A Randomised Phase II Clinical Trial Comparing the Deliverability and Acute Toxicity of Wide Tangent versus Volumetric Modulated Arc Therapy to the Breast and Internal Mammary Chain

AIMS

Inclusion of the internal mammary chain in the radiotherapy target volume (IMC-RT) improves disease-free and overall survival in higher risk breast cancer patients, but increases radiation doses to heart and lungs. Dosimetric data show that either modified wide-tangential fields (WT) or volumetric modulated arc therapy (VMAT) together with [AQ1]voluntary deep inspiration breath hold (vDIBH) keep mean heart doses below 4 Gy in most patients. However, the impact on departmental resources has not yet been documented. This phase II clinical trial compared the time taken to deliver IMC-RT using either WT and vDIBH or VMAT and vDIBH, together with planning time, dosimetry, set-up reproducibility and toxicity.

MATERIALS AND METHODS

Left-sided breast cancer patients requiring IMC-RT were randomised to receive either WT(vDIBH) or VMAT radiotherapy. The primary outcome was treatment time, powered to detect a minimum difference of 75 min (5 min/fraction) between techniques. The population mean displacement, systematic error and random error for cone beam computed tomography chest wall matches in three directions of movement were calculated. Target volume and organ at risk doses were compared between groups. Side-effects, including skin (Radiation Therapy Oncology Group), lung and oesophageal toxicity (Common Terminology Criteria for Adverse Events v 4.03) rates, were compared between the groups over 3 months. Patient-reported outcome measures, including shoulder toxicity at baseline, 6 months and 1 year, were compared.

RESULTS

Twenty-one patients were recruited from a single UK centre between February 2017 and January 2018. The mean (standard deviation) total treatment time per fraction for VMAT treatments was 13.2 min (1.7 min) compared with 28.1 min (3.3 min) for WT(vDIBH). There were no statistically significant differences in patient set-up errors in between groups. The average mean heart dose for WT(vDIBH) was 2.6 Gy compared with 3.4 Gy for VMAT(vDIBH) (P = 0.13). The mean ipsilateral lung V17Gy was 32.8% in the WT(vDIBH) group versus 34.4% in the VMAT group (P = 0.2). The humeral head (mean dose 16.8 Gy versus 2.8 Gy), oesophagus (maximum dose 37.3 Gy versus 20.1 Gy) and thyroid (mean dose 22.0 Gy versus 11.2 Gy) all received a statistically significantly higher dose in the VMAT group. There were no statistically significant differences in skin, lung or oesophageal toxicity within 3 months of treatment. Patient-reported outcomes of shoulder toxicity, pain, fatigue, breathlessness and breast symptoms were similar between groups at 1 year.

CONCLUSION

VMAT(vDIBH) and WT(vDIBH) are feasible options for locoregional breast radiotherapy including the IMC. VMAT improves nodal coverage and delivers treatment more quickly, resulting in less breath holds for the patient. This is at the cost of increased dose to some non-target tissues. The latter does not appear to translate into increased toxicity in this small study.

The stability of imaging biomarkers in radiomics: a framework for evaluation

This paper studies the sensitivity of a range of image texture parameters used in radiomics to: (i) the number of intensity levels, (ii) the method of quantisation to select the intensity levels and (iii) the use of an intensity threshold. 43 commonly used texture features were studied for the gross target volume outlined on the CT component of PET/CT scans of 50 patients with non-small cell lung carcinoma (NSCLC). All cases were quantised for all values between 4 and 128 intensity levels using four commonly used quantisation methods. All results were analysed with and without a threshold range of  -200 HU to 300 HU. Cases were ranked for each texture feature and for all quantisation methods with the Spearman's rank correlation coefficient determined to evaluate stability. Results showed large fluctuations in ranking, particularly for low numbers of levels, differences between quantisation methods and with the use of a threshold, with values Spearman's Rank Correlation for many parameters below 0.2. Our results demonstrated the sensitivity of radiomics features to the parameters used during analysis and highlight the risk of low reproducibility comparing studies with slightly different parameters. In terms of the lung cancer CT datasets, this study supports the use of 128 intensity levels, the same uniform quantiser applied to all scans and thresholding of the data. It also supports several of the features recommended in the literature for such studies such as skewness and kurtosis. A recommended framework is presented for curation of the data analysis process to ensure stability of results.

