Evaluation of microscopic tumor extension in non-small-cell lung cancer for three-dimensional conformal radiotherapy planning

PURPOSE

One of the most difficult steps of the three-dimensional conformal radiotherapy (3DCRT) is to define the clinical target volume (CTV) according to the degree of local microscopic extension (ME). In this study, we tried to quantify this ME in non-small-cell lung cancer (NSCLC).

MATERIAL AND METHODS

Seventy NSCLC surgical resection specimens for which the border between tumor and adjacent lung parenchyma were examined on routine sections. This border was identified with the naked eye, outlined with a marker pen, and the value of the local ME outside of this border was measured with an eyepiece micrometer. The pattern of histologic spread was also determined.

RESULTS

A total of 354 slides were examined, corresponding to 176 slides for adenocarcinoma (ADC) and 178 slides for squamous cell carcinoma (SCC). The mean value of ME was 2.69 mm for ADC and 1.48 mm for SCC (p = 0.01). The usual 5-mm margin covers 80% of the ME for ADC and 91% for SCC. To take into account 95% of the ME, a margin of 8 mm and 6 mm must be chosen for ADC and SCC, respectively. Aerogenous dissemination was the most frequent pattern observed for all groups, followed by lymphatic invasion for ADC and interstitial extension for SCC.

CONCLUSION

The ME was different between ADC and SCC. The usual CTV margin of 5 mm appears inadequate to cover the ME for either group, and it must be increased to 8 mm and 6 mm for ADC and SCC, respectively, to cover 95% of the ME. This approach is obviously integrated into the overall 3DCRT procedure and with other margins.

Conformal radiotherapy (CRT) planning for lung cancer: analysis of intrathoracic organ motion during extreme phases of breathing

PURPOSE

Conformal radiotherapy beams are defined on the basis of static computed tomography acquisitions by taking into account setup errors and organ/tumor motion during breathing. In the absence of precise data, the size of the margins is estimated arbitrarily. The objective of this study was to evaluate the amplitude of maximum intrathoracic organ motion during breathing.

METHODS AND MATERIALS

Twenty patients treated for non-small-cell lung cancer were included in the study: 10 patients at the Institut Curie with a personalized alpha cradle immobilization and 10 patients at Tenon Hospital with just the Posirest device below their arms. Three computed tomography acquisitions were performed in the treatment position: the first during free breathing and the other two during deep breath-hold inspiration and expiration. For each acquisition, the displacements of the various intrathoracic structures were measured in three dimensions.

RESULTS

Patients from the two centers were comparable in terms of age, weight, height, tumor site, and stage. In the overall population, the greatest displacements were observed for the diaphragm, and the smallest displacements were observed for the lung apices and carina. The relative amplitude of motion was comparable between the two centers. The use of a personalized immobilization device reduced lateral thoracic movements (p < 0.02) and lung apex movements (p < 0.02).

CONCLUSION

Intrathoracic organ movements during extreme phases of breathing are considerable. Quantification of organ motion is necessary for definition of the safety margins. A personalized immobilization device appears to effectively reduce apical and lateral displacement.

CT and (18)F-deoxyglucose (FDG) image fusion for optimization of conformal radiotherapy of lung cancers

PURPOSE

To validate a computed tomography (CT) and (18)F-deoxyglucose (FDG) image fusion procedure and to evaluate its usefulness to facilitate target definition and treatment planning in three-dimensional conformal radiation therapy (3D-CRT) for non-small-cell lung cancer.

METHODS AND MATERIALS

Twelve patients were assessed by CT and FDG-coincidence mode dual-head gamma camera (CDET) before radiotherapy. The patients were placed in a similar position during CT and FDG-CDET. Matching was achieved by minimizing the cost function by 3D translation and rotation between four landmarks drawn on the patient's skin. Virtual simulation was performed from image fusion and estimated dose-volume histograms (DVH) were calculated.

RESULTS

Quantitative analysis indicated that the matching error was < 5 mm. Fusion of anatomic and metabolic data corrected staging of lymph nodes (N) for 4 patients and staging of metastases for 1 patient. In these 5 patients, DVH revealed that the lung volume irradiated at 20 Gy (Vl(20)) was decreased by an average of 22.8%, and tumor volume irradiated at the 95% isodose (V(95)) was increased by 22% and 8% for 2 patients, respectively, and was decreased by an average of 59% for 3 patients after fusion. No difference in terms of Vl(20) and V(95) was observed for the other 7 patients.

CONCLUSION

We have validated CT and FDG-CDET lung image fusion to facilitate determination of lung cancer volumes, which improved the accuracy of 3D-CRT.

