The use of a multi-leaf collimator for conformal radiotherapy of carcinomas of the prostate and nasopharynx

Evaluation of the Effect of Various Radiotherapy Modalities on Swallowing Function in Patients With Nasopharyngeal Cancer

The objective of this study was to compare the effect of curative 3-dimensional (3D) conformal radio-chemotherapy and intensity-modulated radiotherapy (IMRT) modalities on swallowing function in patients with nasopharyngeal cancer. Ten patients receiving 3D conformal radiotherapy and 10 patients receiving curative radiotherapy with IMRT, who were admitted for malignancy control for nasopharyngeal cancer, were included in the study. Swallowing functions were determined by flexible fiberoptic endoscopic evaluation. Premature spillage, retention pooling, penetration, aspiration, and reflex cough were evaluated. No statistically significant difference was found between patients receiving 3D conformal radiotherapy and IMRT regarding the scores of premature spillage, retention pooling, penetration, and aspiration with 3, 5, and 10 mL water and 5 mL yoghurt and fish crackers (P > .05). Velopharyngeal insufficiency or delayed onset of swallowing reflex was not found in any of the patients (P > .05). No significant difference was found between the groups in terms of the symptoms regarding subjective evaluation of swallowing (P > .05). Swallowing function did not differ among patients receiving IMRT and 3D conformal radiotherapy. Further studies with a larger sample size are warranted in order to verify the results.

Physical and Dosimetric Aspect of Euromechanics Add-on Multileaf Collimator on Varian Clinac 2100 C/D

BACKGROUND

Before treatment planning and dose delivery, quality assurance of multi-leaf collimator (MLC) has an important role in intensity-modulated radiation therapy (IMRT) due to the creation of multiple segments from optimization process.

OBJECTIVE

The purpose of this study is to assess the quality control of MLC leaves using EBT3 Gafchromic films.

MATERIAL AND METHODS

Leaf Position accuracy and leaf gap reproducibility were checked with Garden fence test. The garden fence test consists of 5 thin bands A) 0.2 Cm width spaced at 2 Cm intervals and B) 1 Cm width spaced at 1 Cm intervals. Each leaf accuracy was analyzed with measuring the full-width half-maximum (FWHM). Maximum and average leaf transmission were measured with gafchromic EBT3 films from Ashland for both 6 MV and 18 MV beams.

RESULTS

Leaf positions were found to be in a range between 1.78 - 2.53 mm, instead of nominal 2 mm for the test A and between 9.09 - 10.36 mm, instead of nominal 10 mm for the test B. The Average radiation transmission of the MLC was noted 1.79% and 1.98% of the open 10x10 Cm2 field at isocenter for 6 MV and 18 MV beams, respectively. Maximum radiation transmission was noted 4.1% and 4.4% for 6 MV and 18 MV beams, respectively.

CONCLUSION

In this study, application of gafchromic EBT3 films for the quality assurance of Euromechanics multileaf collimator was studied. Our results showed that the average leaf leakage and positional accuracy of this type of MLC were in the acceptance level based on the Protocols.

Optical and x-ray technology synergies enabling diagnostic and therapeutic applications in medicine

X-ray and optical technologies are the two central pillars for human imaging and therapy. The strengths of x-rays are deep tissue penetration, effective cytotoxicity, and the ability to image with robust projection and computed-tomography methods. The major limitations of x-ray use are the lack of molecular specificity and the carcinogenic risk. In comparison, optical interactions with tissue are strongly scatter dominated, leading to limited tissue penetration, making imaging and therapy largely restricted to superficial or endoscopically directed tissues. However, optical photon energies are comparable with molecular energy levels, thereby providing the strength of intrinsic molecular specificity. Additionally, optical technologies are highly advanced and diversified, being ubiquitously used throughout medicine as the single largest technology sector. Both have dominant spatial localization value, achieved with optical surface scanning or x-ray internal visualization, where one often is used with the other. Therapeutic delivery can also be enhanced by their synergy, where radio-optical and optical-radio interactions can inform about dose or amplify the clinical therapeutic value. An emerging trend is the integration of nanoparticles to serve as molecular intermediates or energy transducers for imaging and therapy, requiring careful design for the interaction either by scintillation or Cherenkov light, and the nanoscale design is impacted by the choices of optical interaction mechanism. The enhancement of optical molecular sensing or sensitization of tissue using x-rays as the energy source is an important emerging field combining x-ray tissue penetration in radiation oncology with the molecular specificity and packaging of optical probes or molecular localization. The ways in which x-rays can enable optical procedures, or optics can enable x-ray procedures, provide a range of new opportunities in both diagnostic and therapeutic medicine. Taken together, these two technologies form the basis for the vast majority of diagnostics and therapeutics in use in clinical medicine.

Dosimetric comparison between three-dimensional conformal radiotherapy (3D-CRT) and intensity-modulated radiotherapy (IMRT) in the treatment of different stages of nasopharyngeal carcinoma

Background

This work aims to compare the dosimetric performance of three-dimensional conformal radiotherapy (3D-CRT), a relatively available technique in developing countries, to intensity-modulated radiotherapy (IMRT) in the treatment of different stages of nasopharyngeal carcinoma (NPC).

Materials and Methods

According to the diagnostic stages, 40 NPC patients were divided into two equal groups. Three planning techniques such as 3D-CRT, seven-field IMRT (7F-IMRT) and nine-field IMRT (9F-IMRT) were compared. Dose prescriptions of 70 and 66 Gy were delivered in 35 fractions to gross planning target volume (PTV1) and bilateral retropharyngeal carcinoma (PTV2), respectively.

Results

Stage I dose data for almost all of the three investigated planning techniques obey the international recommendations. The dose delivered to PTV1 and PTV2 for 3D-CRT and 7F-IMRT are statistically similar, whereas 9F-IMRT is significantly better than 3D-CRT. For organs at risk (OARs), the delivered dose is significantly better for 9F-IMRT compared with the other two techniques, whereas 7F-IMRT is significantly better than 3D-CRT.

Conclusions

3D-CRT is an acceptable alternative treatment technique for stage I NPC patients in developing countries suffering from the lack of advanced radiotherapy treatment techniques. 3D-CRT and 7F-IMRT have comparable performance in PTVs, while 9F-IMRT is superior in PTVs and OARs.

Effective Penumbra and Scalloping Effect: A Dosimetric Study in Multifield Radiotherapy with Multileaf Collimator for Prostate Cancer Treatment

Aims and background

The aim of the study was to test whether the multileaf collimator can be used to replace divergent alloy blocks for field shaping and to evaluate the dosimetric impact of the scalloping effect and the effective penumbra in multifield isocentric configurations routinely used at our department to treat early stage prostate tumor in supine patients. Deviations between measurements and treatment planning system calculations are also discussed in order to estimate the accuracy of effective penumbra calculations carried out by the software in the presence of blocks or a multileaf collimator.

