid water. Beam data were collected using a diamond detector in a scanning water tank and planned dose distributions were verified using LiF TLDs and film. A small, shaped phantom was also constructed to confirm field shaping accuracy using portal images.

RESULTS

Mean transmission through the closed multi-leaves was 1.9 +/- 0.1% and leakage between leaves was 2.8 +/- 0.15%. Between opposing leaves abutting along the central beam-axis transmission was approximately 15 +/- 3%, but was reduced to a mean of 4.5 +/- 0.6% by moving the abutmen position 4.5 cm off-axis. Beam penumbrae were effectively constant as a function of increasing square field size and asymmetric fields and was seen to vary non-linearly when shaped to diagonal, straight edges. TMR, OAR and relative output beam data measurements of circular m3 fields were comparable to conventional, circular stereotactic collimators. Multiple, conformal field dose distributions were calculated with good spatial and dosimetric accuracy, with the planned 90% isodose curves agreeing with measurements to within 1-2 mm and to +/- 3% at isocentre. Portal films agreed with planned beams eye-view field shaping to within 1 mm.

CONCLUSIONS

The m3 micro multi-leaf collimator is a stable, high precision field-shaping device suitable for small-field, radiosurgery applications. Dose distributions can be accurately calculated by a planning system using only a few beam data parameters.

Stereotactically guided conformal radiotherapy for meningiomas

PURPOSE

Stereotactically guided conformal radiotherapy, (SCRT) is a high precision technique of conformal radiotherapy (RT) which reduces the volume of normal tissue irradiated compared to conventional RT and may lead to a reduction in long-term toxicity We describe the technique and the preliminary results in patients with inoperable, residual or recurrent meningiomas.

MATERIAL AND METHODS

From July 1993 to November 1997, 24 patients (median age: 56 years, range: 28-72) with base of skull (n = 21). falx or upper skull (n = 3) meningiomas were treated with SCRT. The technique employed immobilization in a Gill-Thomas-Cosman (GTC) frame and CT localization with a Brown-Roberts-Wells (BRW) fiducial system for stereotactic space definition. The planning target volume (PTV) was defined as gross tumour volume (GTV) and a 0.5-1 cm margin. Treatment was delivered with three (12 patients) or four non-coplanar conformal fixed fields (12 patients) Conformal blocking was achieved either with lead alloy blocks (n = 11) or with a multi-leaf collimator (MLC) (n = 13). Patients were treated on a 6 MV linear accelerator to doses of 50-55 Gy, in 30-33 daily fractions. The treatments were carried out as part of a routine work of a busy radiotherapy department.

RESULTS

Median GTV for 24 meningiomas was 21.7 cm3 (range: 4.4-183 cm3). SCRT was well tolerated with minimal toxicity Three months after the end of radiotherapy, seven of 15 patients with neurological deficit had an improvement and eight remained unchanged. Two patients experienced early side effects (one VII nerve palsy, one Addisonian state). At a median follow-up of 13-months (range: 3-43) the 1 year progression free survival and overall survival are 100%. which is within the range expected for conventional fractionated radiotherapy for meningiomas.

CONCLUSIONS

SCRT is a feasible technique of high precision conformal RT for patients with meningiomas. Potential advantages in tumour control, survival and toxicity over conventional RT, require evaluation in long-term prospective studies.

Beam shaping for SRT/SRS

Field shaping for stereotactic radiosurgery and stereotactic radiotherapy has evolved from static field shaping techniques applied to static or arc fields and now includes dynamic field shaping definition which can be dynamically modified during the arc. This allows greater conformation of dose to the target volume while minimizing dose to surrounding normal tissue. This results in treatment to a single isocenter, which simplifies the treatment planning and dose delivery, thereby minimizing treatment time and improving patient comfort and satisfaction during the treatment. A number of optimization techniques remain to be investigated.