Breast radiotherapy in the lateral decubitus position: A technique to prevent lung and heart irradiation

PURPOSE

To present an original technique for breast radiotherapy, with the aim of limiting lung and heart irradiation, satisfying quality assurance criteria.

METHODS AND MATERIAL

An original radiotherapy technique for breast irradiation has been developed at the Institute Curie in January 1996. It consists of isocentric breast irradiation in the lateral decubitus position (isocentric lateral decubitus [ILD]). This technique is indicated for voluminous or pendulous breasts needing breast irradiation only. Thin carbon fiber supports and special patient positioning devices have been developed especially for this technique. In vivo measurements were performed to check the dose distribution before the routine use of the technique.

RESULTS

ILD has been successfully implemented in routine practice, and 500 patients have been already treated. Breast radiotherapy is performed using a dose of 50 Gy at ICRU point in 25 fractions. ILD shows good homogeneity of the dose in breast treatment volume, treatment fields are perpendicular to the skin ensuring its protection, and extremely low dose is delivered to the underlying lung and heart.

CONCLUSION

In cases of voluminous breasts or patients with a history of lung and heart disease, our technique provides several advantages over the conventional technique with opposing tangential fields. This technique improves the dose homogeneity according to the ICRU recommendations.

Variation of sensitometric curves of radiographic films in high energy photon beams

Film dosimetry is an important tool for the verification of irradiation techniques. The shape of the sensitometric curve depends on the type of film as well as on the irradiation and processing conditions. Existing data concerning the influence of irradiation geometry on the sensitometric curve are conflicting. In particular the variation of optical density, OD, with field size and depth in a phantom shows large differences in magnitude between various authors. This variation, as well as the effect of beam energy and film plane orientation on OD, was therefore investigated for two types of film, Kodak X-Omat V and Agfa Structurix D2. Films were positioned in a solid phantom, either perpendicular or (almost) parallel to the beam axis, and irradiated to different dose levels using various photon beams (Co-60, 6 MV, 15 MV, 18 MV, 45 MV). It was found that the sensitometric curves of the Kodak film derived at different depths are almost identical for the four x-ray beams. For the Kodak film the differences in OD with depth are less than 2%, except for the Co-60 beam, where the difference is about 4% at 10 cm depth for a 15 cm x 15 cm field. The slope of the sensitometric curve of the Agfa film is somewhat more dependent on photon beam energy, depth and field size. The sensitometric curves of both types of film are almost independent of the film plane orientation, except for shallow depths. For Co-60 and for the same dose, the Kodak and Agfa films gave at dose maximum an OD lower by 4% and 6%, respectively, for the parallel compared to the perpendicular geometry. Good dosimetric results can be obtained if films from the same batch are irradiated with small to moderate field sizes (up to about 15 cm x 15 cm), at moderate depths (up to about 15 cm), using a single calibration curve, e.g., for a 10 cm x 10 cm field.

PSA bounce after permanent implant prostate brachytherapy may mimic a biochemical failure: A study of 295 patients with a minimum 3-year followup

PURPOSE

To assess the frequency and features of the PSA bounce phenomenon in a series of patients treated with permanent implant brachytherapy for prostate cancer, and to evaluate the percentage of cases in which this bounce could have mimicked a biochemical relapse according to the American Society for Therapeutic Radiology and Oncology consensus criteria.

METHODS AND MATERIALS

From January 1999 to December 2001, 295 patients were treated with a permanent prostate implantation (real-time technique, with free (125)I seeds) by the Paris Institut Curie/Hospital Cochin/Hospital Necker Paris group. Duration of followup is 40.3 months (9-66 months). PSA level was reported at intervals not exceeding 6 months. Bounce was defined by temporary elevation in PSA level, followed by a spontaneous decrease.

RESULTS

In our series, 161 patients (55%) showed a transitory PSA increase (bounce) of at least 0.1 ng/mL; 145 patients (49%) a bounce of 0.2 ng/mL; 93 patients (32%) a bounce of 0.4 ng/mL; and 43 patients (15%) a bounce of at least 1 ng/mL. Mean PSA bounce was 0.8 ng/mL (0.1-4.1), and mean time to bounce was 19 months. Thirty-two patients (11% of total) presented three successive PSA increases, and therefore were to be considered as experiencing a biochemical relapse according to the American Society for Therapeutic Radiology and Oncology (ASTRO) consensus criteria. Among those 32 patients, 18 (56%) subsequently showed, without any treatment, a complete normalization of their PSA. In multivariate analysis, age <70 (p<0.0001) and D90>200Gy (p<0.003) were identified as independent factors for a PSA bounce of at least 0.4 ng/mL.

