Impact of setup variability on incidental lung irradiation during tangential breast treatment

Dosimetric Effects of the Interfraction Variations during Whole Breast Radiotherapy: A Prospective Study

Introduction

The aim of this work was to assess the dosimetric impact of the interfraction variations during breast radiotherapy.

Materials and methods

Daily portal imaging measurements were prospectively performed in 10 patients treated with adjuvant whole breast irradiation (50 Gy/25 fractions). Margins between the clinical target volume and the planning target volume (PTV) were 5 mm in the three dimensions. Parameters of interest were the central lung distance (CLD) and the inferior central margin (ICM). Daily movements were applied to the baseline treatment planning (TP1) to design a further TP (TP2). The PTV coverage and organ at risk exposure were measured on both TP1 and TP2, before being compared.

Results

A total of 241 portal images were analyzed. The random and systematic errors were 2.6 and 3.7 mm for the CLD, 4.3 and 6.9 mm for the ICM, respectively. No significant consequence on the PTV treatments was observed (mean variations: +0.1%, p = 0.56 and −1.8%, p = 0.08 for the breast and the tumor bed, respectively). The ipsilateral lung and heart exposure was not significantly modified.

Conclusion

In our series, the daily interfraction variations had no significant effect on the PTV coverage or healthy tissue exposure during breast radiotherapy.

Analysis of couch position tolerance limits to detect mistakes in patient setup

This work investigates the use of the tolerance limits on the treatment couch position to detect mistakes in patient positioning and warn users of possible treatment errors. Computer controlled radiotherapy systems use the position of the treatment couch as a surrogate for patient position and a tolerance limit is applied against a planned position. When the couch is out of tolerance a warning is sent to a user to indicate a possible mistake in setup. A tight tolerance may catch all positioning mistakes while as the same time sending too many warnings; while a loose tolerance will not catch all mistakes. We develop a statistical model of the absolute position for the three translational axes of the couch. The couch position for any fraction is considered a random variable x(i). The ideal planned couch position x(p) is unknown before a patient starts treatment and must be estimated from the daily positions x(i). As such x(p) is also a random variable. The tolerance, tol, is applied to the difference between the daily and planned position, d(i) = x(i) - x(p). The di is a linear combination of random variables and therefore the density of di is the convolution of distributions of xi and xp. Tolerance limits are based on the standard deviation of d(i) such that couch positions that are more than 2 standard deviation away are considered out of tolerance. Using this framework we investigate two methods of setting x(p) and tolerance limits. The first, called first day acquire (FDA), is to take couch position on the first day as the planned position. The second is to use the cumulative average (CumA) over previous fractions as the planned position. The standard deviation of d(i) shrinks as more samples are used to determine x(p) and so the tolerance limit shrinks as a function of fraction number when a CumA technique is used. The metrics of sensitivity and specificity were used to characterize the performance of the two methods to correctly identify a couch position as in or out of tolerance. These two methods were tested using simulated and real patient data. Five clinical sites with different indexed immobilization were tested. These were whole brain, head and neck, breast, thorax and prostate. Analysis of the head and neck data shows that it is reasonable to model the daily couch position as a random variable in this treatment site. Using an average couch position for x(p) increased the sensitivity of the couch interlock and reduced the chances of acquiring a couch position that was a statistical outlier. Analysis of variation in couch position for different sites allowed the tolerance limit to be set specifically for a site and immobilization device. The CumA technique was able to increase the sensitivity of detecting out of tolerance positions while shrinking tolerance limits for a treatment course. Making better use of the software interlock on the couch positions could have a positive impact on patient safety and reduce mistakes in treatment delivery.

Assessment of setup accuracy in patients receiving postmastectomy radiotherapy using electronic portal imaging

PURPOSE

The aim of this study was to investigate the setup accuracy for patients undergoing postmastectomy radiotherapy using electronic portal imaging.

MATERIALS AND METHODS

Ten patients undergoing radiotherapy via tangent (TG), supraclavicular-axillary (SA), and internal mammary (IM) fields were included. To explore the setup accuracy, distances between chosen landmarks were taken as reference parameters (RPs). The difference between measured RPs on simulation films and electronic portal images (EPIs) was calculated as the setup error.

