Three-dimensional dose distribution of tangential breast irradiation: results of a multicentre phantom dosimetry study

Correlation of hot spot to breast separation in patients treated with postlumpectomy tangent 3D-CRT using field-in-field technique and mixed photon energies

Radiotherapy to an intact breast was previously determined to have a positive correlation between breast separation measurement and hot spot dose. As breast separation measurement increased, hot spot dose increased. The purpose of this retrospective study was to determine if this correlation persisted despite current techniques including field-in-field (FiF) blocking and mixed photon energies. Radiation treatment plans on unilateral intact breasts from 90 female, early stage breast cancer patients treated with lumpectomy were analyzed. Plans were created using 3-dimensional conformal radiation therapy (3D-CRT) nondivergent opposing tangent beams, FiF technique, and 6 MV with or without higher energy photons. Data collected included breast separation measurement, hot spot point dose and location, number of beams, photon energy, clinical target volume (CTV) coverage and breast volume coverage. Correlations between breast separation measurement and each of these values were determined. The positive correlation between breast separation measurement and hot spot dose persisted despite incorporating FiF and mixed photon energies. Correlations were also found between breast separation and the number of beams as well as breast separation and photon energy. Larger breast separations tended to be treated with additional beams of higher photon energy. There were no correlations found between breast separation and CTV or breast volume coverage. The data in this study suggested the medical dosimetrist should expect hot spots above prescription dose of 106%, 107%, and 108% for small, medium and large breast separation sizes respectively. Additionally, adding a high energy photon bean may be indicated with medium and large breast separations.

Age, breast cancer subtype approximation, and local recurrence after breast-conserving therapy

PURPOSE

Prior results of breast-conserving therapy (BCT) have shown substantial rates of local recurrence (LR) in young patients with breast cancer (BC).

PATIENTS AND METHODS

We studied 1,434 consecutive patients with invasive BC who received BCT from December 1997 to July 2006. Ninety-one percent received adjuvant systemic therapy; no patients received trastuzumab. Five BC subtypes were approximated: estrogen receptor (ER) or progesterone receptor (PR) positive, HER2 negative, and grades 1 to 2 (ie, luminal A); ER positive or PR positive, HER2 negative, and grade 3 (ie, luminal B); ER or PR positive, and HER2 positive (ie, luminal HER2); ER negative, PR negative, and HER2 positive (ie, HER2); and ER negative, PR negative, and HER2 negative (ie, triple negative). Actuarial rates of LR were calculated by using the Kaplan-Meier method.

RESULTS

Median follow-up was 85 months. Overall 5-year cumulative incidence of LR was 2.1% (95% CI, 1.4% to 3.0%). The 5-year cumulative incidence of LR was 5.0% (95% CI, 3.0% to 8.3%) for age quartile 23 to 46 years; 2.2% (95% CI, 1.0% to 4.6%) for ages 47 to 54 years; 0.9% (95% CI, 0.3% to 2.6%) for ages 55 to 63 years; and 0.6% (95% CI, 0.1% to 2.2%) for ages 64 to 88 years. The 5-year cumulative incidence of LR was 0.8% (95% CI, 0.4% to 1.8%) for luminal A; 2.3% (95% CI, 0.8% to 5.9%) for luminal B; 1.1% (95% CI, 0.2% 7.4%) for luminal HER2; 10.8% (95% CI, 4.6% to 24.4%) for HER2; and 6.7% (95% CI, 3.6% to 12.2%) for triple negative. On multivariable analysis, increasing age was associated with decreased risk of LR (adjusted hazard ratio, 0.97; 95% CI, 0.94 to 0.99; P = .009).

CONCLUSION

In the era of systemic therapy and BC subtyping, age remains an independent prognostic factor after BCT. However, the risk of LR for young women appears acceptably low.