Methods

The multileaf collimator installed on a dual energy (6 and 18 MV) linear accelerator Varian CLINAC 2100 C/D is an add-on component positioned below the standard jaws, with 40 computer-controlled opposed pairs of tungsten leaves. Transmission, effective penumbra and scalloping measurements were performed with films placed at different depths in a (30 × 30 × 20 cm3) acrylic phantom. A laser scanning photo-densitometer was used to obtain the optical density and the relative dose profile. Effective penumbra and scalloping effect measured data were tested on a software phantom; the phantom, automatically performed with the treatment planning system, was a regular parallelepiped measuring 30 × 30 × 20 cm3 and the acrylic electronic density value.

Results

For one multileaf collimator-shaped field, the width of effective penumbra was about 2 mm wider than penumbra for cerrobend blocks, at a 45° angle between leaf motion direction and the field edge. Collimator rotation, automatically performed by the treatment planning system, by minimizing the sum of over- and underblocked areas, reduces the differences between the multileaf collimator and blocks. The differences between measured and treatment planning system calculated data were within the treatment planning system dose calculation accuracy limits, as recommended in ICRU Report No. 42.

Conclusions

Penumbra and, for the multileaf collimator, scalloping effect values seem to depend on the gantry angle, i.e., on the fields path of entry. The values for even fields are higher than the those for odd fields, because the dose gradient at the target edges is steeper for a single field than for two opposite fields, and the interplay of doses from the individual beams increases the distance between the isodose levels. Therefore, in order to reduce scalloping and effective penumbra values, it would be better to rotate the collimators, above all in even-number field techniques. In particular, the six-field technique, used mostly in prostate treatment, shows the same effective penumbra values with the multileaf collimator, with proper collimator rotation, and in blocked fields.

The Role of A Conventional Simulator in Multileaf-Plan Simulation: A Proposal

Modern computer networks provide satisfying levels of data recording and verification between the treatment planning system (TPS) and the accelerators, while the main weakness of the preparation chain remains the simulation. When a conventional simulator is employed, it may adversely affect the three-dimensional treatment planning system (3DPS) process because of the difficulty to document the leaf positions on the simulator location films and on the patient's skin. With a conventional simulator, hard copies of the DRRs of each field and CT scans at isocenter level are needed.

In an attempt to transfer more information displayed from a BEV perspective from the 3DPS to simulator radiographs, this study aimed to reduce the quality loss by using a 2D conventional simulator in a 3DPS process.

We realized an acetate photocopy of TPS data for each field, from a BEV perspective, containing: DRR, wire frames of the PTV, organs at risk and MLC aperture. The photocopies, with an appropriate magnification factor to obtain a correct projective value (ratio 1:1) at isocenter level, are carefully placed on the radiographic images on the same hard copy which allows us to better understand possible setup errors and obliges us to correct these.

The method provides reliable documentation, facilitates treatment verification, and fulfils the criteria for MLC simulation. It is accurate, simple, and very inexpensive.

Dosimetric effect of Elekta Beam modulator micromultileaf in three-dimensional conformal radiotherapy and intensity-modulated radiotherapy for prostate cancer

The purpose of this study is to analyze the dosimetric effect of Elekta Beam Modulator in 3-dimensional conformal radiation therapy (3DCRT) and in intensity-modulated radiation therapy (IMRT) for localized prostate cancer. We compared treatment plans developed with 2 different Elekta multileaf collimators (MLC): Beam Modulator micro-MLC (mMLC) (4-mm leaf width at the isocenter) and standard MLC (10-mm leaf width at the isocenter). The comparison was performed for 15 patients with localized prostate cancer in 3DCRT and IMRT delivery; a total of 60 treatment plans were processed. The dose-volume histograms were used to provide the quantitative comparison between plans. In particular, we analyzed differences between rectum and bladder sparing in terms of a set of appropriate Vx (percentage of organ at risk [OAR] volume receiving the x dose) and differences between target conformity and coverage in terms of coverage factor and conformation number. Our analysis demonstrates that in 3DCRT there is an advantage in the use of Elekta Beam Modulator mMLC in terms of organ sparing; in particular, a significant decrease in rectal V60 and V50 (p = 0.001) and in bladder V70 and V65 (p = 0.007 and 0.002, respectively) was found. Moreover, a better target dose conformity was obtained (p = 0.002). IMRT plans comparison demonstrated no significant differences between the use of the 4 or 10-mm MLCs. Our analysis shows that in 3DCRT the use of the Elekta Beam Modulator mMLC gives a gain in target conformity and in OARs dose sparing whereas in IMRT plans there is no advantage.

Role of radiation therapy in the management of pancreatic cancer

Local failure remains a major issue for patients with both resectable and locally advanced pancreatic cancer. The role of radiation therapy in the management of this disease is less clear and represents an area of some controversy. The objective of this review is to present the rationale for radiation therapy in pancreatic cancer, as well as to discuss the potential limitations and caveats of the currently available studies.

Phantom dosimetric study of nondivergent aluminum tissue compensator using ion chamber, TLD, and gafchromic film

Anatomic contour irregularity and tissue inhomogeneity in head-and-neck radiotherapy can lead to significant dose inhomogeneity due to the presence of hot and cold spots across the treatment volumes. Missing tissue compensators (TCs) can overcome this dose inhomogeneity. The current study examines the capacity of 2-dimensional (2D) custom aluminum TCs fabricated at our hospital to improve the dose homogeneity across the treatment volume. The dosimetry of the 2D custom TCs was carried out in a specially designed head-and-neck phantom for anterior-posterior (AP) and posterior-anterior (PA) fields with an ion chamber, thermoluminscence dosimeters (TLDs), and film. The results were compared for compensated and uncompensated plans generated from the Eclipse treatment planning system. On average, open-field plans contained peak doses of 117%, optimally wedged-plans contained peak doses of 113%, and custom-compensated plans contained peak doses of 105%. The dose variation between prescribed and measured dose at midplane of the phantom was observed as high as 17%, which was reduced to 3.2% for the customized TC during ionometric measurements. It was further confirmed with TLDs, in a sagittal plane, that the high-dose region of 13.3% was reduced to 2.3%. The measurements carried out with the ion chamber, TLDs, and film were found in good agreement with each other and with Eclipse. Thus, a custom-made 2D TC is capable of reducing hot spots to improve overall dose homogeneity across the treatment volume.