CONCLUSIONS

The observed rate of 32% of patients showing a PSA bounce of at least 0.4 ng/mL in our series is in good agreement with what has been previously reported in the literature. Among 32 patients fulfilling the classical ASTRO criteria for a biochemical relapse, 18 (56%) subsequently showed a spontaneous PSA decrease, questioning the ASTRO consensus for the biochemical followup of patients undergoing permanent implant prostate brachytherapy.

Breast irradiation in the lateral decubitus position: technique of the Institut Curie

Breast irradiation in the lateral decubitus (LD) position is a technique used at the Institut Curie for more than 30 years in the breast-conserving management of patient with breast cancer. This technique is described in detail in this article. The patient's position allows the breast to flatten over a support, hence providing a rather homogeneous thickness throughout the treated volume. Dose at mid-thickness on the beam axis can be easily determined from entrance and exit dose measurements. Disadvantages and advantages of the LD technique are discussed. We presently recommend this technique for patients with large breasts (more than 6 cm thickness in LD position).

Experimental determination and verification of the parameters used in a proton pencil beam algorithm

We present an experimental procedure for the determination and the verification under practical conditions of physical and computational parameters used in our proton pencil beam algorithm. The calculation of the dose delivered by a single pencil beam relies on a measured spread-out Bragg peak, and the description of its radial spread at depth features simple specific parameters accounting individually for the influence of the beam line as a whole, the beam energy modulation, the compensator, and the patient medium. For determining the experimental values of the physical parameters related to proton scattering, we utilized a simple relation between Gaussian radial spreads and the width of lateral penumbras. The contribution from the beam line has been extracted from lateral penumbra measurements in air: a linear variation with the distance collimator-point has been observed. Analytically predicted radial spreads within the patient were in good agreement with experimental values in water under various reference conditions. Results indicated no significant influence of the beam energy modulation. Using measurements in presence of Plexiglas slabs, a simple assumption on the effective source of scattering due to the compensator has been stated, leading to accurate radial spread calculations. Dose measurements in presence of complexly shaped compensators have been used to assess the performances of the algorithm supplied with the adequate physical parameters. One of these compensators has also been used, together with a reference configuration, for investigating a set of computational parameters decreasing the calculation time while maintaining a high level of accuracy. Faster dose computations have been performed for algorithm evaluation in the presence of geometrical and patient compensators, and have shown good agreement with the measured dose distributions.

Automatic exposure control in multichannel CT with tube current modulation to achieve a constant level of image noise: Experimental assessment on pediatric phantoms

Automatic exposure control (AEC) systems have been developed by computed tomography (CT) manufacturers to improve the consistency of image quality among patients and to control the absorbed dose. Since a multichannel helical CT scan may easily increase individual radiation doses, this technical improvement is of special interest in children who are particularly sensitive to ionizing radiation, but little information is currently available regarding the precise performance of these systems on small patients. Our objective was to assess an AEC system on pediatric dose phantoms by studying the impact of phantom transmission and acquisition parameters on tube current modulation, on the resulting absorbed dose and on image quality. We used a four-channel CT scan working with a patient-size and z-axis-based AEC system designed to achieve a constant noise within the reconstructed images by automatically adjusting the tube current during acquisition. The study was performed with six cylindrical poly(methylmethacrylate) (PMMA) phantoms of variable diameters (10-32 cm) and one 5 years of age equivalent pediatric anthropomorphic phantom. After a single scan projection radiograph (SPR), helical acquisitions were performed and images were reconstructed with a standard convolution kernel. Tube current modulation was studied with variable SPR settings (tube angle, mA, kVp) and helical parameters (6-20 HU noise indices, 80-140 kVp tube potential, 0.8-4 s. tube rotation time, 5-20 mm x-ray beam thickness, 0.75-1.5 pitch, 1.25-10 mm image thickness, variable acquisition, and reconstruction fields of view). CT dose indices (CTDIvol) were measured, and the image quality criterion used was the standard deviation of the CT number measured in reconstructed images of PMMA material. Observed tube current levels were compared to the expected values from Brooks and Di Chiro's [R.A. Brooks and G.D. Chiro, Med. Phys. 3, 237-240 (1976)] model and calculated values (product of a reference value multiplied by a dose ratio measured with thermoluminescent dosimeters). Our study demonstrates that this AEC system accurately modulates the tube current according to phantom size and transmission to achieve a stable image noise. The system accurately controls the tube current when changing tube rotation time, tube potential, or image thickness, with minimal variations of the resulting noise. Nevertheless, CT users should be aware of possible changes of tube current and resulting dose and quality according to several parameters: the tube angle and tube potential used for SPR, the x-ray beam thickness (tube current decreases and image noise increases when doubling x-ray beam thickness), the pitch value (a pitch decrease leads to a higher dose but also to a higher noise), and the acquisition field of view (FOV) (tube current is lower when using the small acquisition FOV compared to the large one, but the use of small acquisition FOV at 120 kVp leads to a peculiar increase of tube current and CTDIvol).