Abstract GS4-05: Dose escalated simultaneous integrated boost radiotherapy for women treated by breast conservation surgery for early breast cancer: 3-year adverse effects in the IMPORT HIGH trial (CRUK/06/003)

Background

IMPORT HIGH is a randomised, multi-centre phase III trial testing dose escalated simultaneous integrated boost (SIB) against sequential boost each delivered by intensity modulated radiotherapy (IMRT) for early stage breast cancer with higher risk of local relapse. The primary endpoint was initially breast induration at 3 years, requiring 840 patients; accrual was extended (target 2568) with the new primary endpoint of local relapse. We report adverse effects (AE) at 3 years.

Methods

Women age ≥18 after breast conservation surgery for pT1-3 pN0-pN3a M0 invasive carcinoma were eligible. Randomisation was 1:1:1 between 40Gy/15F to whole breast (WB) + 16Gy/8F sequential photon boost to tumour bed (40+16Gy), 36Gy/15F to WB, 40Gy to partial breast + 48Gy (48Gy) or + 53Gy (53Gy) in 15F SIB to tumour bed. AEs were assessed annually by clinicians in all patients and in a planned sub-set (840) of patients by photographs at 3 years and by patients at 6 months, 1 and 3 years. AE scores were dichotomised as none/mild vs marked for photographs and none/mild vs moderate/marked for patients and clinicians. Fisher's exact tests compared groups; principal comparison (protocol-specified) between 53Gy and 48Gy (p<0.01 defined as statistical significance).

Results

2617 women consented between 03/2009 and 09/2015 from 39 UK radiotherapy centres. Median follow-up was 49.1 (IQR 36.8-63.2) months. Median age was 49 (IQR 44-56); 9%, 38% & 53% were tumour grade 1, 2 & 3 respectively; 30% were node positive. 66% received chemotherapy and 73% endocrine therapy. 3-year AE data were available for 2017 clinician assessments, 641 photographs and 842 patient assessments. Proportions of patients with marked AEs were low overall. Rates of moderate/marked AEs at 3 years were broadly similar between the randomised groups; with a suggestion of a slightly increased risk for breast induration in 53Gy compared with control (borderline significance).

AE at 3 years 40+16Gy n(%)48Gy n(%)53Gy n(%)ClinicianBreast induration;N656668654None451 (69)483 (72)445 (68)Mild167 (25)141 (21)146 (22)Moderate32 (5)42 (6)56 (9)Marked6 (1)2 (1)7 (1)P-value 0.57010.0102  0.0443Breast shrinkage;N655669654None442 (68)472 (71)448 (69)Mild167 (26)161 (24)166 (25)Moderate40 (6)33 (5)35 (5)Marked6 (1)3 (1)5 (1)P-value 0.25410.5772  0.6373Breast distortion;N656669654None451 (69)464 (69)442 (68)Mild169 (26)170 (25)170 (26)Moderate33 (5)32 (5)38 (6)Marked3 (1)3 (1)4 (1)P-value 0.90310.4862  0.4113PatientChange in breast appearance;N287264285None38 (13)50 (19)58 (20)Mild164 (57)151 (57)142 (50)Moderate57 (20)45 (17)54 (19)Marked28 (10)18 (7)31 (11)P-value 0.14910.9992  0.1243PhotographChange in breast appearance;N218210213None183 (84)185 (88)177 (83)Mild25 (11)23 (11)32 (15)Marked10 (5)2 (1)4 (2)P-value 0.03610.1732  0.6853148Gy v 40+16Gy; 253Gy v 40+16Gy; 353Gy v 48Gy

Conclusions

These results represent the largest and most mature reported AE outcomes of breast SIB within a clinical trial. At 3 years, rates of moderate/marked AEs were similar between SIB IMRT and WB + sequential boost IMRT delivered over 3 and 4.5 weeks respectively.

Citation Format: Coles CE, Griffin CL, Kirby AM, Haviland JS, Titley JC, Benstead K, Brunt AM, Chan C, Ciurlionis L, Din OS, Donovan EM, Eaton DJ, Harnett AN, Hopwood P, Jefford ML, Jenkins PJ, Lee CE, McCormack M, Sherwin L, Syndikus I, Tsang Y, Twyman NI, Ventikaraman R, Wickers S, Wilcox MH, Bliss JM, Yarnold JR. Dose escalated simultaneous integrated boost radiotherapy for women treated by breast conservation surgery for early breast cancer: 3-year adverse effects in the IMPORT HIGH trial (CRUK/06/003) [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr GS4-05.