Determination of critical dosimetric parameters for breast radiation using forward-planned segmented fields for intensity modulation

The purpose of this study was to determine factors associated with acute skin toxicity from breast radiation for optimizing forward-planned intensity modulation. Treatment plans in 100 patients who received breast radiation using three-dimensional treatment planning were analyzed. Fifty-two patients were treated with tangent fields using wedges (nonsegmented), and 48 patients were treated with forward-planned fields segmented by a multileaf collimator to modulate intensity. Clinical and dosimetric variables were recorded. Acute skin toxicity was prospectively documented using a standard scale. Body weight, breast target volume, maximum body dose (encompassing 10 mL), and volume of body receiving >50 Gy and 55 Gy (V50Gy, and V55Gy) were associated with acute toxicity. Patients treated with segmented plans had significantly larger breast targets and were treated to lower prescription isodoses, confounding comparison with nonsegmented plans. Consequently, datasets from patients treated with segmented plans were used to design new nonsegmented plans for paired comparison. Segmented plans were superior with respect to dosimetric endpoints predictive of toxicity in this paired comparison. Limitations of 55 Gy for maximum body dose and 1100 mL for V50Gy appeared to be appropriate values to guide forward treatment planning of segmented fields.

The use of in vivo thermoluminescent dosimeters in the quality assurance programme for the START breast fractionation trial

BACKGROUND AND PURPOSE

The use of in vivo dosimetry for patient measurement is recommended in many publications. It provides an additional check to verify that the dose delivered to the patient corresponds to the prescribed dose. In the context of a clinical trial investigating the effects of different fractionation regimens, it is imperative that the dose given is that prescribed to ensure that noise in the data between centres does not mask the results of the trial. The methodology for in vivo measurement in a clinical trial of breast radiotherapy was developed and verified.

MATERIALS AND METHODS

A cohort of patients in the STAndardisation of breast RadioTherapy (START) trial was monitored using postal thermoluminescent dosimeters chips (TLD). All TLD were processed and analysed at Mount Vernon Hospital. Patients for in vivo measurements were identified at randomisation as a random 1 in 9 samples for the first 2500 patients randomised (282 TLD) increasing to 1 in 3 thereafter. The TLD were left in place for the duration of the tangential field treatment and thus a composite entrance and exit dose was recorded.

RESULTS

TLD measurements were performed on 429 patients from 33 hospitals. The average ratio of dose measured using TLD to that prescribed was 0.99+/-0.04. Eight patients had initial measurements more than 10% different to the prescribed dose. The mean TLD results for a given centre correlated well with dose measurements performed using an ionisation chamber in a breast shaped phantom at that centre as part of the START trial audit.

CONCLUSION

Thermoluminescence dosimetry has provided useful quality assurance information on the doses received by patients in centres participating in the START trial.

Radiotherapy quality assurance: time for everyone to take it seriously

Like high-risk industries, radiotherapy requires intense attention to detail, alertness, precision, and adequate human and material resources to minimise the risk of irreversible consequences. Clinical trials data such as that generated by the Quality Assurance programme of the Radiotherapy Group of the European Organization for Research and Treatment of Cancer (EORTC) in this issue of the Journal have been instrumental in identifying problems with technical quality, the understanding of which can have a direct impact on improving the quality of care in the community. Consistency in absolute dosimetry, dose delivery, volume definition and reproducibility are paramount in radiotherapy quality assurance and have become even more important with the advent of conformal therapy. Extension of these principles to other oncological disciplines has added an additional dimension of improvement. Waiting times and measures of access must also be monitored if overall quality at the population level is to be assessed and enhanced. Lessons should be learned from clinical trials methodology in the use of intervention-specific guidelines, physician education and real time audit of treatment planning decisions. In the future, novel approaches, such as web based systems may further improve education and audit. Wider application and audit of evidence-based management guidelines about the use radiotherapy will bring to standard clinical practice the quality benefits that are considered a basic minimum standard for clinical trials.

Three-dimensional distribution of radiation within the breast: an intercomparison of departments participating in the START trial of breast radiotherapy fractionation

PURPOSE

To examine the ability of computer planning systems to calculate the dose to the breast correctly in three dimensions. Both the absolute dose at the center of the breast and the accuracy of the isodose distributions were investigated.

