A study testing the routine use of ultrasound measurements when selecting the electron energy for breast boost radiotherapy

Discrepancies in determining electron energy for lumpectomy boost treatment

The aim of this study was to compare lumpectomy cavity depth measurements obtained through ultrasound (U/S) and retrospective computed tomography (CT). Twenty-five patients with stage T1-2 invasive breast cancer formed the cohort of this study. Their U/S and CT measurements were converted into electron energy and compared. The mean U/S depth was 3.6 ± 1.3 cm, while the mean CT depth was 4.9 ± 1.9 cm; the listed error ranges are one standard deviation. Electron energies for treatment ranged from 6 MeV to 12 MeV based on the U/S determination. There was no significant correlation between cavity depths measured by U/S and CT (R²= 0.459, P < 0.002). Furthermore, only 20% of CT-based electron energy determinations matched the corresponding U/S determinations. This ratio increased to 40% when taking into account an upper limit based on the depth of organs at risk below the cavity. The study shows that there is a significant discrepancy between cavity depths determined by U/S and CT. It also supports the concept that post-lumpectomy radiotherapy boosts should be tailored according to the needs and comfort of individual practices and institutions.

Can radiographic plain film be used to determine the depth of the tumour bed in the absence of surgical clips for breast boost planning

Purpose

A number of studies have demonstrated the importance of using surgical clips to define the tumour bed in breast boost radiotherapy. In the absence of such clips, other techniques suggested to improve boost location have included CT and ultrasound (US). Determination of the depth of the tumour bed is important in the selection of electron energy. This study was conducted to prospectively compare the depth of the lumpectomy cavity as defined by ultrasound to radiographic plain film evaluation of the anterior border of the pectoralis muscle.

Materials and Methods

Forty-one breast-cancer patients treated at the Division of Therapeutic Radiology and Oncology, Department of Radiology, Faculty of Medicine, Chiang Mai University between December 2004 and December 2006 were prospectively identified as having no surgical clips within the lumpectomy cavity. All patients underwent both US evaluation of the depth of tumour bed (D1) and radiographic evaluation of the depth of the anterior border of the pectoralis muscle (D2). These depth dimensions (D1 and D2) were compared using a paired t-test. The correlation of both methods was analyzed by Pearson correlation test.

Results

Depth dimensions by US were shorter than the radiographic film method in 85% of patients. The absolute mean difference of the depth (radiographic films minus US) was 0.129 cm. A paired t-test demonstrated that the difference between these two methods to be not statistically significant (p= 0.27). The absolute difference of depth between the two methods ranged from 0 to 0.5 cm. A significant correlation was found between US and radiographic film measurements (p<0.01).

Conclusion

Plane radiographic film evaluation of the anterior border of the pectoralis muscle can be used to define the depth of the tumour bed in patients who have no surgical clips. However, the plane radiographic film method determines only the depth, not the transverse and longitudinal dimensions of the tumour bed. Additional information from US is needed to delineate the target volume for the tumour bed boost. In the absence of surgical clips, the authors recommend integration of both methods in breast boost planning process.

Anatomical imaging for radiotherapy

The goal of radiation therapy is to achieve maximal therapeutic benefit expressed in terms of a high probability of local control of disease with minimal side effects. Physically this often equates to the delivery of a high dose of radiation to the tumour or target region whilst maintaining an acceptably low dose to other tissues, particularly those adjacent to the target. Techniques such as intensity modulated radiotherapy (IMRT), stereotactic radiosurgery and computer planned brachytherapy provide the means to calculate the radiation dose delivery to achieve the desired dose distribution. Imaging is an essential tool in all state of the art planning and delivery techniques: (i) to enable planning of the desired treatment, (ii) to verify the treatment is delivered as planned and (iii) to follow-up treatment outcome to monitor that the treatment has had the desired effect. Clinical imaging techniques can be loosely classified into anatomic methods which measure the basic physical characteristics of tissue such as their density and biological imaging techniques which measure functional characteristics such as metabolism. In this review we consider anatomical imaging techniques. Biological imaging is considered in another article. Anatomical imaging is generally used for goals (i) and (ii) above. Computed tomography (CT) has been the mainstay of anatomical treatment planning for many years, enabling some delineation of soft tissue as well as radiation attenuation estimation for dose prediction. Magnetic resonance imaging is fast becoming widespread alongside CT, enabling superior soft-tissue visualization. Traditionally scanning for treatment planning has relied on the use of a single snapshot scan. Recent years have seen the development of techniques such as 4D CT and adaptive radiotherapy (ART). In 4D CT raw data are encoded with phase information and reconstructed to yield a set of scans detailing motion through the breathing, or cardiac, cycle. In ART a set of scans is taken on different days. Both allow planning to account for variability intrinsic to the patient. Treatment verification has been carried out using a variety of technologies including: MV portal imaging, kV portal/fluoroscopy, MVCT, conebeam kVCT, ultrasound and optical surface imaging. The various methods have their pros and cons. The four x-ray methods involve an extra radiation dose to normal tissue. The portal methods may not generally be used to visualize soft tissue, consequently they are often used in conjunction with implanted fiducial markers. The two CT-based methods allow measurement of inter-fraction variation only. Ultrasound allows soft-tissue measurement with zero dose but requires skilled interpretation, and there is evidence of systematic differences between ultrasound and other data sources, perhaps due to the effects of the probe pressure. Optical imaging also involves zero dose but requires good correlation between the target and the external measurement and thus is often used in conjunction with an x-ray method. The use of anatomical imaging in radiotherapy allows treatment uncertainties to be determined. These include errors between the mean position at treatment and that at planning (the systematic error) and the day-to-day variation in treatment set-up (the random error). Positional variations may also be categorized in terms of inter- and intra-fraction errors. Various empirical treatment margin formulae and intervention approaches exist to determine the optimum strategies for treatment in the presence of these known errors. Other methods exist to try to minimize error margins drastically including the currently available breath-hold techniques and the tracking methods which are largely in development. This paper will review anatomical imaging techniques in radiotherapy and how they are used to boost the therapeutic benefit of the treatment.