The UK HeartSpare Study (Stage II): Multicentre Evaluation of a Voluntary Breath-hold Technique in Patients Receiving Breast Radiotherapy

AIMS

To evaluate the feasibility and heart-sparing ability of the voluntary breath-hold (VBH) technique in a multicentre setting.

MATERIALS AND METHODS

Patients were recruited from 10 UK centres. Following surgery for early left breast cancer, patients with any heart inside the 50% isodose from a standard free-breathing tangential field treatment plan underwent a second planning computed tomography (CT) scan using the VBH technique. A separate treatment plan was prepared on the VBH CT scan and used for treatment. The mean heart, left anterior descending coronary artery (LAD) and lung doses were calculated. Daily electronic portal imaging (EPI) was carried out and scanning/treatment times were recorded. The primary end point was the percentage of patients achieving a reduction in mean heart dose with VBH. Population systematic (Σ) and random errors (σ) were estimated. Within-patient comparisons between techniques used Wilcoxon signed-rank tests.

RESULTS

In total, 101 patients were recruited during 2014. Primary end point data were available for 93 patients, 88 (95%) of whom achieved a reduction in mean heart dose with VBH. Mean cardiac doses (Gy) for free-breathing and VBH techniques, respectively, were: heart 1.8 and 1.1, LAD 12.1 and 5.4, maximum LAD 35.4 and 24.1 (all P<0.001). Population EPI-based displacement data showed Σ =+1.3-1.9 mm and σ=1.4-1.8 mm. Median CT and treatment session times were 21 and 22 min, respectively.

CONCLUSIONS

The VBH technique is confirmed as effective in sparing heart tissue and is feasible in a multicentre setting.

The effect of image guidance on dose distributions in breast boost radiotherapy

AIMS

To determine the effect of image-guided radiotherapy on the dose distributions in breast boost treatments.

MATERIALS AND METHODS

Computed tomography images from a cohort of 60 patients treated within the IMPORT HIGH trial (CRUK/06/003) were used to create sequential and concomitant boost treatment plans (30 cases each). Two treatment plans were created for each case using tumour bed planning target volume (PTV) margins of 5 mm (achieved with image-guided radiotherapy) and 8 mm (required for bony anatomy verification). Dose data were collected for breast, lung and heart; differences with margin size were tested for statistical significance.

RESULTS

A median decrease of 29 cm(3) (range 11-193 cm(3)) of breast tissue receiving 95% of the prescribed dose was observed where image-guided radiotherapy margins were used. Decreases in doses to lungs, contralateral breast and heart were modest, but statistically significant (P < 0.01). Plan quality was compromised with the 8 mm PTV margin in one in eight sequential boost plans and one third of concomitant boost plans. Tumour bed PTV coverage was <95% (>91%) of the prescribed dose in 12 cases; in addition, the required partial breast median dose was exceeded in nine concomitant boost cases by 0.5-3.7 Gy.

CONCLUSIONS

The use of image guidance and, hence, a reduced tumour bed PTV margin, in breast boost radiotherapy resulted in a modest reduction in radiation dose to breast, lung and heart tissues. Reduced margins enabled by image guidance were necessary to discriminate between dose levels to multiple PTVs in the concomitant breast boost plans investigated.

Second cancer incidence risk estimates using BEIR VII models for standard and complex external beam radiotherapy for early breast cancer

PURPOSE

To compare organ specific cancer incidence risks for standard and complex external beam radiotherapy (including cone beam CT verification) following breast conservation surgery for early breast cancer.

