Medical Treatment Using Computed Tomography (CT)
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Treatment Site / Diagnosis: Left Breast
Treatment Modality: 2 Dimensional tangential
Type of Patient Data: Computed tomography and virtual simulation
Prescription Isodose: 40GY in 15#
Computed tomography (CT) was employed as an imaging modality for this treatment. In general, CT is the standard imaging modality employed. This is due to the ability of CT to provide a 3 Dimensional image of the tumour and the ordinary anatomy in that area, CT is particularly effective in visualising bony anatomy. It also provides the electron density data essential to enable accurate dose calculation and planning. Although CT scanning is the standard for treatment localisation, magnetic resonance imagining (MRI) are superior in defining soft tissue and tumour metastasis and so it is now advisable to employ CT-MRI fusion imaging in order to improve the accuracy of tumour localisation for treatment (Bhide and Nutting 2010).
Tangential fields are normally used in the treatment of tumours of the breast. Virtual simulation (v-sim) is employed in order to generate lateral and medial tangential fields. V-sim ensures accurate field matching and acceptable coverage of the breast tissue, the chest wall and the surgical or mastectomy scar (Barrett and Dobbs et al. 2009 and Moran and Haffty 2009).
The choice of beam energy is heavily dependent on patient size and separation; in general the chosen beam energy is 6 Megavoltage. For patients with a separation greater than 22cm, higher energy beams are usually used in order to improve dose homogeneity. Higher energy beams reduce the skin sparing effect of a lower energy; therefore care should be taken to ensure there is sufficient coverage of the superficial breast, mastectomy scar and clinical boarders (Barrett, Dobbs et al. 2009).
The breast is traditionally treated by means of two dimensional conformal tangential beams. The beam arrangement is a lateral and medial beam. The field boarders are marked up clinically, usually by a specialist radiographer. The lateral beam is usually at mid axilla and the medial beam at sternal level. The beams are at tangents to each other avoid irradiation of the heart, lung, dose uniformity and no dose overlap to the contralateral breast (Lee and Harris 2009).
Segmented beams were also selected in order to improve Planning Target Volume (PTV) coverage and allow for Multi Leaf Collimator (MLC) shielding without compromising coverage of the PTV whilst shielding out undesired hot spots that naturally occur in the inframammary fold of the breast tissue (Nakamura, Hatanaka et al. 2011).
According to a study by Purdy 2004 ICRU 50 guidelines states that the isodose distribution within the PTV must be between 95% - 107% .In order to achieve optimum dose distribution wedged beams are employed, wedges are tissue compensators that account for missing tissue in order to improve the homogeneity of the dose distribution (Barrett, Dobbs et al. 2009). This is particularly important in breast treatments due to the contour of the breast. The anterior surface of the breast is less dense than the tissue toward the chest wall, hotspots tend to occur around the areola for this reason wedges in this plan are 60 â° and orientated with the thick end anterior in order to distribute the dose away from the nipple and conform more homogenously to the chest wall. There are also wedges on the segments in order to improve dose homogeneity in the superior/ inferior direction and to ensure acceptable PTV coverage (Haffty, Buchholz et al. 2008).
There is acceptable coverage of the CTV, in breast treatment, the aim is to treat all the breast tissue to the deep fascia the 95% isodose should conform to the chest wall but not include the pectoralis major (Barrett, Dobbs et al. 2009). The breast tissue is covered by the 95% isodose line and it adheres well to the muscles of the chest wall. There are no hot spots present within the plan due to the optimal use of tissue compensators as mentioned above. According to a study by Purdy in 2004, ICRU guidelines isodse distribution must be kept between 95% - 107%, MLC shielding on the segments were employed in order to shield any hotspots present within the CTV without compromising target coverage or causing the plan to become too cold.
The critical organs that were contoured were the left lung. Although there is no dose volume histogram associated with breast treatments, there should be no more than 2cm of lung volume included in the treatment field in order to prevent late toxicities such as lung fibrosis and pneumonitis. It is also advisable to contour the heart on left sided breast treatments.
This plan is clinically acceptable, however as the treatment is being delivered to the left side, the heart should be taken into consideration. Deep inspiration breath hold (DIBH) is becoming more common for left sided breast treatments. DIBH involves treating the patient on inspiration and breath hold through coaching either auditory or visually or both. During inspiration the breast tissue is lifted off the chest wall and thus results in less cardiac tissue and lung being irradiated (Vikström, Hjelstuen et al. 2011).
Not all patients are suitable for DIBH if they cannot remain in breath hold for the length of time it takes to deliver the beam. However it is still important to remove the heart from the high dose area, this is achievable by the use of cardiac shielding created by MLCS. Although this has shown a reduction in the dose received by the heart, it also risks underdoing of the target (Bartlett, Yarnold et al. 2013).
Haffty, B. G., T. A. Buchholz, et al. (2008). "Should intensity-modulated radiation therapy be the standard of care in the conservatively managed breast cancer patient?" Journal of Clinical Oncology 26(13): 2072-2074.
Nakamura, N., S. Hatanaka, et al. (2011). "Quantification of cold spots caused by geometrical uncertainty in field-in-field techniques for whole breast radiotherapy." Japanese journal of clinical oncology 41(9): 1127-1131.
Vikström, J., M. H. Hjelstuen, et al. (2011). "Cardiac and pulmonary dose reduction for tangentially irradiated breast cancer, utilizing deep inspiration breath-hold with audio-visual guidance, without compromising target coverage." Acta Oncologica 50(1): 42-50.
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