RESULTS

A total of 30 simulation films and 120 EPIs were evaluated. In the SA field, calculated RPs were lung length (LL), clavicle-field center perpendicular distance, and clavicle-field center transverse distance. The mean of the standard deviations (SDs) of the random errors (sigma) for these parameters were 4.7, 7.3, and 7.6; and the SDs of the systematic errors (Sigma) were 6.8, 4.4, and 13.5, respectively. In the TG fields, the calculated RPs were the central lung distance (CLD), maximum lung distance (MLD), and central soft-tissue distance (CSTD). In the medial TG field, the sigma values for these parameters were 3.4, 3.6, and 4.1, respectively; and the sigma values were 6.6, 2.6, and 3.4, respectively. In the lateral TG field, Sigma values for the calculated RPs were 2.4, 3.2, and 3.3l, respectively; and the Sigma values were 5.6, 3.6, and 4.8, respectively.

CONCLUSION

CLD, MLD, and CSTD in TG fields and LL in SA fields are easily identifiable and are helpful for detecting setup errors using EPIs in patients undergoing postmastectomy radiotherapy.

Moving to a High-tech Approach to the Irradiation of Early Breast Cancer: Is It Possible to Balance Efficacy, Morbidity and Resource Use?

There is substantial evidence documenting the potential morbidity associated with radiotherapy in early breast cancer. An appraisal of current standard radiation practice is therefore necessary, given that women are surviving longer, have an improved quality of life, and are overcoming subsequent side-effects caused by postoperative irradiation. New technology allows the application of more complex approaches. This discussion paper considers some of the benefits of the widespread use of new complex approaches, such as intensity-modulated radiotherapy (IMRT) in the light of staffing and equipment shortfalls, and possible consequences on waiting times for treatment. The discussion is considered under the following themes: (1) which women with breast cancer benefit from complex treatment approaches? (2) What is the role of treatment accuracy in limiting morbidity? And (3) what is the potential effect of complex breast irradiation approaches on service delivery? In the UK, and globally, many departments are struggling to meet waiting-time guidelines. The use of more complex approaches for breast irradiation may increase this difficulty. However, a number of simple technical changes can be used to enhance efficacy and reduce levels of normal tissue morbidity. A sub-set of women who are at greatest risk from normal tissue morbidity or reduced cosmesis should be accurately defined in order to allow departments to plan their treatment strategies with optimal use of resources.

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.

Comparaison simulation classique–simulation virtuelle au cours d'une irradiation mammaire : étude prospective sur 14 patientes

PURPOSE

To compare conventional 2D simulation and virtual simulation on 14 patients with breast cancer.

PATIENTS AND METHODS

Patients were simulated for treatment using standard procedure. They subsequently underwent CT scan in the treatment position. The CTV was defined as breast tissue. The PTV was obtained by adding a 3D margin of 1 cm around CTV. Organs at risk (lungs and heart) were outlined. Ballistics and dose distribution obtained with the two planning methods were compared.

RESULTS

With conventional simulation, 95% of CTV received 95% of the dose prescribed. Virtual simulation significantly improved dosimetric coverage of PTV without increasing irradiation volume of lung and heart. In 2D simulation, using three slices allowed optimisation by adjusting wedge angle. The five-slice plan was a much better predictor of the maximum dose regions when compared to the three-slice plan. Using entire CT data didn't give any benefit.

CONCLUSION

Variations in CTV delineation and PTV definition limit interest of virtual simulation. In classic simulation, a 5 CT slice-plan can be used to optimise dose distribution.

Evaluation of set-up deviations during the irradiation of patients suffering from breast cancer treated with two different techniques

PURPOSE

To compare reproducibility of set-up for two different treatment techniques for external irradiation of the breast.

METHODS AND MATERIALS

In total, the analysis comprised 56 pairs of portal and simulator films for 14 consecutive patients treated following breast conserving therapy and 98 pairs of portal and simulator films for 20 consecutive patients treated after mastectomy. For the first group the tangential field technique (TF technique) was used, for the second the inverse hockey stick technique (IHS technique). Evaluation of the treatment reproducibility was performed in terms of systematic and random error calculated for the whole groups, comparison of set-up accuracy by means of comparison of cumulative distribution of the length of the displacement vector.