Evaluation of dosimetric effect of leaf position in a radiation field of an 80 leaf multileaf collimator fitted to the LINAC head as tertiary collimator

This study evaluates changes in the dosimetric characteristics of a Varian Millennium 80‐leaf multileaf collimator (MLC) in a radiation field. In this study, dose rate, scatter factor, percentage depth dose, surface dose and dose in the buildup region, beam profile, flatness and symmetry, and penumbra width measurements were made for 6‐MV and 15‐MV photon beams. Analysis of widths between 50% dose levels of the beam profiles to reflect the field size at the level of profile measurement shows a significant difference between the fields defined by MLC and/or jaws and MLC (zero gap) and the fields defined by jaws only. The position of the MLC leaves in the radiation field also significantly affects scatter factors. A new relationship has, therefore, been established between the scatter factors and the position of the MLC, which will indeed be useful in the dose calculation for irregular fields. Penumbra widths increase with field size and were higher for fields defined by jaws and/or MLC than jaws and MLC (zero gap) by 1.5 mm to 4.2 mm and 3.8 mm to 5.0 mm, for 6‐MV, and 1.5 mm to 2.4 mm and 3.0 mm to 5.6 mm, for 15‐MV, at 20% to 80% and 10% to 90% levels, respectively. The surface dose and the dose in the buildup region were smaller for fields defined by jaws and MLC (zero gap) than the fields defined by jaws and/or MLC for both photon energies. No significant differences were found in percentage depth dose beyond dmax, beam profiles above 80% dose level, and flatness and symmetry for both energies. The results of this study suggest that while one collects linear accelerator beam data with a MLC, the effects of the positions of the MLC leaves play an important role in dosimetric characteristics of 3D conformal radiation therapy as well as intensity‐modulated radiotherapy.

PACS number: 87.53.Dq

Elongated beamlets: a simple technique for segment and MU reduction for sMLC IMRT delivery on accelerators utilizing 5 mm leaf widths

The focus of this work is to demonstrate the effects of using an elongated beamlet to achieve similar dose conformity as achieved with a square beamlet while reducing the number of segments and subsequent MU required. A series of 10 patients were planned for IMRT delivery to the prostate using minimum beamlet sizes of 5x5 mm2 (default scheme), 10x5 mm2 with the short axis parallel to the prostate-rectum interface (scheme 1), and 10x5 mm2 with the short axis perpendicular to the prostate-rectum interface (scheme 2). All other parameters between plans were left unchanged. Plans were appropriately normalized and evaluated for R65, R40, conformity index, total number of segments and MU. All plans were generated using the Corvus inverse planning system. The average number of segments in this study decreased by approximately 49% for both schemes 1 and 2. The subsequent number of MU required decreased by approximately 34.6%. The resultant modified modulation scaling factor (MSFmod) decreased by approximately 34.3%. Additionally, we found that each isodose distribution using scheme 2 would still meet our clinical acceptance criteria with no visible degradation in the dose distribution as compared with the default scheme. In conclusion, we have demonstrated that it is possible to achieve similar results as those obtained using a 5x5 mm2 beamlet with respect to target coverage and critical structure sparing by using strategically oriented elongated beamlets. This technique directly translates to a decreased MSF(mod) allowing for decreased leakage dose to the patient, a decreased risk of exceeding secondary shielding limits in pre-existing vaults, and shorter treatment times.

Intensity modulated radiation therapy with multileaf collimators of different leaf widths: a comparison of achievable dose distributions

PURPOSE

A planning study to analyze the impact of different leaf widths on the achievable dose distributions with intensity modulated radiation therapy (IMRT).

METHODS

Five patients (3 intra- and 2 extra-cranial) with projected planning target volume (PTV) sizes smaller than 10 cm by 10 cm were re-planned with four different multileaf collimators (MLC). Two internal collimators with an isocentric leaf width of 4 and 10 mm and two add-on collimators with an isocentric leaf width of 2.75 and were evaluated. The inverse treatment planning system KonRad (Siemens Medical Solutions) was used to create IMRT 'step & shoot' plans. For each patient the same arrangement of beams and the same parameters for the optimization were used for all MLCs. The beamlet size for all treatment plans was chosen to coincide with the leaf width of the respective MLC. To evaluate the treatment plans 3D dose distributions and dose volume histograms were analyzed. As indicators for the quality of the PTV dose distribution the minimum dose, maximum dose and the standard deviation were used. For the organs at risk (OAR) the equivalent uniform dose (EUD) was calculated. To measure the dose coverage of the PTV the volume (V(90)) that received doses higher than 90% of the prescribed dose was calculated where for the conformity the dose conformity index given by Baltas et al. was determined.

RESULTS

The MLC with the smallest leaf width yields the best mean value of all five patients for the PTV coverage and for the conformity. For the MLCs with the same leaf width, the add-on MLC leads to superior treatment plans than the internal MLC. This is due to the sharper penumbra of the add-on MLC. The number of IMRT field segments to deliver increased by approximately a factor of two if 2. MLC leafs are used instead of the standard 10 mm leafs. In case of the para-spinal patients the EUD value for the spinal cord is only reduced slightly by using MLCs with leaf widths smaller than 5 mm. For the intra-cranial the EUD value for some organs improved with reduced leaf widths while for some organs the 10 mm MLC leafs give comparable values.

CONCLUSION

As expected the MLC with the smallest leaf width always yields the best PTV coverage. Reducing the leaf width from 4 to 2.75 mm results in a slight enhancement of the PTV coverage. With the selected organ parameters no significant improvement for most OAR was found. The disadvantage of the reduction of the leaf width is the increasing number of segments due to the more complex fluence patterns and therefore an increased delivery time.

Acute radiation toxicity assessment of a 3-D conformal head and neck radiation treatment technique

Patients undergoing radiation treatment for head and neck cancer have many critical structures within or adjacent to the treatment area. Avoiding these critical structures is more efficient and easier using 3-D conformal planning. At Sir Charles Gairdner Hospital an investigation into the acute radiation toxicity was undertaken when the head and neck treatment technique was changed from the conventional 2-D plan to the conformal 3-D plan. Although the primary target volume (PTV), fraction size and overall dose remain the same for the two techniques, differing beam configurations mean that treatment fields enter and exit through different parts of the head and neck. Ten patients were initially assessed to gain a baseline appearance of their head and neck region, and then graded weekly according to the toxicity criteria during treatment and at 4, 8 and 12 weeks after treatment. The results of the assessment indicate that there is no increase in toxicity as a result of treating using the conformal head and neck technique.

Radiation oncology: a century of achievements

Over the twentieth century the discipline of radiation oncology has developed from an experimental application of X-rays to a highly sophisticated treatment of cancer. Experts from many disciplines - chiefly clinicians, physicists and biologists - have contributed to these advances. Whereas the emphasis in the past was on refining techniques to ensure the accurate delivery of radiation, the future of radiation oncology lies in exploiting the genetics or the microenvironment of the tumour to turn cancer from an acute disease to a chronic disease that can be treated effectively with radiation.