Comparison of conformal radiation therapy techniques within the dynamic radiotherapy project 'Dynarad'

The objective of the dynamic radiotherapy project 'Dynarad' within the European Community has been to compare and grade treatment techniques that are currently applied or being developed at the participating institutions. Cervical cancer was selected as the tumour site on the grounds that the involved organs at risk, mainly the rectum and the bladder, are very close to the tumour and partly located inside the internal target volume. In this work, a solid phantom simulating the pelvic anatomy was used by institutions in Belgium, France, Greece, Holland, Italy, Sweden and the United Kingdom. The results were evaluated using both biological and physical criteria. The main purpose of this parallel evaluation is to test the value of biological and physical evaluations in comparing treatment techniques. It is demonstrated that the biological objective functions allow a much higher conformality and a more clinically relevant scoring of the outcome. Often external beam treatment techniques have to be combined with intracavitary therapy to give clinically acceptable results. However, recent developments can reduce or even eliminate this need by delivering more conformal dose distributions using intensity modulated external dose delivery. In these cases the reliability of the patient set-up procedure becomes critical for the effectiveness of the treatment.

Assessment of organ absorbed doses and estimation of effective doses from pediatric anthropomorphic phantom measurements for multi-detector row CT with and without automatic exposure control

This study was designed to measure organ absorbed doses from multi-detector row computed tomography (MDCT) on pediatric anthropomorphic phantoms, calculate the corresponding effective doses, and assess the influence of automatic exposure control (AEC) in terms of organ dose variations. Four anthropomorphic phantoms (phantoms represent the equivalent of a newborn, 1-, 5-, and 10-y-old child) were scanned with a four-channel MDCT coupled with a z-axis-based AEC system. Two CT torso protocols were compared: a first protocol without AEC and constant tube current-time product and a second protocol with AEC using age-adjusted noise indices. Organ absorbed doses were monitored by thermoluminescent dosimeters (LiF: Mg, Cu, P). Effective doses were calculated according to the tissue weighting factors of the International Commission on Radiological Protection (). For fixed mA acquisitions, organ doses normalized to the volume CT dose index in a 16-cm head phantom (CTDIvol16) ranged from 0.6 to 1.5 and effective doses ranged from 8.4 to 13.5 mSv. For the newborn-equivalent phantom, the AEC-modulated scan showed almost no significant dose variation compared to the fixed mA scan. For the 1-, 5- and 10-y equivalent phantoms, the use of AEC induced a significant dose decrease on chest organs (ranging from 61 to 31% for thyroid, 37 to 21% for lung, 34 to 17% for esophagus, and 39 to 10% for breast). However, AEC also induced a significant dose increase (ranging from 28 to 48% for salivary glands, 22 to 51% for bladder, and 24 to 70% for ovaries) related to the high density of skull base and pelvic bones. These dose increases should be considered before using AEC as a dose optimization tool in children.

On-axis and off-axis primary dose component in high energy photon beams

The depth dose of the primary dose component, on axis and off axis of six different x-ray beams, has been determined from transmission measurements in narrow beam geometry with and without flattening filter using a Perspex column of a cross section large enough to ensure electronic equilibrium. In order to derive the primary photon fluence, a correction for the scatter from the column has been applied according to the following method: A number of spectra taken from the literature have been used for computing a scatter coefficient Sc at different depths by convolution of dose spread arrays. Using the relationship between Sc and the single attenuation coefficient mu i to represent each entire spectrum, it has been possible to correct the experimental transmission curves iteratively, until the corresponding values of mu were stabilized and representative of the primary. The measured attenuation coefficients were found to have a linear increase as a function of the distance from the central axis for all the energies and types of linear accelerators. For the same nominal energy, this increase is different from one accelerator to another. The same phenomenon was observed for the attenuation coefficients obtained without the flattening filter in the same experimental conditions. The results are tentatively interpreted considering the angular variation of bremsstrahlung energy spectra with and without a flattening filter as calculated by a Monte Carlo method and they are consistent and useful to take accurately into account the softening of the beam as the off-axis distance increases.