METHODS AND MATERIALS

Measurements were performed in a water-filled breast phantom using an ionization chamber. Thirty-six sets of data obtained during the Standardization of Breast Radiotherapy breast fractionation trial quality assurance program were included in the analysis. The planning systems were grouped according to the algorithms used on the basis of the definitions given in International Commission on Radiation Units and Measurements Report No. 24.

RESULTS

Thirty-two of the 36 planning systems overestimated the dose to the center of the breast, with a mean measured/calculated dose ratio of 0.979 (SD 0.013). The relative dose within 2 cm of the lung was also overestimated.

CONCLUSION

Only one algorithm (collapsed cone) investigated in this study was able to calculate the dose at the center of the breast correctly in tangential breast radiotherapy. With modern algorithms, it is important to include a correction for the lower density of the lung, because the dose close to the interface between breast and lung tissue will also be lower than anticipated.

Optimizing breast cancer treatment efficacy with intensity-modulated radiotherapy

PURPOSE

To present our clinical experience using intensity-modulated radiation therapy (IMRT) to improve dose uniformity and treatment efficacy in patients with early-stage breast cancer treated with breast-conserving therapy.

METHODS AND MATERIALS

A total of 281 patients with Stage 0, I, and II breast cancer treated with breast-conserving therapy received whole breast RT after lumpectomy using our static, multileaf collimator (sMLC) IMRT technique. The technical and practical aspects of implementing this technique on a large scale in the clinic were analyzed. The clinical outcome of patients treated with this technique was also reviewed.

RESULTS

The median time required for three-dimensional alignment of the tangential fields and dosimetric IMRT planning was 40 and 45 min, respectively. The median number of sMLC segments required per patient to meet the predefined dose-volume constraints was 6 (range 3-12). The median percentage of the treatment given with open fields (no sMLC segments) was 83% (range 38-96%), and the median treatment time was <10 min. The median volume of breast receiving 105% of the prescribed dose was 11% (range 0-67.6%). The median breast volume receiving 110% of the prescribed dose was 0% (range 0-39%), and the median breast volume receiving 115% of the prescribed dose was also 0%. A total of 157 patients (56%) experienced Radiation Therapy Oncology Group Grade 0 or I acute skin toxicity; 102 patients (43%) developed Grade II acute skin toxicity and only 3 (1%) experienced Grade III toxicity. The cosmetic results at 12 months (95 patients analyzable) were rated as excellent/good in 94 patients (99%). No skin telengiectasias, significant fibrosis, or persistent breast pain was noted.

CONCLUSION

The use of intensity modulation with our sMLC technique for tangential whole breast RT is an efficient method for achieving a uniform and standardized dose throughout the whole breast. Strict dose-volume constraints can be readily achieved resulting in both uniform coverage of breast tissue and a potential reduction in acute and chronic toxicities. Because the median number of sMLC segments required per patient is only 6, the treatment time is equivalent to conventional wedged-tangent treatment techniques. As a result, widespread implementation of this technology can be achieved with minimal imposition on clinic resources and time constraints.

A novel approach of dose mapping using a humanoid breast phantom in radiotherapy

A study of dose mapping techniques to investigate the dose distribution throughout a planned target volume (PTV) in a humanoid breast phantom exposed to a 6 MV photon beam similar to that of treatment conditions is described. For tangential breast irradiation using a 6 MV accelerator beam, the dose is mapped at various locations within the PTV using thermoluminescent dosemeters (TLDs) and radiographic films. An average size perspex breast phantom with the ability to hold the dosemeters was made. TLDs were exposed after packing them in various locations in a particular slice, as planned by the treatment planning system (TPS). To map the dose relative to the isocenter, films were exposed after tightly packing them in between phantom slices, parallel to the central axis of the beam. The dose received at every location was compared with the given dose as generated by the TPS. The mapped dose in each location in the isocentric slice from superficial to deep region was found to be in close agreement with the TPS generated dose to within +/-2%. Doses at greater depths and distant medial and lateral ends, however, were found to be lower by as much as 9.4% at some points. The mapped dose towards the superior region and closest inferior region from the isocenter was found to agree with those for TPS. Conversely, results for the farthest inferior region were found to be significantly different with a variance as much as 17.4% at some points, which is believed to be owing to the variation in size and shape of the contour. Results obtained from films confirmed this, showing similar trends in dose mapping. Considering the importance of accurate doses in radiotherapy, evaluating dose distribution using this technique and tool was found to be useful. This provides the opportunity to choose a technique and plan to provide optimum dose delivery for radiotherapy to the breast.