On empirical methods to determine scatter factors for irregular MLC shaped beams

Multileaf collimators (MLCs) are in clinical use for more than a decade and are a well accepted tool in radiotherapy. For almost each MLC design different empirical or semianalytical methods have been presented for calculating output ratios in air for irregularly shaped beams. However, until now no clear recommendations have been given on how to handle irregular fields shaped by multileaf collimators for independent monitor unit (MU) verification. The present article compares different empirical methods, which have been proposed for independent MU verification, to determine (1) output ratios in air (Sc) and (2) phantom scatter factors (Sp) for irregular MLC shaped fields. Ten dedicated field shapes were applied to five different types of MLCs (Elekta, Siemens, Varian, Scanditronix, General Electric). All calculations based on empirical relations were compared with measurements and with calculations performed by a treatment planning system with a fluence based algorithm. For most irregular MLC shaped beams output ratios in air could be adequately modeled with an accuracy of about 1%-1.5% applying a method based on the open field aperture defined by the leaf and jaw setting combined with the equivalent square formula suggested by Vadash and Bjärngard [P. Vadash and B. E. Bjärngard, Med. Phys. 20, 733-734 (1993)]. The accuracy of this approach strongly depends on the inherent head scatter characteristics of the accelerator in use and on the irregular field under consideration. Deviations of up to 3% were obtained for fields where leaves obscure central parts of the flattening filter. Simple equivalent square methods for Sp calculations in irregular fields did not provide acceptable results (deviations mostly >3%). Sp values derived from Clarkson integration, based on published tables of phantom scatter correction factors, showed the same accuracy level as calculations performed using a pencil beam algorithm of a treatment planning system (in a homogeneous media). The separation of head scatter and phantom scatter contributions is strongly recommended for irregular MLC shaped beams as both contributions have different factors of influence. With rather simple methods Sc and Sp can be determined for independent MU calculation with an accuracy better than 1.5% for most clinical situations encountered in conformal radiotherapy.

Stereotactic IMRT for prostate cancer: dosimetric impact of multileaf collimator leaf width in the treatment of prostate cancer with IMRT

The focus of this work is the dosimetric impact of multileaf collimator (MLC) leaf width on the treatment of prostate cancer with intensity-modulated radiation therapy (IMRT). Ten patients with prostate cancer were planned for IMRT delivery using two different MLC leaf widths--4mm and 10mm--representing the Radionics micro-multileaf collimator (mMLC) and Siemens MLC, respectively. Treatment planning was performed on the XKnifeRT2 treatment-planning system (Radionics, Burlington, MA). All beams and optimization parameters were identical for the mMLC and MLC plans. All the plans were normalized to ensure that 95% of the planning target volume (PTV) received 100% of the prescribed dose. The differences in dose distribution between the two different plans were assessed by dose-volume histogram (DVH) analysis of the target and critical organs. We specifically compared the volume of rectum receiving 40 Gy (V40), 50 Gy (V50), 60 Gy (V60), the dose received by 17% and 35% of rectum (D17 and D35), and the maximum dose to 1 cm3 of the rectum for a prescription dose of 74 Gy. For the urinary bladder, the dose received by 25% of bladder (D25), V40, and the maximum dose to 1 cm3 of the organ were recorded. For PTV we compared the maximum dose to the "hottest" 1 cm3 (Dmax1 cm3) and the dose to 99% of the PTV (D99). The dose inhomogeneity in the target, defined as the ratio of the difference in Dmax1 cm3 and D99 to the prescribed dose, was also compared between the two plans. In all cases studied, significant reductions in the volume of rectum receiving doses less than 65 Gy were seen using the mMLC. The average decrease in the volume of the rectum receiving 40 Gy, 50 Gy, and 60 Gy using the mMLC plans was 40.2%, 33.4%, and 17.7%, respectively, with p < 0.0001 for V40 and V50 and p < 0.012 for V60. The mean dose reductions for D17 and D35 for the rectum using the mMLC were 20.4% (p < 0.0001) and 18.3% (p < 0.0002), respectively. There were consistent reductions in all dose indices studied for the bladder. The target dose inhomogeneity was improved in the mMLC plans by an average of 29%. In the high-dose range, there was no significant difference in the dose deposited in the "hottest" 1 cm3 of the rectum between the two plans for all cases (p > 0.78). In conclusion, the use of the mMLC for IMRT of the prostate resulted in significant improvement in the DVH parameters of the prostate and critical organs, which may improve the therapeutic ratio.

Geometric leaf placement strategies

Geometric leaf placement strategies for multileaf collimators (MLCs) typically involve the expansion of the beam's-eye-view contour of a target by a uniform MLC margin, followed by movement of the leaves until some point on each leaf end touches the expanded contour. Film-based dose-distribution measurements have been made to determine appropriate MLC margins--characterized through an index d90--for multileaves set using one particular strategy to straight lines lying at various angles to the direction of leaf travel. Simple trigonometric relationships exist between different geometric leaf placement strategies and are used to generalize the results of the film work into d90 values for several different strategies. Measured d90 values vary both with angle and leaf placement strategy. A model has been derived that explains and describes quite well the observed variations of d90 with angle. The d90 angular variations of the strategies studied differ substantially, and geometric and dosimetric reasoning suggests that the best strategy is the one with the least angular variation. Using this criterion, the best straightforwardly implementable strategy studied is a 'touch circle' approach for which semicircles are imagined to be inscribed within leaf ends, the leaves being moved until the semicircles just touch the expanded target outline.

Commissioning, evaluation, quality assurance and clinical application of a virtual micro MLC technique

Multileaf collimators (MLCs) are a valuable tool in modern radiation therapy, offering flexible and convenient field shaping. One disadvantage, however, is the undulation of the dose distribution at the edge shaped by the leaves due to the finite leaf width. An attempt to reduce the effect of this undulation is the objective of the commercial linear accelerator package HD270, which incorporates three-dimensional couch translation together with leaf adjustment to emulate finer leaf widths. In this paper we report on the commissioning and evaluation of this feature, together with the development of a process for quality assurance, as well as description of a clinical application of this technique. It is concluded that this technique could be applied reliably to situations currently utilizing MLC for shielding, with little added cost in treatment time, provided that a comprehensive quality assurance program is in place to monitor the performance of this complicated procedure.

Reirradiation of nasopharyngeal carcinoma with intracavitary mold brachytherapy: an effective means of local salvage

PURPOSE

To assess the role of intracavitary mold brachytherapy in salvaging local failure of nasopharyngeal carcinoma (NPC).

METHODS AND MATERIALS

The outcomes of 118 consecutive NPC patients with local failure treated with mold brachytherapy between 1989 and 1996 were retrospectively reviewed. Eleven patients received additional external radiotherapy.