METHOD

Doses from breast radiotherapy and kilovoltage cone beam CT (CBCT) exposures were obtained from thermoluminescent dosimeter measurements in an anthropomorphic phantom in which the positions of radiosensitive organs were delineated. Five treatment deliveries were investigated: (i) conventional tangential field whole breast radiotherapy (WBRT), (ii) noncoplanar conformal delivery applicable to accelerated partial beast irradiation (APBI), (iii) two-volume simultaneous integrated boost (SIB) treatment, (iv) forward planned three-volume SIB, and (v) inverse-planned three volume SIB. Conformal and intensity modulated radiotherapy methods were used to plan the complex treatments. Techniques spanned the range from simple methods appropriate for patient cohorts with a low cancer recurrence risk to complex plans relevant to cohorts with high recurrence risk. Delineated organs at risk included brain, salivary glands, thyroid, contralateral breast, left and right lung, esophagus, stomach, liver, colon, and bladder. Biological Effects of Ionizing Radiation (BEIR) VII cancer incidence models were applied to the measured mean organ doses to determine lifetime attributable risk (LAR) for ages at exposure from 35 to 80 yr according to radiotherapy techniques, and included dose from the CBCT imaging.

RESULTS

All LAR decreased with age at exposure and were lowest for brain, thyroid, liver, and bladder (<0.1%). There was little dependence of LAR on radiotherapy technique for these organs and for colon and stomach. LAR values for the lungs for the three SIB techniques were two to three times those from WBRT and APBI. Uncertainties in the LAR models outweigh any differences in lung LAR between the SIB methods. Constraints in the planning of the SIB methods ensured that contralateral breast doses and LAR were comparable to WBRT, despite their added complexity. The smaller irradiated volume of the ABPI plan contributed to a halving of LAR for contralateral breast compared with the other plan types. Daily image guided radiotherapy (IGRT) for a left breast protocol using kilovoltage CBCT contributed <10% to LAR for the majority of organs, and did not exceed 22% of total organ dose.

CONCLUSIONS

Phantom measurements and calculations of LAR from the BEIR VII models predict that complex breast radiotherapy techniques do not increase the theoretical risk of second cancer incidence for organs distant from the treated breast, or the contralateral breast where appropriate plan constraints are applied. Complex SIB treatments are predicted to increase the risk of second cancer incidence in the lungs compared to standard whole breast radiotherapy; this is outweighed by the threefold reduction in 5 yr local recurrence risk for patients of high risk of recurrence, and young age, from the use of radiotherapy. APBI may have a favorable impact on risk of second cancer in the contralateral breast and lung for older patients at low risk of recurrence. Intensive use of IGRT increased the estimated values of LAR but these are dominated by the effect of the dose from the radiotherapy, and any increase in LAR from IGRT is much lower than the models' uncertainties.

Clinical implementation of kilovoltage cone beam CT for the verification of sequential and integrated photon boost treatments for breast cancer patients

OBJECTIVE

The objective of this study was to formulate a practical method for the use of cone beam CT (CBCT) for the verification of sequential and integrated tumour bed boosts for early breast cancer patients.

METHODS

Partial arc scan geometries were assessed on a treatment unit. Imaging dose measurements on an Elekta Synergy CBCT system were made in a CT dose phantom for scan parameters 100 kV, 25 mA and 40 ms with an S20 collimator. The protocol was used to verify the setup of a cohort of 38 patients, all of whom had surgical clips inserted in the tumour bed. Setup errors with and without an extended no action level (eNAL) protocol were calculated.

RESULTS

Arcs from 260° to 85° (left breast) and 185° to 15° (right breast) were found sufficient to image fiducial markers and anatomy whilst accounting for the physical limits of the equipment. A single treatment and imaging isocentre was found by applying simple constraints: isocentre <8 cm from midline and isocentre-couch distance <30 cm. Contralateral breast doses were ∼2 mGy per scan (right breast) and ∼12 mGy (left breast). Both mean population systematic error and mean population random error were 3 mm prior to correction. The systematic error reduced to 1.5 mm using an eNAL correction protocol, implying that a 5-mm setup margin could be achieved.

CONCLUSION

An image-guided verification protocol using CBCT for breast cancer boost plans was implemented successfully. Setup errors were reduced with an acceptable imaging dose to the contralateral breast.