RESULTS

In the IHS and TF techniques for medial and lateral fields, displacement larger than 5 mm occurred in 28.3, 15.8 and 25.4%, respectively. For the IHS technique, the systematic errors for lateral and cranial-caudal direction were 1.9 and 1.7 mm, respectively (1 SD), the random errors for lateral and cranial-caudal direction were 2.0 and 2.5 mm. For the TF technique, the systematic errors for ventral-dorsal and cranial-caudal direction were 2.6 and 1.3 mm for medial field and 3.7 and 0.7 mm for lateral fields, respectively, the random errors for lateral and cranial-caudal direction were 2.2 and 1.0 mm for medial field and 2.9 and 1.1 for lateral field, respectively. Rotations were negligible in the IHS technique. For the TF technique the systematic and random components amounted to about 2.0 degrees (1 SD).

CONCLUSIONS

Both the inverse hockey stick and standard tangential techniques showed good reproducibility of patients' set-up with respect to cranial-caudal direction. For the TF technique, the accuracy should be improved for the medial field with respect to the ventral-dorsal direction.

Reliability of portal control procedure in irradiation of breast cancer: A Bayesian analysis

BACKGROUND AND PURPOSE

To determine whether the information gathered from a fixed number of periodic verification films accurately reflects the true imprecision in patient positioning during the whole radiation therapy of early breast cancer.

PATIENTS AND METHODS

A total of 204 medial and lateral treatment fields were evaluated in 102 breast cancer patients treated with conservative surgery and radiation therapy. For each treatment field, the central lung distance was measured on portal films obtained from one simulation and four treatment controls at weekly intervals during breast irradiation. Systematic and random errors in patient positioning throughout all treatment fractions were estimated from the available controls using Bayesian methods.

RESULTS

The average systematic and random errors during treatment controls were 2.7 and 1.9 mm, respectively. For these mean control values, the probabilities that the true systematic and random errors remain below 5mm during all treatment fractions were 99 and 100%, respectively.

CONCLUSIONS

Reproducibility of patient positioning was supported by a virtually null probability for systematic or random errors greater than 5 mm during the whole radiation therapy. Weekly verification films seem to be sufficient to estimate patient positioning errors with high accuracy in radiotherapy of early breast cancer.

Technical factors associated with radiation pneumonitis after local +/- regional radiation therapy for breast cancer

PURPOSE

To assess the incidence of, and clinical factors associated with, symptomatic radiation pneumonitis (RP) after tangential breast/chest wall irradiation with or without regional lymph node treatment.

METHODS AND MATERIALS

The records of 613 patients irradiated with tangential photon fields for breast cancer with >6 months follow-up were reviewed. Clinically significant RP was defined as the presence of new pulmonary symptoms requiring steroids. Data on clinical factors previously reported to be associated with RP were collected, e.g., tamoxifen or chemotherapy exposure and age. The central lung distance (CLD) and the average of the superior and inferior mid lung distance (ALD) in the lateral tangential field were measured on simulator films as a surrogate for irradiated lung volume. Many patients were treated with partly wide tangential fields that included a heart block shielding a part of the lower lung.

RESULTS

RP developed in 15/613 (2.4%) patients. In the univariate analysis, there was an increased incidence of RP among patients treated with local-regional radiotherapy (RT) (4.1%) vs. those receiving local RT only (0.9%) (p = 0.02), and among patients receiving chemotherapy (3.9%) vs. those not treated with chemotherapy (1.4%) (p = 0.06). According to multivariate analysis, only the use of nodal RT remained independently associated with RP (p = 0.03). There was no statistically significant association between ranked CLD or ALD measurements and RP among patients treated with nodal irradiation with tangential beams. However, there was a statistically nonsignificant trend for increasing rates of RP with grouped ALD values: below 2 cm (4% RP rate), between 2 and 3 cm (6%), and above 3 cm (14%).

CONCLUSIONS

RP was an uncommon complication, both with local and local-regional RT. The addition of regional lymph node irradiation slightly increased the incidence of RP among patients treated with the partly wide tangential field technique. Concern for RP should, however, not deter patients with node-positive breast cancer from receiving local-regional RT.