Prostate cancer brachytherapy: is real-time ultrasound-based dosimetry predictive of subsequent CT-based dose distribution calculation? a study of 450 patients by the Institut Curie/Hospital Cochin (Paris) Group

PURPOSE

Real-time ultrasound (US)-based dosimetry performed during (125)I loose seed implantation provides the radiation oncologist with an estimation of the dose distribution at seed insertion. However, for a number of reasons, this distribution may not reflect the real (reference) dosimetry as determined by subsequent CT, usually performed 1-2 months after implantation. The present study compared the two dosimetry data sets (US and CT) to evaluate how predictive extemporaneous US-based dosimetry can be of the real dose distribution.

METHODS AND MATERIALS

A total of 450 patients with prostate cancer were treated with loose (125)I seed implantation between June 1999 and October 2002 by the Institut Curie/Hospital Cochin (Paris) Group. The mean patient age was 65 years. Most patients (74%) had Stage T1c; the stage did not exceed T2b for the others. All patients had a prostate-specific antigen level of <15 ng/mL and was <10 ng/mL for 72%; 84% had a Gleason score of < or =6 and did not exceed 7 for the others; and 56% were treated with neoadjuvant hormonal therapy for a mean of 4.3 months. All patients were treated with loose seed implantation. Real-time US-based dosimetry was performed intraoperatively for all patients. CT-based dosimetry was performed 2 months after implantation, using the VariSeed software. The minimal dose to 90% of the outlined volume (D(90)) and percentage of volume receiving at least 100% of the prescribed dose (V(100)) were calculated with the two methods and compared for all patients.

RESULTS

On CT-based dosimetry, the D(90) was found to be > or =145 Gy (range, 115-240 Gy) in all patients except one. A large majority (86%) of patients showed a CT-based V(100) of >95%, and 48% had a V(100) of >98%. The mean CT-based D(90)/US-based D(90) ratio was 1.0 (range, 0.66-1.33). For 89% of the patients, the difference between the two values was <20% and for 62% was <10%. The mean CT-based V(100)/US-based V(100) ratio was 0.98 (range, 0-1.02), with 89% of patients showing a difference of <5%.

CONCLUSION

Our results indicate that the D(90) and V(100) values obtained intraoperatively with our real-time US-based dosimetry are in reasonable agreement with the subsequent values obtained with CT-based dosimetry performed 2 months after implantation. Recent innovations in our dose planning software allowed better control of the longitudinal seed position and could still improve the correlation between real-time US-based dosimetry and the subsequent CT-based dose distribution.

ICRP publication 112. A report of preventing accidental exposures from new external beam radiation therapy technologies

Disseminating the knowledge and lessons learned from accidental exposures is crucial in preventing re-occurrence. This is particularly important in radiation therapy; the only application of radiation in which very high radiation doses are deliberately given to patients to achieve cure or palliation of disease. Lessons from accidental exposures are, therefore, an invaluable resource for revealing vulnerable aspects of the practice of radiotherapy, and for providing guidance for the prevention of future occurrences. These lessons have successfully been applied to avoid catastrophic events with conventional technologies and techniques. Recommendations, for example, include the independent verification of beam calibration and independent calculation of the treatment times and monitor units for external beam radiotherapy, and the monitoring of patients and their clothes immediately after brachytherapy. New technologies are meant to bring substantial improvement to radiation therapy. However, this is often achieved with a considerable increase in complexity, which in turn brings opportunities for new types of human error and problems with equipment. Dissemination of information on these errors or mistakes as soon as it becomes available is crucial in radiation therapy with new technologies. In addition, information on circumstances that almost resulted in serious consequences (near-misses) is also important, as the same type of events may occur elsewhere. Sharing information about near-misses is thus a complementary important aspect of prevention. Lessons from retrospective information are provided in Sections 2 and 4 of this report. Disseminating lessons learned for serious incidents is necessary but not sufficient when dealing with new technologies. It is of utmost importance to be proactive and continually strive to answer questions such as 'What else can go wrong', 'How likely is it?' and 'What kind of cost-effective choices do I have for prevention?'. These questions are addressed in Sections 3 and 5 of this report. Section 6 contains the conclusions and recommendations. This report is expected to be a valuable resource for radiation oncologists, hospital administrators, medical physicists, technologists, dosimetrists, maintenance engineers, radiation safety specialists, and regulators. While the report applies specifically to new external beam therapies, the general principles for prevention are applicable to the broad range of radiotherapy practices where mistakes could result in serious consequences for the patient and practitioner.

