Reisha Willis Treatment Planning Project March 2015 Lung

1 Reisha Willis Treatment Planning Project March 2015 Lung Treatment Planning Project Cancer is a group of diseases that involve abnormal cell growth that in turn can lead to invasion into other parts of the body. Lung cancer is the leading cause of cancer deaths in the United States, among both men and women. Lung cancer claims more lives each year than do colon, prostate, ovarian and breast cancers combined¹. When patients receive a new diagnosis, the first question is what type of treatment do they need. Treatment of lung cancer include surgery, chemotherapy and radiation therapy.​
Besides attacking the tumor, radiotherapy can help to relieve some of the symptoms the tumor causes such as shortness of breath. When used as an initial treatment instead of surgery, radiotherapy may be given alone or combined with chemotherapy. ​
Not all patients are candidates for surgery due to the size and location of the tumor as well as the patient's overall health. When surgery is not an option, they turn to chemotherapy and radiation. ​
Radiation therapy is the delivery of focused high­energy x­rays (photons), gamma rays or atomic particles. It affects cells that are rapidly dividing—such as cancer cells—much more than those that are not. Most cancers, including lung tumors, are made of cells that divide more rapidly than those in normal lung tissue, holding out the hope that the tumor can be eliminated without damaging surrounding normal tissues. Radiotherapy acts by attacking the genetic material—or DNA—within tumor cells, making it impossible for them to grow and create more cancer cells. Normal body cells may also be damaged but they are able to repair themselves and function properly once again. The key strategy is to give daily doses of radiation large enough to kill a high percentage of the rapidly dividing cancer cells, while at the same time minimizing damage to the more slowly dividing normal tissue cells in the same area². When a patient comes in for radiation a CT simulation is done to give the physician a 3­dimensional image of the tumor and surrounding organs at risk (OR). From that data set, the physician will contour what they want treated. That usually includes the tumor or tumor bed, 2 positive lymph nodes, and at risk lymph nodes. From there a dosimetrist will plan the treatment and decide which is the best way to deliver the dose prescribed by the physician while sparing healthy tissue. When deciding on how to treat a lung tumor, the main OR to spare will be healthy lung tissue, the spinal cord and the heart. The beam angles are chosen by taking into account these structures. Before conformal techniques were standard, an anterior and posterior beam configuration were used to spinal cord tolerance then a “boost” was given using obliqued angles to avoid the spinal cord.​
The volume of irradiated lung parenchyma is smaller with anterior and posterior fields than with oblique fields, and the former should therefore be used until spinal cord tolerance is reached. One disadvantage of using anterior and posterior fields is the dose gradient within the fields resulting from the slope of the chest in the caudal­cephalad direction. The dose in the spinal cord near the cephalad margin of the fields is therefore of particular concern because of it is consistently higher than the calculated central axis dose. The dose differential depends on the steepness of the slope, the off axis ratio, and the beam energy³. A higher beam energy is needed for most lungs. The plan being presented below is of a lung tumor volume in the left side being treated with an anterior/posterior technique. Tumor volume was increased by 0.5cm to create a Planning Target Volume (PTV). The lungs, spinal cord and heart were contoured in addition to the PTV. If sufficient margins have been allowed for in the localization of PTV, the beam apertures are then shaped to conform and adequately cover the PTV within 95%­105% isodose surface relative to prescribed dose⁴. Patient motion, including that of tumor volume, critical organs, and external fiducial marks during imaging, simulation, and treatment, can give rise to systematic as well as random errors that must be accounted for when designing the PTV. Multi­leaf Collimators (MLCs) are used to shape the irradiated field to the shape of the PTV. A 2 cm margin is given between the MLC’s and PTV to adequately cover the tumor with the 95% isodose line. Two plans were created, one with heterogeneity corrections to account for the difference in tissue density as compared to water and the second is planned with heterogeneity corrections turned off, which treated all tissues within the body as water equivalent. 3 As you can see from the isodose lines in this first plan (figure 1), the heterogeneity correction takes into account the difference in tissue densities of the lung and the surrounding tissue. Lung than water. The anterior beam is weighted heavier and there is no wedge needed. The prescribed dose is 200cGy per day for a total of 35 treatments. The monitor units (MUs) needed to deliver the prescribed dose is 129 for the anterior field and 99 for the posterior field. The second plan, shown in figure 3, shows the first plan but with heterogeneity corrections turned off. Since the algorithm is treating all tissues the same, it shows that the dose is more concentrated on the anterior portion of the patients body. The MUs are slightly different as well. As radiation passes through matter, the x­ray beam undergoes a gradual reduction in the number of photons or exposure rate⁵. This is the attenuation. The denser the material, the more it attenuates the radiation beam. And the human body is not a homogeneous structure. It consists of varying quantities of air, fat, water, muscle, and bone, each with its own absorption properties⁵. This is why there is a difference seen when heterogeneity corrections are turned off. It is important to account for the inhomogeneities of the human body when calculating a radiation therapy plan. It is more accurate and gives an accurate account of what dose the tumor is receiving and the OR are receiving. If the second plan was used, an underdose of the anterior portion of the tumor may occur. 4 Figures Figure 1. 3D View with DVH of plan with heterogeneity corrections Figure 2. Monitor Unit and Dose Parameter Report for 1st plan 5 Figure 3. ​
3D View with DVH of plan with no heterogeneity corrections Figure 4. Monitor Unit and Dose Parameter Report for 2nd plan 6 References 1.Mayo Clinic. Lung Cancer website .​
http://www.mayoclinic.org/diseases­conditions/lung­cancer/basics/definition/con­20025531​
. assessed March 1, 2015. 2. RadiologyInfo.org. Lung Cancer Treatment. http://www.radiologyinfo.org/en/info.cfm?pg=lung­cancer­therapy​
. assessed March 1, 2015 3.​
Bentel GC. Radiation Therapy Planning. Second ed. New York. McGraw­Hill. 1996 th​
4. ​
Khan F. ​
The Physics of Radiation Therapy​
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Ed. Baltimore, MD: Lippincott Williams and Wilkins; 2014. 59 5. Washington CM, Leaver D. ​
Principles and Practice of Radiation Therapy.​
3rd ed.St. Louis,Mo:2010