A simulation of the effects of set-up error and changes in breast volume on conventional and intensity-modulated treatments in breast radiotherapy

The effect of interfractional patient movement on dosimetry has been investigated for breast radiotherapy. Errors in patient set-up and changes in breast volume were simulated individually to determine how each contributes to the total dosimetric error. Two treatment techniques were investigated: a conventional treatment and an intensity-modulated treatment delivered using compensators. Six patients were investigated and anterior-posterior (AP) and superior-inferior (SI) displacements were simulated by displacing the isocentre in both directions by 2, 5 and 10 mm. A model of the breast was developed from the six patients to simulate changes in breast volume. In this model, the breast was described as a set of semi-ellipses. The volume of the breast was changed by varying the magnitude of the semi-major and semi-minor axes. Anisotropic changes in breast volume were also investigated. The dosimetric error was evaluated for each dose plan by calculating the volume outside the 95-105% dose range resulting from the simulations. A number of parameters describing the size and shape of the breast were also investigated to determine whether a susceptibility of outline sets to interfractional patient movement could be predicted. A parameter describing the increase in the breast volume outside the 95-105% dose range was calculated for AP a

Evaluation of a standard breast tangent technique: a dose-volume analysis of tangential irradiation using three-dimensional tools

PURPOSE

A thorough dose-volume analysis of a standard tangential radiation technique has not been published. We evaluated the adequacy of a tangential radiation technique in delivering dose to the breast and regional lymphatics, as well as dose delivered to underlying critical structures.

METHODS AND MATERIALS

Treatment plans of 25 consecutive women with breast cancer undergoing lumpectomy and adjuvant breast radiotherapy were studied. Patients underwent two-dimensional (2D) treatment planning followed by treatment with standard breast tangents. These 2D plans were reconstructed without modification on our three-dimensional treatment planning system and analyzed with regard to dose-volume parameters.

RESULTS

Adequate coverage of the breast (defined as 95% of the target receiving at least 95% of the prescribed dose) was achieved in 16 of 25 patients, with all patients having at least 85% of the breast volume treated to 95% of the prescribed dose. Only 1 patient (4%) had adequate coverage of the Level I axilla, and no patient had adequate coverage of the Level II axilla, Level III axilla, or the internal mammary lymph nodes.

CONCLUSION

Three-dimensional treatment planning is superior in quantification of the dose received by the breast, regional lymphatics, and critical structures. The standard breast tangent technique delivers an adequate dose to the breast but does not therapeutically treat the regional lymph nodes in the majority of patients. If coverage of the axilla or internal mammary lymph nodes is desired, alternate beam arrangements or treatment fields will be necessary.

Computer-assisted decision making in portal verification--optimization of the neural network approach

PURPOSE

Conventional portal verification requires that a qualified radiation oncologist make decisions as to the set-up acceptability. This scheme is no longer sustainable with the large numbers of images available on-line and stringent time constraints. Therefore the objective of this study was to develop, optimize, and evaluate on clinical data an artificial intelligence decision-making tool for portal verification. The tool, based on the artificial neural network (ANN) approach, should approximate, as closely as possible, portal verification assessments made by a radiation oncologist expert.

METHODS AND MATERIALS

A total of 328 electronic portal images of tangential breast irradiations were included in the study. A radiation oncologist expert evaluated these images and rated the treatment set-up acceptability on a scale from 0 to 10. Translational and rotational errors in the placement of the radiation field boundaries formed seven-dimensional feature vectors that represented each of the 328 portal images/treatments. The feature vectors were used as inputs to a three-layer, feedforward ANN. The neural network was trained on the oncologist's ratings.

RESULTS

The rms discrepancy between the ANN and the expert's ratings was 1.05 rating points. Using the decision threshold equal to 5 for both sets of ratings, the ANN classifier was capable of detecting 100% of the portals classified as "unacceptable" by the expert. Only 6.5% of portals acceptable to the oncologist were misclassified as "unacceptable" by the ANN.

CONCLUSION

The results of this study indicate the feasibility of using the ANN portal image classifier as an automated assistant to the radiation oncologist. Its role would be to recommend an appropriate decision as to the acceptability or otherwise of a given treatment set-up depicted in a portal image.