Fully 4D list-mode reconstruction applied to respiratory-gated PET scans

(18)F-fluoro-deoxy-glucose ((18)F-FDG) positron emission tomography (PET) is one of the most sensitive and specific imaging modalities for the diagnosis of non-small cell lung cancer. A drawback of PET is that it requires several minutes of acquisition per bed position, which results in images being affected by respiratory blur. Respiratory gating techniques have been developed to deal with respiratory motion in the PET images. However, these techniques considerably increase the level of noise in the reconstructed images unless the acquisition time is increased. The aim of this paper is to evaluate a four-dimensional (4D) image reconstruction algorithm that combines the acquired events in all the gates whilst preserving the motion deblurring. This algorithm was compared to classic ordered subset expectation maximization (OSEM) reconstruction of gated and non-gated images, and to temporal filtering of gated images reconstructed with OSEM. Two datasets were used for comparing the different reconstruction approaches: one involving the NEMA IEC/2001 body phantom in motion, the other obtained using Monte-Carlo simulations of the NCAT breathing phantom. Results show that 4D reconstruction reaches a similar performance in terms of the signal-to-noise ratio (SNR) as non-gated reconstruction whilst preserving the motion deblurring. In particular, 4D reconstruction improves the SNR compared to respiratory-gated images reconstructed with the OSEM algorithm. Temporal filtering of the OSEM-reconstructed images helps improve the SNR, but does not achieve the same performance as 4D reconstruction. 4D reconstruction of respiratory-gated images thus appears as a promising tool to reach the same performance in terms of the SNR as non-gated acquisitions while reducing the motion blur, without increasing the acquisition time.

Lung volume assessment for a cross-comparison of two breathing-adapted techniques in radiotherapy

PURPOSE

To assess the validity of gated radiotherapy of lung by using a cross-check methodology based on four-dimensional (4D)-computed tomography (CT) exams. Variations of volume of a breathing phantom was used as an indicator.

METHODS AND MATERIALS

A balloon was periodically inflated and deflated by a medical ventilator. The volume variation (DeltaV) of the balloon was measured simultaneously by a spirometer, taken as reference, and by contouring 4D-CT series (10 phases) acquired by the real-time position management system (RPM). Similar cross-comparison was performed for 2 lung patients, 1 with free breathing (FB), the other with deep-inspiration breath-hold (DIBH) technique.

RESULTS

During FB, DeltaV measured by the spirometer and from 4D-CT were in good agreement: the mean differences for all phases were 8.1 mL for the balloon and 10.5 mL for a patient-test. End-inspiration lung volume has been shown to be slightly underestimated by the 4D-CT. The discrepancy for DeltaV between DIBH and end-expiration, measured from CT and from spirometer, respectively, was less than 3%.

CONCLUSIONS

Provided that each slice series is correctly associated with the proper breathing phase, 4D-CT allows an accurate assessment of lung volume during the whole breathing cycle (DeltaV error <3% compared with the spirometer signal). Taking the lung volume variation into account is a central issue in the evaluation and control of the toxicity for lung radiation treatments.

A model for the lateral penumbra in water of a 200-MeV proton beam devoted to clinical applications

An experimental approach for modeling the lateral penumbra of a proton beam has been investigated. Measurements were made with a silicon diode in a water tank. Several geometrical configurations (phantom position, collimator-to-surface distance, collimator diameter, bolus thickness, air gap, etc.) and beam characteristics (range, modulation, etc.) have been studied. The results show that the lateral penumbra is almost independent of the beam modulation and the diameter of the collimator. The use of scaled variables for depth and penumbra allows us to represent the increase in penumbra with depth for any configuration with a second order polynomial function, provided that the penumbra at the entrance of the medium and at the depth of the range are known.

Incidence of anterior pituitary deficiency after radiotherapy at an early age: study in retinoblastoma

Thirty-one patients treated for retinoblastoma in the first few years (3 months to 3 years and 6 months) of life were studied 2 to 15 years later. Radiotherapy delivered 1 300 to 6 500 rads to the hypothalamo-pituitary area. Growth deficiency was documented in 30% of all cases. Other pituitary deficiencies were the exception. The critical dose for GH insufficiency is between 2 000 and 3 000 rads, as in older children or adults. Our study does not support the hypothesis that the hypothalamo-pituitary area is more sensitive to radiation at an early age. Furthermore, conservative therapy of retinoblastoma leads to double lateral irradiation and will increase the number of GH deficient children after retinoblastoma.

Automatic three-dimensional expansion of structures applied to determination of the clinical target volume in conformal radiotherapy

PURPOSE

A method is provided for the automatic calculation of the Clinical Target Volume (CTV) by automatic three dimensional (3D) expansion of the Gross Tumor Volume (GTV), keeping a constant margin M in all directions and taking into account anatomic obstacles.