A Computed Tomography-based Protocol vs Conventional Clinical Mark-up for Breast Electron Boost

AIMS

Computed tomography planning of whole breast radiotherapy (WBRT) improves breast coverage and reduces the normal tissue dose. Computed tomography planning may increase tumour bed boost treatment accuracy. The aims of this investigation were (1) to compare the breast boost volume treated with clinical mark-up with the volume delineated with computed tomography planning and (2) to study tumour bed volume changes between the initial planning computed tomography scan and a second computed tomography scan at the time of breast boost mark-up.

MATERIALS AND METHODS

Women receiving adjuvant WBRT and an electron boost after breast-conserving surgery were eligible. As per standard practice, WBRT was computed tomography planned while the boost electron portal was clinically defined. Electron field borders were then traced with wire and a second computed tomography scan was carried out in the boost treatment position. Post-surgical radiological abnormalities were contoured to create a tumour bed clinical target volume (CTV) on both scans (CTV1 and CTV2). A 1cm margin to CTV2 defined the planning target volume (PTV). The proportions of the CTV2 and PTV receiving 90% (V90) and 80% (V80) of the dose were calculated. Changes in volume between CTV1 and CTV2 were analysed.

RESULTS

Data from 47 eligible patients were analysed. The mean V90 for the PTV was 61%. Lower electron energy (P<0.001) and small field sizes (P=0.004) were associated with a low V90. The mean CTV decreased by 4.3 cm3 (P=0.014) and was smaller in those with a long surgery to computed tomography interval (P=0.008). On average, the 90% isodose covered 61 cm3 of normal tissue.

CONCLUSIONS

Conventional clinical breast boost planning is inaccurate. Electron boost computed tomography planning together with appropriate surgical clip placement and the use of mammograms and pathological information should provide optimal coverage of the tumour site. The boost could usually be planned from the initial computed tomography scan.

Ultrasound-Based Localization

Ultrasound is a noninvasive, relatively easy, rapid, and real-time imaging technique for organ targeting for radiotherapy. Its application has been developed to a greater extent in prostate cancer than in other sites in which it has been shown to improve the accuracy of daily treatment delivery. With the move toward dose escalation and the need to maximally spare the adjacent critical structures through more conformal therapy and smaller field margins, an innovative technique for accurate and reproducible tumor targeting is mandatory. Basic ultrasound principles and organ location lend themselves well to the application of this modality in prostate cancer. Promising results using daily ultrasound-guided B-mode acquisition and targeting for patients with upper abdominal tumors suggest an area for additional trials and study. For breast cancer radiotherapy, ultrasound serves to define involved primary and nodal sites, especially in patients in whom surgical evaluation will not be the first therapeutic step.

Correlating sonography, mammography, and pathology in the assessment of breast cancer size

BACKGROUND

With the increasing use of neoadjuvant and minimally invasive therapy, the accuracy of preoperative determination of breast tumor size becomes important. Therefore, we undertook this study to compare mammography and ultrasonography (US).

METHODS

A total of 180 invasive breast cancer patients were prospectively examined by mammography and US; 146 eligible patients had tumors visualized by both modalities.

RESULTS

In 69% of cases, US was better than or equivalent to mammography in determining tumor size. Both underestimated tumor size; mean (median) underestimation was 3.8 +/- 0.7 mm (1.7 mm) by US and 3.5 +/- 0.9 mm (2 mm) by mammogram. Maximal tumor dimension was accurate within 5 mm in 65% of cases by mammography and 75% of cases by US. For mammographically determined size (versus pathologic size) correlation, r, was 0.4 and for US it was 0.63 and improved for only T1 and T2 tumors.

CONCLUSIONS

These data suggest that US is more accurate than mammography in assessing breast cancer size.