An investigation into methods of IMRT planning applied to breast radiotherapy

The purpose of this study was to investigate methods used to modulate dose distributions in radiotherapy planning, to determine the fundamental features of these and to establish the attainable dose uniformity. Published modulation methods were categorized, and a simple physical model devised to predict the weight of the wedged beam and the relative dose distribution for each category. Each technique was applied to patient data with planning target volume sizes ranging from below 500 cm(3) to 2200 cm(3). The spatial distribution of high-dose regions in the breast, and maximum dose for the heart and lung, were determined for each plan. The dose uniformity was analysed by evaluating the volume of the breast (V(I)) receiving <95% and <105% of the prescribed dose. The difference between V(105%) and V(95%) for each method for each patient data set was also calculated. The simple model predicted the trend in percentage weight of the wedge beam and the form of the dose distribution in the transverse plane with the modulation method. Improvements in the dose uniformity were seen for the majority of modulation methods. The magnitude of the change was between 5.6% and 11.1% (p<0.05) of the breast volume for breast sizes above 500 cm(3). Some modulation methods introduced high dose at the chest wall. In conclusion, the majority of the methods improved dose uniformity for breast sizes of 500 cm(3) or greater. No method showed a clear advantage over the others. The use of modulation methods should be governed by consideration of its effects relative to a simple wedge plan.

Accuracy and precision of an external-marker tracking-system for radiotherapy treatments

The purpose of this work was to determine the accuracy and precision of a real-time motion-tracking system (Osiris+) for the monitoring of external markers used on patients receiving radiotherapy treatments. Random and systematic errors in the system were evaluated for linear (1D), circular (2D) and elliptical (3D) continuous motions, and for a set of static positions offset from an origin. A Wellhofer beam data measurement system and a computer controlled platform (which could be programmed to give motion in 3D) were used to move a hemi-spherical test object. The test object had four markers of the type used on patients. Three markers were aligned in the central plane and a fourth was positioned out of plane. Errors were expressed as deviations from the planned positions at the sampled time points. The marked points on the test object were tracked for the linear motion case with a variation from the true position of less than +/-1 mm, except for two extreme situations. The variation was within +/-2 mm when the lights were dimmed and when the amplitude of the movement was +/-5.0 cm. The 2D circular motion was tracked with a standard deviation of 1 mm or less over four cycles. The sampling rates of the system were found to be 0.3-0.4 s when it was monitoring actively and 1.5-1.6 s otherwise. The recorded Osiris+ measurements of known static positions were within +/-1 mm of the value from the computer controlled platform moving the test object. The elliptical motions in 3D were tracked to +/-1 mm in two directions (Y,Z), and generally to within +/-2 mm for the third direction (X); however, specific marked points could display an error of up to 5 mm at certain positions in X. The overall displacement error for the 3D motion was +/-1 mm with a standard deviation of 2.5 mm. The system performance is satisfactory for use in tracking external marker motion during radiotherapy treatments.

Comparison of a lung fitting algorithm with CT data for tangential fields in radiotherapy of the breast

A method of estimating the shape and position of the lung in tangential breast fields is presented for patients who have not been CT scanned. Using the Osiris system, the external contour is obtained optically, and an estimated lung structure superimposed on the transverse outlines based on the measured lung depth in the tangential fields and an analysis of the typical lung shapes obtained from CT images. The accuracy of this fit was determined by comparison with a set of 64 CT images imported into the Osiris system. Dose distributions were calculated by two treatment planning systems: ADAC Pinnacle and GE Target2. The computed dose distributions for 6 MV photons were compared against measured doses in a specialized breast phantom. For the worst case of lung fit compared with CT, the dosimetric error (based upon ADAC Pinnacle calculations) was 2.0% in the shadow of the lung. For the complete patient data set, the relative dose errors to these points were reduced from a mean value of 8.4% and standard deviation (SD)=1.8% (no lung correction) to a mean of 0.2% and SD=1.0% (lung correction using fitted lung). It was also found that for every 1 cm of lung path length the dose to the breast along that path length increased by approximately 1%. The results of these investigations indicated that the lung fit model was satisfactory for routine clinical use, so that good dosimetric results can be obtained using lung correction without the need for CT imaging.