METHODS AND MATERIALS

Our model uses a description of the GTV from contours (polygons) defined in a series of parallel slices obtained from Computed Tomography (CT) or Magnetic Resonance Imaging (MRI). Each slice is considered sequentially, including those slices located apart from the GTV at a distance smaller than M. In the current slice a two-dimensional (2D) expansion is performed by transforming each vertex of the polygon into a circle with a radius equal to M, and each segment into a rectangle with a height equal to 2M. A cartesian millimetric grid is then "projected" onto the slice and a specific value is assigned at each point depending if the point is internal to the 2D expansion. The influence in the current slice of any slice located at a distance delta z smaller than M is taken into account by applying a 2D expansion using a margin [formula: see text]. Additional contours representative of various "barriers" stopping the expansion process can also be defined.

RESULTS

The method has been applied to cylindrical and spherical structures and has proven to be successful, provided that the slice thickness is small enough. For usual slice thicknesses and margins, it gives a slight overestimation of the additional volume (around 5%) due to the choice that the calculated target volume would not be less than the expected volume. It has been shown that for a spherical volume, a 2D expansion performed slice by slice leads to a volume up to 80% smaller than that obtained by 3D expansion.

CONCLUSIONS

This tool, which mimics the tumor cell spreading process, has been integrated in our treatment-planning software and used clinically for conformal radiotherapy of brain and prostatic tumors. It has been found to be extremely useful, not only saving time but also allowing a precise determination of the CTV which would be impossible to do manually.

Quality assurance in conformal radiotherapy: DYNARAD consensus report on practice guidelines

BACKGROUND AND PURPOSE

Conformal radiotherapy has only recently been widely implemented. Although not all aspects have yet been adequately proven, it is generally recognized that maintaining a high degree of precision throughout the process is critical to the treatment outcome while the focus for quality assurance and quality improvement will need to concentrate more on human factors, procedures, communication, organization and training. A general consensus document on quality assurance guidelines for institutions that deliver conformal radiotherapy treatments to patients has been elaborated within the framework of the DYNARAD/BIOMED concerted action on conformal radiotherapy. The present paper aims to highlight those issues that were identified as of specific importance to conformal radiotherapy. The work reported here further details this guidance by direct correlation with the issues involved in the special case of conformal radiotherapy.

METHODS

The DYNARAD document has been drafted in the form of a desktop guide comprising six sets of guidelines and is based on the ESTRO advisory report on 'Quality Assurance in Radiotherapy'.

RESULTS AND CONCLUSIONS

The document has been endorsed by the DYNARAD group of institutions. As such it can form the basis for further discussions and enter into the subsequent phase of expanding its consensus basis.

Dose calculation and verification of intensity modulation generated by dynamic multileaf collimators

While the development of inverse planning tools for optimizing dose distributions has come to a level of maturity, intensity modulation has not yet been widely implemented in clinical use because of problems related to its practical delivery and a lack of verification tools and quality assurance (QA) procedures. One of the prerequisites is a dose calculation algorithm that achieves good accuracy. The purpose of this work was twofold. A primary-scatter separation dose model has been extended to account for intensity modulation generated by a dynamic multileaf collimator (MLC). Then the calculation procedures have been tested by comparison with carefully carried out experiments. Intensity modulation is being accounted for by means of a 2D (two-dimensional) matrix of correction factors that modifies the spatial fluence distribution, incident to the patient. The dose calculation for the corresponding open field is then affected by those correction factors. They are used in order to weight separately the primary and the scatter component of the dose at a given point. In order to verify that the calculated dose distributions are in good agreement with measurements on our machine, we have designed a set of test intensity distributions and performed measurements with 6 and 20 MV photons on a Varian Clinac 2300C/D linear accelerator equipped with a 40 leaf pair dynamic MLC. Comparison between calculated and measured dose distributions for a number of representative cases shows, in general, good agreement (within 3% of the normalization in low dose gradient regions and within 3 mm distance-to-dose in high dose gradient regions). For absolute dose calculations (monitor unit calculations), comparison between calculation and measurement reveals good agreement (within 2%) for all tested cases (with the condition that the prescription point is not located on a high dose gradient region).

