Radiotherapy is a cancer therapy with the use of radioisotopes. Radiotherapy utilizes ionizing radiation such as X-rays, gamma radiation or high-energy electrons to kill as many cancer cells as possible with the least negative effect on the surrounding healthy tissue. External radiation is radiotherapy using radiation sources from outside the patient’s body. The patient is in a seated or lying position, and the source of external ionizing radiation is directed to a particular body part. External radiation can be done by using the Linear Accelerator (LINAC) or Cobalt-60 machines. LINAC emits photons or electrons with an energy of 6 MeV or 10 MeV.
The working principle of LINAC is to accelerate electrons so that the kinetic energy increases from 4 MeV to 25 MeV.听In LINAC, electrons are accelerated to follow a straight path in a special structure called听Accelerating waveguide.听LINAC is currently equipped with several choices of radiation: electron and photon beams.听The control system on photons and electrons produced by LINAC uses a computer, a body scanning system using the听Electronic Portal Imaging Device听(EPID), regulating radiation doses using the听Multi Leaf Collimator听(MLC) and setting the dosage fully using intensity modulation.听Treatment Planning System听Technology(TPS) on LINAC uses 3D calculations with MLC system, which can produce precise shapes in the radiation听treatment听to听target听volume.听TPS or radiation planning systems are all processes for determining the design of angular geometry, radiation energy, number of radiation fields, immobility and dosimetry of patients to be given in the听radiation听treatment.
3 Dimensional Conformal Radiotherapy听Technology听(3 DCRT) can increase the number of the radiation field and the shape of the MLC that can be applied to the target by protecting normal tissue.听The dose output from LINAC is in the form of a听Monitor Unit听(MU) which is equivalent to cGy.听The amount of MU depends on the amount of dose that will be given, the depth of cancer, the calibration of monitor, the size of听the听collimator听settings, the field size听and other variables.
Therapy with LINAC is done by maximizing the dose distribution in cervical cancer and minimizing the dose distribution in surrounding healthy organs, bladder听and听rectum.听叠濒补诲诲别谤听补苍诲听谤别肠迟耻尘听are critical organs around the cervix at risk for radiation, also known as听Organ at Risk(OAR).听The effective dose is a radiation dose that considers tissue weight factors which are subject to radiation and radiation weighting factors from various radiation sources.听Each radiation source has a different radiation weighting factors.听Radiation will also have different effects on different tissues.听Giving excessive doses to OAR will cause damage to its constituent tissue.听Each OAR has a tolerance dose depending on the sensitivity of the tissue making up the organ.听Accuracy of dose determination is necessary for therapy using LINAC.
The Groupe Europen de Curieth茅rapie and the European Society for Radiotherapy & Oncology听(GEC-ESTRO听)听recommended a reference point for determining OAR doses, on听bladder听and听rectum.听Acute side effects of radiation therapy on the听bladder听are inflammation, mucositis,听interstitial fibrosis,听and contractures while for the rectum听include bleeding, tenesmus, urgency,听and diarrhea (Billingham听et al听., 2009).听Therefore, the radiation dose of听bladder听and听rectum may not exceed the dose tolerance.听Separate consideration is needed in the next action for OAR dosage.听The dose of the听bladder听and听rectum听can be evaluated by looking at both dose in the听Dose Volume Histogram听(DVH)听bladder听and the听2cc听rectum听(D2cc).
Author: Suryani Dyah Astuti
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