RESULTS

All molds were tailor-made, and the whole procedure was performed under local anesthesia. Pharyngeal recess dissection was routinely performed to allow direct contact of the radioactive source with the pharyngeal recess, a common site of local failure. Initially, the molds were preloaded with 192Ir wires, but since 1992, the sources have been manually afterloaded; the mold has also been redesigned for better conformity, ease of insertion, and radiation safety. Using brachytherapy alone, 50-55 Gy was given for recurrence in 4-7 days; for persistence, 40 Gy was administered. The overall complete remission rate was 97%. The rates of 5-year local control, relapse-free survival, disease-specific survival, overall survival, and major complication were 85%, 68.3%, 74.8%, 61.3%, and 46.9%, respectively. Major complications included nasopharyngeal necrosis with headache, necrosis of cervical vertebrae with atlantoaxial instability, temporal lobe necrosis, and palsy of the cranial nerves. The afterloaded mold was as effective as the preloaded version, but with fewer complications.

CONCLUSIONS

Intracavitary mold brachytherapy was effective in salvaging NPC with early-stage local persistence or first recurrence.

Design considerations for a computer controlled multileaf collimator for the Harper Hospital fast neutron therapy facility

The d(48.5) + Be neutron beam from the Harper Hospital superconducting cyclotron is collimated using a unique multirod collimator (MRC). A computer controlled multileaf collimator (MLC) is being designed to improve efficiency and allow for the future development of intensity modulated radiation therapy with neutrons. For the current study the use of focused or unfocused collimator leaves has been studied. Since the engineering effort associated with the leaf design and materials choice impacts significantly on cost, it was desirable to determine the clinical impact of using unfocused leaves in the MLC design. The MRC is a useful tool for studying the effects of using focused versus unfocused beams on beam penumbra. The effects of the penumbra for the different leaf designs on tumor and normal tissue DVHs in two selected sites (prostate and head and neck) was investigated. The increase in the penumbra resulting from using unfocused beams was small (approximately 1.5 mm for a 5 x 5 cm2 field and approximately 7.6 mm for a 25 x 25 cm2 field at 10 cm depth) compared to the contribution of phantom scatter to the penumbra width (5.4 and 20 mm for the small and large fields at 10 cm depth, respectively). Comparison of DVHs for tumor and critical normal tissue in a prostate and head and neck case showed that the dosimetric disadvantages of using an unfocused rather than focused beam were minimal and only significant at shallow depths. For the rare cases, where optimum penumbra conditions are required, a MLC incorporating tapered leaves and, thus, providing focused collimation in one plane is necessary.

The role of intensity-modulated radiotherapy in the treatment of parotid tumors

PURPOSE

To compare intensity-modulated radiotherapy (IMRT) treatment plans with three-dimensional conformal radiotherapy (3D-CRT) plans to investigate the suitability of IMRT for the treatment of tumors of the parotid gland.

METHODS AND MATERIALS

One 3D-CRT treatment plan and 10 IMRT treatment plans with differing beam arrangements were produced for each of nine patient data sets. The plans were compared using regret analysis, dose conformity, dose to organs at risk, and uncomplicated tumor control probability (UTCP).

RESULTS

The target dose was comparable in the 3D-CRT and IMRT plans, although improvements were seen when seven and nine IMRT fields were used. IMRT reduced the mean dose to the contralateral parotid gland and the maximum doses to the brain and the spinal cord, but increased the ipsilateral lens dose in some cases. Each IMRT arrangement produced a higher UTCP than the 3D-CRT plans; the largest absolute difference was 9.6%.

CONCLUSIONS

IMRT is a suitable means for treating cancer of the parotid, and a five-field class solution is proposed. It produced substantial sparing of organs at risk and higher UTCPs than 3D-CRT and should enable dose escalation.

What is the optimum leaf width of a multileaf collimator?

The following question is investigated: How narrow do the leaves of a multileaf collimator have to be such that further reduction of the leaf width does not lead to physical improvements of the dose distribution. Because of the physical principles of interaction between radiation and matter, dose distributions in radiotherapy are generally relatively smooth. According to the theory of sampling, the dose distribution can therefore be represented by a set of evenly spaced samples. The distance between the samples is identified with the distance between the leaf centers of a multileaf collimator. The optimum sampling distance is derived from the 20% to 80% field edge penumbra through the concept of the dose deposition kernel, which is approximated by a Gaussian. The leaf width of the multileaf collimator is considered to be independent from the sampling distance. Two cases are studied in detail: (i) the leaf width equals the sampling distance, which is the regular case, and (ii) the leaf width is twice the sampling distance. The practical delivery of the latter treatment geometry requires a couch movement or a collimator rotation. The optimum sampling distance equals the 20%-80% penumbra divided by 1.7 and is on the order of 1.5-2 mm for a typical 6 MV beam in soft tissue. The optimum leaf width equals this sampling distance in the regular case. A relatively small deterioration results if the leaf width is doubled, while the sampling distance remains the same. The deterioration can be corrected for by deconvolving the fluence profile with an inverse filter.

CONCLUSIONS

With the help of the sampling theory and, more generally, the theory of linear systems, one can find a general answer to the question about the optimum leaf width of a multileaf collimator from a physical point of view. It is important to distinguish between the sampling distance and the leaf width. The sampling distance is more critical than the leaf width. The leaf width can be up to twice as large as the sampling width. Furthermore, the derived sampling distance can be used to select the optimum resolution of both the fluence and the dose grid in dose calculation and inverse planning algorithms.

A design for a dual assembly multileaf collimator

A multileaf collimator for radiation therapy has been designed that splits each leaf bank into two vertically displaced levels with each level consisting of alternate leaves and leaf spaces. The leaves in the upper level shield the spaces in the lower level. Each level can move laterally, in the direction perpendicular to leaf motion by one leaf width. Following lateral movement of one level, the leaves align with the other level and radiation is transmitted through the collimator as multiple slit fields in a grid pattern. This transmission can be used to form an image of the external anatomy and would enable double-exposure portal images to be acquired much more rapidly than at present. These could potentially be acquired during the treatment delivery. The radiation profiles transmitted for image formation through the collimator design were investigated. Individual and grid pattern slit field profiles formed by tungsten and lead alloy collimators were measured with varying slit width, source-collimator distance, collimator-detector distance, and collimation thickness. The slit width was found to have the major influence on the transmitted profiles. As the slit width decreases the profiles become broader than the geometric slit projection resulting in increasing overlap of adjacent profiles. The overlap results in a modulated image of the external anatomy for small slit widths, rather than a sampled or "grid" image for larger widths. The shielding of this design was found to be adequate provided the leaf faces of the adjacent vertically displaced leaves are at least aligned, therefore an overlap or tongue and groove is not required.