Irradiation du cancer du sein : incertitudes liées aux mouvements respiratoires et au repositionnement

Adjuvant Radiotherapy has been shown to significantly reduce locoregional recurrence but this advantage is associated with increased cardiovascular and pulmonary morbidities. All uncertainties inherent to conformal radiation therapy must be identified in order to increase the precision of treatment; misestimation of these uncertainties increases the potential risk of geometrical misses with, as a consequence, underdosage of the tumor and/or overdosage of healthy tissues. Geometric uncertainties due to respiratory movements or set-up errors are well known. Two strategies have been proposed to limit their effect: quantification of these uncertainties, which are then taken into account in the final calculation of safety margins and/or reduction of respiratory and set-up uncertainties by an efficient immobilization or gating systems. Measured on portal films with two tangential fields, CLD (central lung distance), defined as the distance between the deep field edge and the interior chest wall at the central axis, seems to be the best predictor of set-up uncertainties. Using CLD, estimated mean set-up errors from the literature are 3.8 and 3.2 mm for the systematic and random errors respectively. These depend partly on the type of immobilization device and could be reduced by the use of portal imaging systems. Furthermore, breast is mobile during respiration with motion amplitude as high as 0.8 to 10 mm in the anteroposterior direction. Respiratory gating techniques, currently on evaluation, have the potential to reduce effect of these movements. Each radiotherapy department should perform its own assessments and determine the geometric uncertainties with respect of the equipment used and its particular treatment practices. This paper is a review of the main geometric uncertainties in breast treatment, due to respiration and set-up, and solutions proposed to limit their impact.

Polarity effect for various ionization chambers with multiple irradiation conditions in electron beams

The effect of reversing the voltage polarity applied to an ionization chamber has been investigated in electron beams for several types of chambers and several irradiation conditions. It has been found that differences in readings can be significant for cylindrical chambers (about 10%) as well as for plane parallel chambers (20%). The effect is larger for large field sizes than small ones. It generally includes an appreciable stem and cable effect. Differences in readings with both polarities are related to the energy distribution of the electron beam and are greater for lower electron energies than higher. Polarity effect and charge deposit within the chamber wall material appear to be closely connected. This charge deposit, expressed as a proportion of the total collected charge, can be directly derived from double polarities measurements. Careful investigation of the effect should be made to avoid significant error (over 5%) in the determination of the absorbed dose.

The sliding slit test for dynamic IMRT: a useful tool for adjustment of MLC related parameters

For treatments with dynamic intensity modulated radiotherapy (IMRT), the adjustment of multileaf collimator (MLC) parameters affecting both the optimization algorithm and dose distributions is crucial. The main parameters characterizing the MLC are the transmission (T) and the dosimetric leaf separation (DLS). The aim of this study is twofold: a methodology based on the 'sliding slit' test is proposed to determine (T, DLS) combinations inducing the best conformity between calculations and measurements. Secondly, the effects of the MLC adjustment on measured dose and on optimization are presented for different configurations as the chair test and for the patient dosimetric quality control (DQC). Tests were performed with a Varian 23EX linac operated at 20 MV and equipped with a 120 leaf Millenium dynamic collimator. The treatment planning system was CadPlan/Helios (version 6.3.6). Results demonstrated that the sliding width (SW) strongly depends on the (T, DLS) combinations, and the measured dose is a linear function of the SW. Different (T, DLS) combinations induced a good agreement between calculations and measurements. The influence of the MLC calibration was found to be particularly important on the 'sliding slit' test (11.8% for a gap change of 0.8 mm) but not so much on the chair test and on the DQC. To detect small variations in leaf adjustment and to ensure consistency between calculation and actual dose delivered to patients, a daily check called IMRT MU check is proposed.

Quality control of inhomogeneity correction algorithms used in treatment planning systems

PURPOSE

This quality control program has been carried out under the auspices of S.F.P.H. (Socíete Fraņcaise des Physiciens d'Hôpital), to evaluate the performances of radiotherapy treatment planning systems (RTPS) used by different institutions. The aim of this Quality Assurance Programme was: (a) to set up a methodology to assess globally the capability of a given system to perform inhomogeneity corrections in the irradiated medium with external photon beams; (b) to analyze the limitations of the algorithms presently used and especially the two-dimensional (2D) dose calculation possibilities; (c) to check, on a number of systems in clinical use, the validity of the method and the variation of the results as compared to measurements used as reference.

METHODS AND MATERIALS

Phantom (lung equivalent material placed into polystyrene) measurements, using cobalt-60 radiation, were carried out by the authors. The phantoms were circulated among the participating institutes to be scanned, and used as input to the treatment planning computer.

RESULTS

Ten systems were tested in this study, using seven different inhomogeneity correction algorithms implemented in nine different TPS; four of these algorithms are used in a pixel by pixel basis and five of them in a contour basis. Significant discrepancies or inconsistencies have been observed even for sophisticated models supposed to be mostly accurate.

CONCLUSION

The proposed tests and the experimental data provided are very useful as part of a quality-control program. They should be included in the initial extensive validation of TPS before starting clinical use, and should be repeated at regular intervals and at each updating of the program. They have the merit of including the whole procedure, from patient data acquisition to dose distribution printout.