Dose-position and dose-volume histogram analysis of standard wedged and intensity modulated treatments in breast radiotherapy

The aim of this work was to evaluate the positional distribution of dose in a concise manner and to analyse dose-histogram results in tangential breast radiotherapy in 300 patients, randomized to standard wedged or intensity modulated radiotherapy (IMRT), for future correlation with clinical outcome data. A simple method for analysing the dose-position relationship in the treatment volume was used to compare the spatial distribution of dose in patients. The breast was divided into equal thirds (upper, middle and lower) and dose was assessed using three dose bands; 95-105%, >105-110% and >110% of the prescription dose. The effect of using IMRT on the dosimetry was assessed from dose-volume histogram data using the following parameters: percentage of the target volume receiving a dose less than 95%, greater than 105%, either less than 95% or greater than 105% of that prescribed; the mean dose; and the maximum dose. Doses greater than 105% were predominantly in the upper and lower regions of the breast in the standard wedged treatment. 96% of these patients received doses greater than 105% in the upper region of the breast and 70% received doses greater than 105% in the lower breast. Only 4% of patients allocated IMRT received doses greater than 105% in either region. Analysis of dose-volume histogram data showed that IMRT reduced the volume receiving a dose greater than 105% by a mean of 10.7% (p= or <0.001); the mean change in the volume receiving a dose less than 95% was 0.2% (p=0.63). Average mean plan dose was 101.6% for standard treatment and 99.6% for IMRT (p<0.001 for each compared with 100.0% ideal). The mean value of maximum dose was reduced from 111% to 106% in the group of patients randomized to IMRT. A simple method for describing the relationship between dose and position in the breast, which is helpful for the effective correlation of dosimetry and clinical effects, is reported. Further, application of IMRT to the tangential field irradiation of the breast has been demonstrated to reduce high dose regions in both volume and dose level without compromising either minimum dose coverage or mean dose delivered to the breast.

The delivery of intensity modulated radiotherapy to the breast using multiple static fields

BACKGROUND AND PURPOSE

To develop a method of using a multileaf collimator (MLC) to deliver intensity modulated radiotherapy (IMRT) for tangential breast fields, using an MLC to deliver a set of multiple static fields (MSFs).

MATERIALS AND METHODS

An electronic portal imaging device (EPID) is used to obtain thickness maps of medial and lateral tangential breast fields. From these IMRT deliveries are designed to minimize the volume of breast above 105% of prescribed dose. The deliveries are universally-wedged beams augmented with a set of low dose shaped irradiations. Dosimetric and planning QA of this method has been compared with the standard, wedged treatment and the corresponding treatment using physical compensators. Several options for delivering the MSF treatment are presented.

RESULTS

The MSF technique was found to be superior to the standard technique (P value=0.002) and comparable with the compensated technique. Both IMRT methods reduced the volume of breast above 105% dose from a mean value of 12.0% of the total breast volume to approximately 2.8% of the total breast volume.

CONCLUSIONS

This MSF method may be used to reduce the high dose volume in tangential breast irradiation significantly. This may have consequences for long-term side effects, particularly cosmesis.

Painless aortic dissection presenting as high paraplegia: a case report

Acute aortic dissection is a catastrophic episode that usually presents as a sudden, painful, ripping sensation in the chest or back. Physical findings may include loss of pulses and aortic regurgitation. It is associated with neurologic sequelae in as many as one third of patients. Painless dissection occurs in 5% of patients. We report a case of painless aortic dissection, presenting as acute paraplegia. The patient was a 77-year-old woman who presented with paraplegia, with no chest or back pain. On examination, strength was 5/5 in both upper extremities and 0/5 in both lower extremities. Deep tendon reflexes were absent in her legs. She had no voluntary anal contraction. Sensation was absent from T6 through S5. Computed tomography of the chest revealed a type A dissecting aneurysm. The vascular supply to the spinal cord and the differential diagnosis for new onset paraplegia are discussed.

A filter for breast imaging on a radiotherapy X-ray simulator

Simulator radiographs taken as a record of breast radiotherapy planning often show ill defined breast tissue margins because exposure parameters are set to optimize visualization of the chest wall rather than the bulk of the breast. This creates difficulties when using simulator images as reference images in verification by comparing with either portal film or images from an electronic portal imaging device. Our aim was to improve breast images taken at simulation without changing exposure parameters that have been optimized for visualization of the chest wall. This has been achieved via an external filter to be used when taking radiographs with the treatment simulator. The filter is made of stainless steel coated with tin and is shaped to maintain acceptable imaging of the chest wall by covering only the section of field anterior to the chest wall. Radiographs of breast simulations using the filter have been accepted as satisfactory by both clinicians and radiographers. The filter is now in routine clinical use for breast and chest wall treatment simulation.