Monte Carlo verification of IMRT dose distributions from a commercial treatment planning optimization system

The purpose of this work was to use Monte Carlo simulations to verify the accuracy of the dose distributions from a commercial treatment planning optimization system (Corvus, Nomos Corp., Sewickley, PA) for intensity-modulated radiotherapy (IMRT). A Monte Carlo treatment planning system has been implemented clinically to improve and verify the accuracy of radiotherapy dose calculations. Further modifications to the system were made to compute the dose in a patient for multiple fixed-gantry IMRT fields. The dose distributions in the experimental phantoms and in the patients were calculated and used to verify the optimized treatment plans generated by the Corvus system. The Monte Carlo calculated IMRT dose distributions agreed with the measurements to within 2% of the maximum dose for all the beam energies and field sizes for both the homogeneous and heterogeneous phantoms. The dose distributions predicted by the Corvus system, which employs a finite-size pencil beam (FSPB) algorithm, agreed with the Monte Carlo simulations and measurements to within 4% in a cylindrical water phantom with various hypothetical target shapes. Discrepancies of more than 5% (relative to the prescribed target dose) in the target region and over 20% in the critical structures were found in some IMRT patient calculations. The FSPB algorithm as implemented in the Corvus system is adequate for homogeneous phantoms (such as prostate) but may result in significant under or over-estimation of the dose in some cases involving heterogeneities such as the air-tissue, lung-tissue and tissue-bone interfaces.

Comparison of treatment plans involving intensity-modulated radiotherapy for nasopharyngeal carcinoma

PURPOSE

To compare intensity-modulated radiotherapy (IMRT) treatment plans with conventional treatment plans for a case of locally advanced nasopharyngeal carcinoma.

METHODS AND MATERIALS

The study case was planned using two types of IMRT techniques, as well as a three-dimensional conformal radiotherapy technique (3D-CRT), and a traditional treatment method using bilateral opposing fields. These four plans were compared with respect to dose conformality, dose-volume histogram (DVH), dose to the sensitive normal tissue structures, and ease of treatment delivery.

RESULTS

The planned dose distributions were more conformal to the tumor target volume in the IMRT plans than those in the conventional plans. With similar dose coverage of the clinical target volume (CTV), defined as delivery of minimum of 60 Gy to >/= 95% of CTV, the IMRT plans achieved better sensitive normal tissue structure sparing, while concomitantly delivering a minimum dose of 68 Gy to >/= 95% of the gross tumor volume (GTV) at a higher dose per fraction.

CONCLUSIONS

Compared to conventional techniques, IMRT techniques provide improved tumor target coverage with significantly better sparing of sensitive normal tissue structures in the treatment of locally advanced nasopharyngeal carcinoma. With improvement of the delivery efficiency, IMRT should provide the optimal treatment for all nasopharyngeal carcinoma. Further studies are needed to establish the true clinical advantage of this new modality.

Energy- and intensity-modulated electron beams for radiotherapy

This work investigates the feasibility of optimizing energy- and intensity-modulated electron beams for radiation therapy. A multileaf collimator (MLC) specially designed for modulated electron radiotherapy (MERT) was investigated both experimentally and by Monte Carlo simulations. An inverse-planning system based on Monte Carlo dose calculations was developed to optimize electron beam energy and intensity to achieve dose conformity for target volumes near the surface. The results showed that an MLC with 5 mm leaf widths could produce complex field shapes for MERT. Electron intra- and inter-leaf leakage had negligible effects on the dose distributions delivered with the MLC, even at shallow depths. Focused leaf ends reduced the electron scattering contributions to the dose compared with straight leaf ends. As anticipated, moving the MLC position toward the patient surface reduced the penumbra significantly. There were significant differences in the beamlet distributions calculated by an analytic 3-D pencil beam algorithm and the Monte Carlo method. The Monte Carlo calculated beamlet distributions were essential to the accuracy of the MERT dose distribution in cases involving large air gaps, oblique incidence and heterogeneous treatment targets (at the tissue-bone and bone-lung interfaces). To demonstrate the potential of MERT for target dose coverage and normal tissue sparing for treatment of superficial targets, treatment plans for a hypothetical treatment were compared using photon beams and MERT.

A method of improving the spatial resolution of treatments that involve a multileaf collimator

In this paper we present a novel method of reducing the dosimetric effects of the finite leaf width of a multileaf collimator (MLC) in conformal and intensity modulated radiotherapy (IMRT). This is achieved by decomposing the required high-resolution fluence distribution into two orthogonal components, which are delivered with two leaf sweeps with head-twists differing by 90 degrees. Before the decomposition stage, a filter is applied to the required beam to force it to have a constant gradient in the two delivery directions. The component deliveries were found to be very spiky in nature, resulting in very inefficient delivery with the scanning leaves of our MLC. This method was evaluated using film dosimetry of four idealized beams: a 45 degree edge, a circle, a hemispherical intensity modulated beam (IMB) and a sine-like IMB. The measurements showed that this method had significantly reduced the effects of the 1 cm leaf width of our MLC at the 50% isodose level, but there was significant overdosage at the edge of the field and immediately inside the held edge. This method shows promise but further work is required before it may find clinical utility.

Conformal radiotherapy for prostate cancer

This review presents the contribution conformal treatment has made to the external beam treatment of prostate cancer drawing on 10 years of experience with three-dimensional conformal radiation treatment in 1500 patients with prostate cancer at our institution. Major contributions from other institutions and the Radiation Therapy Oncology Group (RTOG) clinical trials group are also noted. Patient immobilization, computed tomography (CT) scan identification of the target in three dimensions and beams-eye-view conformal treatment portals are critical to the process. Higher doses of radiation are associated with a marked increase in cure rates as the dose-response curve is extremely steep. The advantage given by the high dose depending on prognostic group ranges from 14% to 35% in 5-year cures. Conformal techniques protect normal tissues so that late complications are rare and in fact fewer than those observed with standard radiation technique at standard radiation doses. Educational efforts are underway in the USA to assist practising radiation oncologists to deliver the required radiation doses of 75-80 Gy safely. Conformal therapy cures more prostate cancers causing fewer complications than standard treatment technique and standard dose. It must therefore be disseminated into the practice of radiation oncology worldwide.

Progress in 3D conformal radiation treatment of prostate cancer

The development of 3D conformal radiation is reviewed and the proof given of three key hypotheses that result in improved patient results. The Fox Chase outcomes from 3D CRT show 7- and 8-year biochemical freedom of disease rates of 72% for all patients, 84% for PSA < 10 ng/ml, 73% for PSA 10-19.9 ng/ml and 37% for PSA > 20 ng/ml. Dose responses by pretreatment PSA level grouping show steep slopes for patients with PSA> 10 ng/ml. Dose response is also demonstrated for 'favorable' and 'unfavorable' prognostic subgroupings of the classic three pretreatment PSA groups. A matched pair analysis of high-dose 3D CRT versus low-dose 3D CRT shows on multivariate analysis a significant advantage in freedom from distant metastasis. A similar matched comparison of high-dose 3D CRT with low-dose 3D CRT patients and low-dose conventional technique patients shows a significant advantage in distant metastasis, cause-specific survival and overall survival for the high-dose group. Future studies including those using biochemical endpoints to direct differential dose distributions are discussed.