Dosimetric intercomparison for two Australasian clinical trials using an anthropomorphic phantom

INTRODUCTION

Many different factors can affect the accurate delivery of dose to the clinical target volume in radiotherapy. This is particularly important in the context of multicenter clinical trials where different equipment and techniques may be used for supposedly identical treatments. A dosimetric intercomparison employing an anthropomorphic phantom (level III dosimetric intercomparison) can be used to check many of the factors that could affect treatment by mimicking the radiotherapy pathway of a patient as closely as possible.

METHODS AND MATERIALS

An anthropomorphic phantom (ART) was taken to 18 radiotherapy centers in Australia and New Zealand and treated for two different treatment scenarios based on current clinical trials of the Trans-Tasman Radiation Oncology Group (TROG): a two-field treatment of a carcinoma of the tonsil (TROG 91.01), and a four-field prostate treatment (TROG 96.01). The dose distribution was assessed in two consecutive treatments using thermoluminescence dosimeters (TLDs) placed throughout the target volume and in "critical" structures such as the lens of the eye or the rectum. The study also included a check of absolute dose calibration in a slab phantom (level I dosimetric intercomparison). The influence of a variety of treatment parameters on the dose homogeneity in the target and the measured dose in the target and the critical organs was evaluated.

RESULTS

The dose measurements confirmed that in all participating centers the correct dose was delivered to the ICRU reference point (tonsil: 99.8 +/- 2.3%; prostate: 100.9 +/- 1.9% [1 SD]). Also the absolute dose calibration and the mean dose in the target volume were within the specified action levels of plus minus 5% for all participating centers. No influence of shielding, beam modifiers, beam weighting, treatment planning approach (CT, 2D, 3D), and type of equipment used on the dose in the target and its homogeneity could be demonstrated. However, treatment technique and energy used influenced the dose to the critical organs. It was shown that the interpretation of results could be improved by including two complementary treatment scenarios and a level I intercomparison with the level III dosimetric intercomparison.

CONCLUSION

The study demonstrated the feasibility of a level III dosimetric intercomparison service at a cost of approximately $1000(US) per center in Australasia. It confirmed that the dose delivered by all participating centers was as intended in the two treatment scenarios chosen. While this provides reassurance to the oncology community and the general public, the service must be extended to all centers and other potentially more complex treatment scenarios. The present study has built the foundation for this by establishing a baseline and action levels and suggesting improvements in phantom design which will be included in future TROG quality assurance exercises.

The START trial-measurements in semi-anatomical breast and chest wall phantoms

This paper describes dosimetry measurements performed prior to departments entering patients into the START Trial, a breast fractionation trial. Absolute and relative doses were measured in semi-anatomical breast and chest wall phantoms, as part of a quality assurance programme visit. Doses were measured using an ionization chamber and the resulting distributions were compared with those calculated by the department. The mean ratio of measured to calculated dose at the START reference point was found to be 0.981 for the breast phantom and 0.978 for the chest wall phantom. This average measured dose was significantly less than the prescribed dose (p < 0.001). Differences were found between 2D and 3D planning systems and for departments using cobalt 60 beams. A number of departments had deviations of greater than 4%, which was the tolerance applied for this trial. It is essential for dose measurements of this type to be performed for randomized clinical trials involving radiotherapy, particularly where dose fractionation regimes are being compared.