Evaluation of compensation in breast radiotherapy: a planning study using multiple static fields

PURPOSE

A method that uses electronic portal imaging to design intensity-modulated beams for compensation in breast radiotherapy was implemented using multiple static fields in a planning study. We present the results of the study to verify the algorithm, and to assess improvements to the dosimetry.

METHODS AND MATERIALS

Fourteen patients were imaged with computed tomography (CT) and on a treatment unit using an electronic portal imager. The portal imaging data were used to design intensity-modulated beams to give an ideal dose distribution in the breast. These beams were implemented as multiple static fields added to standard wedged tangential fields. Planning of these treatments was performed on a commercial treatment planning system (Target 2, IGE Medical Systems, Slough, U.K.) using the CT data for each patient. Dose-volume histogram (DVH) analysis of the plans with and without multileaf collimator (MLC) compensation was carried out. This work has been used as the basis for a randomized clinical trial investigating whether improvements in dosimetry are correlated with the reduction of long-term side effects from breast radiotherapy.

RESULTS

The planning analysis showed a mean increase in target volume receiving 95-105% of prescribed dose of 7.5% (range -0.8% to 15.9%) when additional MLC compensation was applied. There was no change to the minimum dose for all 14 patient data sets. The change in the volume of breast tissue receiving over 105% of prescribed dose, when applying MLC compensation, was between -1.4% and 11.9%, with positive numbers indicating an improvement. These effects showed a correlation with breast size; the larger the breast the greater the amount of improvement.

CONCLUSIONS

The method for designing compensation for breast treatments using an electronic portal imager has been verified using planning on CT data for 14 patients. An improvement was seen in planning when applying MLC compensation and this effect was greater the larger the breast size.

Radiological thickness measurement using a liquid ionization chamber electronic portal imaging device

We present a method of calibrating the Portal Vision electronic portal imaging device to obtain radiological thickness maps for compensator design. In this method, coefficients are derived to describe the relationship between intensity and thickness for a set of water-equivalent blocks. The effects of four parameters were studied: (a) The dose response of the system was measured and found to be describable by a square-root function. (b) The calibration data and images were taken with a wedge in situ. The effects of using different wedges and different wedge orientations were investigated. The intrinsic accuracy of the accelerator/imager system was found to be 1.9 mm, for both 15 degrees and 30 degrees wedges. Changing the wedge orientation between calibration and imaging and rotating the calibration coefficients accordingly led to an error of 3.5 mm. (c) The variation in detector response with gantry angle was measured and corrected. The residual error in this process was 2.4 mm. (d) The use of a model to correct the effects of imaging with different field sizes was investigated and found to yield a residual error of 2.9 mm. The overall error in image calibrations was approximately 4 mm or 2% in dose. This is considered to be sufficiently small for the intended use of designing compensators for tangential breast irradiation.

Practical implementation of compensators in breast radiotherapy

BACKGROUND AND PURPOSE

A method of using electronic portal imaging to design compensators for tangential breast irradiation has been developed. We describe how this has been implemented.

MATERIALS AND METHODS

The compensator design method generates wedged and unwedged beam weights, in conjunction with templates for multiple lead-sheet compensators and pseudo-CT outlines. The latter describe the breast and lung profiles in a set of transverse slices. The layers of the compensator and pseudo-CT outlines are transferred to a treatment planning system for verification. The accuracy of the planning system for the high transmission blocks used to describe the compensators has been verified using a plotting tank system. Dose volume histogram data and transaxial and sagittal plan slices have been compared for both standard and compensated treatments for a sample set of five patients.

RESULTS

The planning system predicted the dose at depths of 1.5 and 5 cm to within 2% for the compensators tested. The biggest source of discrepancy was a consequence of the planning system requiring blocks to have integer percentage transmission. For all patients studied, the compensated treatment resulted in a significant reduction in the percentage volume outside the 95-105% dose, with an average reduction of 10.2%. The percentage volume outside the 95-107% dose was also reduced by typically 3.4%. The implementation was found to yield a convenient automatic method of designing compensators using electronic portal imaging and verifying the results using a planning system.

CONCLUSIONS

These results indicate that this method of implementation can be used in practice. The dosimetric accuracy of the treatment planning system is limited by the requirement that blocks should be of integer transmission, but this effect is small.