The roles of multileaf collimators and micro-multileaf collimators in conformal and conventional nasopharyngeal carcinoma radiotherapy treatments

The purpose of this work is to study the efficacy and limitations of using standard multileaf collimators (MLCs) and micro-multileaf collimators (mMLCs) in the treatment of nasopharyngeal carcinoma (NPC) by conventional and conformal radiotherapy techniques. The penumbra characteristics of MLC, mMLC, and customized block collimated beams are measured with respect to leaf edge angle, beam energy, treatment depth, and field size and compared with those generated by a commercial three-dimensional planning computer system. Upon verification of the planning system, it is used to evaluate the treatment plans generated with these beam shapers for conventional and conformal NPC treatments. The effective penumbra of a MLC beam is strongly influenced by its edge angle, leaf width, and treatment depth. The suitability of standard MLCs in conventional NPC treatments is determined mainly by the edge angle to be used. For conformal NPC treatments involving six or more fields, dose volume histograms comparable to those of customized beam blocks are obtained with a standard MLC. The mMLC does not have the same restrictions as those on standard MLC but is limited to phase II treatment by its small usable field size. Both standard MLCs and mMLCs can be used to replace customized divergent beam blocks in both conventional and conformal NPC treatments. However, a MLC, due to its larger effective penumbra, may be unsuitable for use in cases when the tumor volumes extend very close to the critical normal structures. A mMLC, on the other hand, is limited by its small maximum field size and can only be used for collimating the facial portals in the second phase treatment.

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.

Theoretical considerations of monitor unit calculations for intensity modulated beam treatment planning

A treatment planning system to compute intensity modulated radiotherapy (IMRT) treatments using inverse planning was investigated. The system was designed to optimize the intensity patterns required to treat a specified target volume with specified normal structure constraints. A beam model that uses the convolution of pencil beams was used to compute the dose distributions. A multileaf collimator leaf-setting sequence intended to produce the intensity pattern was computed along with the monitor units required to deliver each of a number of fixed-gantry modulated fields. Computer calculations are commonly verified using an independent manual procedure. It is difficult to calculate treatment delivery monitor units for this variant of IMRT using manual methods. Since manual calculations are not feasible, it is important both to understand and to verify the calculation of treatment monitor units by the planning system algorithm. A formal analysis was made of the dose calculation model and the monitor unit calculation embedded in the algorithm. Experimental verification of the dose delivered by plans computed with the methodology demonstrated an agreement of better than 4% between the dose model and measurements.

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.

A robust method of multileaf collimator (MLC) leaf-configuration verification

Presents a novel and robust method for leaf-position verification with a multileaf collimator (MLC). On the portal image associated with an MLC-generated treatment field, all true treatment-held-edge lines are either parallel or perpendicular to each other. This unique feature of an MLC treatment field has been fully exploited by the authors' method. Employing a Hough-type transformation as an edge-line-orientation detector and a chamfer-matching method, the authors can find the best matching parameters (including translation, rotation and scaling) adaptively between a prescribed MLC leaf configuration and the actual treatment-held edges generated by the MLC system. This works even if the portal image is partially corrupted by noise or covered by compact bony structures. Comparing these parameters with clinically accepted tolerances, the authors can make a "go-or-no-go" decision quickly.

Assessment of dose inhomogeneities in clinical practice by film dosimetry

AIM

To use portal images acquired in routine circumstances for assessment of midplane dose variations in the patient.

MATERIAL AND METHODS

Optical density readings are performed on routinely acquired Verification films of breast and ear-nose-throat (ENT) cancer patients and these readings are converted into relative doses with the sensitometric curve. ( 1 ) The impact of redistribution is evaluated on films taken close to the patient exit surface and at routine focus film distance. (2) Midplane doses are estimated from film readings to assess dose variations in the patient. The influence of wedges is evaluated. Film measurements doses are compared with calculated exit doses.

RESULTS

(1) In regions with large variations in the distance between the patient exit surface and the film but without inhomogeneities in tissue density, the relative doses distributions read on films acquired at large focus-film-distance (FFD) are proportional to exit doses. In regions with flat exit surfaces but with inhomogeneities in tissue density, the redistribution has only a small impact. (2) Large variations in relative midplane doses were found in both breast (85-115%) and ENT (-3.6 to +15%) patients. The application of a wedge was shown to increase dose homogeneity in the midplane. A good agreement (differences < 3%) was found between exit doses obtained from film readings and exit doses calculated with the treatment planning system (TPS).

CONCLUSION

Films acquired in routine circumstances at large FFD can be used to obtain information on exit doses and to assess midplane doses in breast and ENT, without the use of a TPS. Film dosimetry can also provide a quality assurance tool to check actually delivered doses in patients by comparing exit doses estimated on film to expected exit doses calculated by the TPS.

Update on the treatment of prostate cancer with external beam irradiation

BACKGROUND

We review the recent changes in the radiotherapeutic management of clinically localized prostate cancer, including the implementation of three-dimensional (3-D) conformal radiation therapy (3DCRT), biochemical disease-free survival (bNED control) using conventional and 3DCRT techniques, and the morbidity of these treatment strategies.

METHODS

The components of 3DCRT are discussed, including patient immobilization, 3-D treatment planning, multileaf collimation, and electronic portal imaging. bNED control rates from institutions using conventional and 3DCRT techniques are compared. The gastrointestinal (GI) and genitourinary (GU) morbidity from prospective trials using conventional doses of radiation are compared to data from 3DCRT series. bNED control rates stratified by pretreatment prostate-specific antigen (PSA) are compared between surgical and radiation series.

RESULTS

bNED control rates (3-5 years) for patients treated with conventional and 3DCRT techniques ranged from 44-70% and 30-72% with pretreatment PSA levels 4-10 and 10-20, respectively. Although direct comparisons are difficult between treatment modalities, no difference in bNED control stratified by pretreatment PSA was observed between surgical and radiation patients.

CONCLUSIONS

Patients with clinically localized prostate cancer treated with 3DCRT demonstrate durable bNED control at 5 years. Conformal radiation techniques, multileaf collimation, electronic portal imaging, and patient immobilization have reduced acute and chronic GI and GU morbidity while allowing safe dose escalation in an effort to further improve local control and overall survival.

Physical and dosimetric aspects of a multileaf collimation system used in the dynamic mode for implementing intensity modulated radiotherapy

The use of a multileaf collimator in the dynamic mode to perform intensity modulated radiotherapy became a reality at our institution in 1995. Unlike treatment with static fields using a multileaf collimator, there are significant dosimetric issues which must be assessed before dynamic therapy can be implemented. We have performed a series of calculations and measurements to quantify head scatter for small fields, collimator transmission, and the transmission through rounded leaf ends. If not accounted for, these factors affect the delivered dose to the prostate by 5%-20% for a typical plan. Data obtained with ion chambers and radiographic film are presented for both 6 and 15 MV x-ray beams. The impact on the delivered dose of the mechanical accuracy of the multileaf collimator, achieved during leaf position calibration and maintained during dose delivery, is also discussed.