Accuracy of alignment in breast irradiation: a retrospective analysis of clinical practice

The objective of the study was to determine the accuracy of patient positioning in radiotherapy for breast cancer. Portal images were obtained using a fast electronic megavoltage radiotherapy imaging system in 30 cases of breast cancer. Quantitative analysis of 530 megavolt portal images and comparison with 30 digitized simulation films were performed. Five linear measurements were taken for each simulation and verification film. Central lung distance (CLD) is the distance from the dorsomedial beam edge to the inner thoracic wall in the central plane of the beam. Cranial lung distance (CrLD) is the distance from the dorsomedial beam edge to the inner thoracic wall in the plane of the beam at 4 cm from the central plane. Central beam edge to skin distance (CBESD) is the distance from the skin to the ventrolateral beam edge in the central plane of the beam. The central irradiated width (CIW) is defined as the distance from the dorsomedial beam edge to the skin. The craniocaudal distance (CCD) is defined as the distance from a particular landmark to the caudal field border. Concerning patient position in the field, mean standard deviations of the difference between simulation and treatment images were 3.9 mm for the CLD, 3.2 mm at +4 cm, 3.6 mm for the CIW, 3.3 mm for the CBESD, 3.8 mm for the CCD. In 90% of all set-up for treatment, errors were less than 1 cm. The variation of the CLD was the largest set-up error. This parameter is clinically the most significant. Future treatment delivery should be improved by introducing patient positioning devices such as thermoplastic shells. The electronic portal imaging device (EPID) appears to be an adequate tool to study the accuracy of treatment set-ups like this.

Lung and heart dose volume analyses with CT simulator in radiation treatment of breast cancer

PURPOSE

Radiation pneumonitis and cardiac effects are directly related to the irradiated lung and heart volumes in the treatment fields. The central lung distance (CLD) from a tangential breast radiograph is shown to be a significant indicator of ipsilateral irradiated lung volume. Retrospective analysis of the pattern of dose volume of lung and heart with actual volume data from a CT simulator in the treatment of breast cancer is presented with respect to CLD.

METHODS AND MATERIALS

The heart and lung volumes in the tangential treatment fields were analyzed in 108 consecutive cases (52 left and 56 right breast) referred for CT simulation. All patients in this study were immobilized and placed on an inclined breast board in actual treatment setup. Both arms were stretched over head to avoid collision with the scanner aperture. Radiopaque marks were placed on the medial and lateral borders of the tangential fields. All patients were scanned in spiral mode with slice width and thickness of 3 mm each, respectively. The lung and heart structures as well as irradiated areas were delineated on each slice and respective volumes were accurately measured. The treatment beam parameters were recorded and the digitally reconstructed radiographs (DRRs) were generated for the measurement of the CLD and analysis.

RESULTS

Using CT data the mean volume and standard deviation of left and right lungs were 1307.7+/-297.7 cm3 and 1529.6+/-298.5 cm3, respectively. The magnitude of irradiated volume in left and right lung is nearly equal for the same CLD that produces different percent irradiated volumes (PIV). The left and right PIV lungs are 8.3+/-4.7% and 6.6+/-3.7%, respectively. The PIV data have shown to correlate with CLD with second- and third-degree polynomials; however, in this study a simple straight line regression is used to provide better confidence than the higher order polynomials. The regression lines for the left and right breasts are very different based on actual CT data. The slopes of regression lines for the left and right lung are 0.6%/mm and 0.5%/mm, respectively which is statistically different with thep value of 0.01. A maximum heart PIV of >3.0% is observed in 80% of the patients. The heart PIV is inversely correlated with gantry angle and weakly correlated with CLD.

CONCLUSIONS

The CT-simulator provides accurate volumetric information of the heart and lungs in the treatment fields. The lung PIV is directly correlated to the CLD (0.6%/mm and 0.5%/mm for the left and right lungs). Left and right lungs have different volumes and hence, different regression lines are recommended. An additional 12% lung volume could be irradiated in the supraclavicular field. Heart volume is not correlated with the CLD. The heart PIV is associated to the beam angle. Heart volume may not be accurately visualized in a tangential radiograph; however, this can be easily seen in a DRR with contour delineation and can be minimized with proper beam parameters iteratively with a virtual simulator. Lung and heart PIV along with dose volume histograms (DVH) are essential in reducing pulmonary and cardiac complications.