Improving dose homogeneity in routine head and neck radiotherapy with custom 3-D compensation

BACKGROUND AND PURPOSE

Anatomic contour irregularity and tissue inhomogeneity can lead to significant radiation dose variation across the complex treatment volumes found in the head and neck (H&N) region. This dose inhomogeneity can routinely create focal hot or cold spots of 10-20% despite beam shaping with blocks or beam modification with wedges. Since 1992, we have implemented the routine use of 3-D custom tissue compensators fabricated directly from CT scan contour data obtained in the treatment position in order to improve dose uniformity in patients with tumors of the H&N.

MATERIALS AND METHODS

Between July 1992 and January 1997, 160 patients receiving comprehensive H&N radiotherapy had 3-D custom compensators fabricated for their treatment course. Detailed dosimetric records have been analyzed for 30 cases. Dose uniformity across the treatment volume and clinically relevant maximum doses to selected anatomic sub-sites were examined with custom-compensated, uncompensated and optimally-wedged plans.

RESULTS

The use of 3-D custom compensators resulted in an average reduction of dose variance across the treatment volume from 19+/-4% for the uncompensated plans to 5+/-2% with the use of 3-D compensators. Optimally-wedged plans were variable, but on average a 10+/-3% dose variance was noted. For comprehensive H&N treatment which encompassed the larynx within the primary field design, the peak doses delivered were reduced by 5-15% with 3-D custom compensation as compared to optimal wedging.

CONCLUSIONS

The use of 3-D custom tissue compensation can improve dose homogeneity within the treatment volume for H&N cancer patients. Maximum doses to clinically important structures which often receive greater than 105-110% of the prescribed dose are routinely reduced with the use of 3-D custom compensators. Improved dose uniformity across the treatment volume can reduce normal tissue complication profiles and potentially allow for delivery of higher total doses in an attempt to enhance locoregional tumor control.

Beam characteristics of a retrofitted double-focused multileaf collimator

Multileaf collimators (MLCs) are generally believed to be convenient and cost-effective tools for intensity modulation and conformal therapy. They are becoming a standard feature on new accelerators; however, the older units can be retrofitted with modern MLCs. Before such a unit can be clinically used, the beam characteristics must be verified. In this study the beam characteristics of a Siemens double-focused MLC retrofitted to an MD2 linear accelerator are presented. The head leakage along with inter- and intra-leaf radiation transmission were measured using film. The collimator (Sc), phantom (Sp), total (Scp) scatter factors, central axis depth dose, beam profiles for off-axis ratios, penumbra, and surface dose were evaluated for square, rectangular, and irregularly shaped fields. The maximum head leakage was estimated to be < 0.05% in any plane at a distance of 1 m and maximum transmission through the MLC leaves was estimated to be < 1.4% and < 1.1% for the 10 MV and 6 MV beams, respectively. The maximum differences between pre- and post-MLC installation data for the Sc and Scp were < or = 0.7% and < or = 1.4%, respectively. Similarly, the percent depth dose data for all fields and both beam energies were within 1.5% of the original data. The beam profiles measured at various depths were also in agreement with those of the pre-MLC installation data. The measured beam penumbra (20%-80%) showed a range of 7.8 mm-11.0 mm for the 6 MV and 8.4 mm-11.1 mm for the 10 MV beams from smallest to largest fields. These ranges differ by less than a millimeter from those of the old data. The surface dose measurements were slightly lower than the conventional jaw values suggesting that MLC does not produce significant electron contamination. It is concluded that the retrofitted MLC maintains the integrity of the original beam and may provide a cost-effective conformal therapy.

Clinical use of a simulation-multileaf collimator

BACKGROUND

At the University of Lübeck, radiotherapy is delivered by a 6/18-MV linear accelerator. Using the integrated multileaf collimator, irradiation of individually shaped treatment fields is possible in place of alloy blocks. Due to unsatisfactory pretherapeutic review of the radiation-field-specific multileaf collimator (MLC) configuration, we developed a simulation-multileaf collimator (SMLC) and assessed its feasibility at different tumor sites.

MATERIAL AND METHODS

The SMLC is made of a perspex carrier with 52 horizontal sliding leaves. The position of each leaf is calculated by a 3D treatment-planning computer. The technician manually adjusts the leaves according to the beams-eye-view plot of the planning computer. Consequently, the SMLC is mounted on the therapy simulator at a distance of 64.8 cm from the focus. The treatment fields and the position of the leaves are documented by X-ray films.

RESULTS

Using the SMLC, radiation oncologists are able to review exactly the leaf configuration of each MLC-shaped radiation field and to correlate the MLC-shaped radiation field with the treated volume, the organs at risk and the port films acquired by the Portal Vision system.

CONCLUSION

The SMLC is a new tool to review radiation planning that uses an MLC in daily routine. The use of the SMLC improves the documentation and the quality assurance. It accelerates the treatment field review at the linear accelerator by comparing the SMLC simulator films with the portal images.

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.

Multiple two-dimensional versus three-dimensional PTV definition in treatment planning for conformal radiotherapy

PURPOSE

To demonstrate the need for a fully three-dimensional (3D) computerized expansion of the gross tumour volume (GTV) or clinical target volume (CTV), as delineated by the radiation oncologist on CT slices, to obtain the proper planning target volume (PTV) for treatment planning according to the ICRU-50 recommendations.

MATERIALS AND METHODS

For 10 prostate cancer patients two PTVs have been determined by expansion of the GTV with a 1.5 cm margin, i.e. a 3D PTV and a multiple 2D PTV. The former was obtained by automatically adding the margin while accounting in 3D for GTV contour differences in neighbouring slices. The latter was generated by automatically adding the 1.5 cm margin to the GTV in each CT slice separately; the resulting PTV is a computer simulation of the PTV that a radiation oncologist would obtain with (the still common) manual contouring in CT slices. For each patient the two PTVs were compared to assess the deviations of the multiple 2D PTV from the 3D PTV. For both PTVs conformal plans were designed using a three-field technique with fixed block margins. For each patient dose-volume histograms and tumour control probabilities (TCPs) of the (correct) 3D PTV were calculated, both for the plan designed for this PTV and for the treatment plan based on the (deviating) 2D PTV.

RESULTS

Depending on the shape of the GTV, multiple 2D PTV generation could locally result in a 1 cm underestimation of the GTV-to-PTV margin. The deviations occurred predominantly in the cranio-caudal direction at locations where the GTV contour shape varies significantly from slice to slice. This could lead to serious underdosage and to a TCP decrease of up to 15%.

CONCLUSIONS

A full 3D GTV-to-PTV expansion should be applied in conformal radiotherapy to avoid underdosage.