question: how does cancer not always relapse after radiation treatment
comment: The cells in your body are constantly winning wars vs cancer cells. Your body sees cancer cell mutation as foreign and destroys them before they spread. Sometimes some slip through and multiply, in which can spread to parts of the body and start taking over. Radiation helps destroy cancer cells (and normal cells) so eventually the balance can be maintained again.
passages: In 10-20% of cases, FL appears limited to single radiation field, does not involve the bone marrow, and is therefore regarded as localized early-stage FL. In these cases, which are sometimes classified as Ann Arbor stage I (i.e. disease limited to a single restricted region) or stage II (i.e. disease restricted to two sites that are on the same side of the diaphragm), radiation therapy achieves 10 year overall survival rates of 60-80% and median overall survival times of 19 years. It seems likely that many of the relapses in these cases are due to undetected disease outside of the radiation field at the time of radiation treatment. The use of PET/CT imaging is strongly recommended to insure that the FL is localized. In any case, the excellent results achieved with radiation therapy strongly support its use in localized disease. The use of an immunotherapeutic agent such as Rituximab alone or in combination with a chemotherapeutic regimen such as CVP (i.e. cyclophosphamide, vincristine, prednisone and rituximab) in cases of localized, early-stage disease may be appropriate choices for some of these early-stage patients. However, the latter approach is recommended for cases of localized disease in which the disease extends beyond a single field: 56% of patients treated in this manner had progression-free survival at 10 years while patients treated with other regimens had progression free survivals of 41%. Nonetheless, overall survival did not differ between the two groups.
   Section::::Host response to different treatment modalities for cancer.:Host response to radiation therapy.
  
  Radiotherapy is a well-established treatment modality for several cancer types. However, relapses after radiotherapy are often more aggressive and associated with poor prognosis. Cumulative evidence shows that the host response to radiotherapy is a contributing factor to this effect. Tumors implanted in pre-irradiated tissue grow with slower kinetics, however, paradoxically exhibit enhanced invasive and metastatic properties, a phenomenon known as the “tumor bed effect”. This enhanced aggressiveness is attributed to radiation-induced modifications of the tumor microenvironment, including enhanced angiogenesis and recruitment of pro-metastatic bone marrow cells and macrophages
   The frequency of CT scans during surveillance should ensure that relapses are detected at an early stage while minimizing the radiation exposure.
  
  For patients treated for stage I non-seminoma, a randomised trial (Medical Research Council TE08) showed that, when combined with the standard surveillance tests described above, 2 CT scans at 3 and 12 months were as good as 5 over 2 years in detecting relapse at an early stage.
   Radiation therapy has been used both as a primary treatment and combined with surgery or drugs. It is usually reserved for patients who have tumor remaining after surgery. These patients often also receive medication to lower GH levels. Radiation therapy is given in divided doses over four to six weeks. This treatment lowers GH levels by about 50 percent over 2 to 5 years. Patients monitored for more than 5 years show significant further improvement. Radiation therapy causes a gradual loss of production of other pituitary hormones with time. Loss of vision and brain injury, which have been reported, are very rare complications of radiation treatments.
   The cells which are actively undergoing cell cycle are targeted in cancer therapy as the DNA is relatively exposed during cell division and hence susceptible to damage by drugs or radiation. This fact is made use of in cancer treatment; by a process known as debulking, a significant mass of the tumor is removed which pushes a significant number of the remaining tumor cells from G to G phase (due to increased availability of nutrients, oxygen, growth factors etc.). Radiation or chemotherapy following the debulking procedure kills these cells which have newly entered the cell cycle.
   Typically, people who experience a relapse in their ALL after initial treatment have a poorer prognosis than those who remain in complete remission after induction therapy. It is unlikely that the recurrent leukemia will respond favorably to the standard chemotherapy regimen that was initially implemented, and instead these patients should be trialed on reinduction chemotherapy followed by allogeniec bone marrow transplantation. These patients in relapse may also receive blinatumomab, as it has shown to increase remission rates and overall survival rates, without increased toxic effects.
   The most common site of recurrence is in the vagina; vaginal relapses of endometrial cancer have the best prognosis. If relapse occurs from a cancer that has not been treated with radiation, EBRT is the first-line treatment and is often successful. If a cancer treated with radiation recurs, pelvic exenteration is the only option for curative treatment. Palliative chemotherapy, cytoreductive surgery, and radiation are also performed. Radiation therapy (VBT and EBRT) for a local vaginal recurrence has a 50% five-year survival rate. Pelvic recurrences are treated with surgery and radiation, and abdominal recurrences are treated with radiation and, if possible, chemotherapy. Other common recurrence sites are the pelvic lymph nodes, para-aortic lymph nodes, peritoneum (28% of recurrences), and lungs, though recurrences can also occur in the brain (<1%), liver (7%), adrenal glands (1%), bones (4–7%; typically the axial skeleton), lymph nodes outside the abdomen (0.4–1%), spleen, and muscle/soft tissue (2–6%).
   Late side effects occur months to years after treatment and are generally limited to the area that has been treated. They are often due to damage of blood vessels and connective tissue cells. Many late effects are reduced by fractionating treatment into smaller parts.
  
  BULLET::::- Fibrosis
  
  BULLET::::- Epilation
  
  BULLET::::- Dryness
  
  BULLET::::- Lymphedema
  
  BULLET::::- Cancer
  
  BULLET::::- Cardiovascular disease
  
  BULLET::::- Cognitive decline
  
  BULLET::::- Radiation enteropathy
  
  BULLET::::- Radiation-induced polyneuropathy
  
  Section::::Side effects.:Cumulative side effects.
   Section::::History.
   After stable remission was induced, the standard of care previously was to undergo 2 years of maintenance chemotherapy with methotrexate, mercaptopurine and ATRA. A significant portion of patients relapsed without consolidation therapy. In the 2000 European APL study, the 2-year relapse rate for those that did not receive consolidation chemotherapy (ATRA not included) therapy was 27% compared to 11% in those that did receive consolidation therapy (p0.01). Likewise in the 2000 US APL study, the survival rates in those receiving ATRA maintenance was 61% compared to just 36% without ATRA maintenance. 
   Radiation therapy often is considered for WHO grade I meningiomas after subtotal (incomplete) tumor resections. The clinical decision to irradiate after a subtotal resection is somewhat controversial, as no class I randomized, controlled trials exist on the subject. Numerous retrospective studies, however, have suggested strongly that the addition of postoperative radiation to incomplete resections improves both progression-free survival (i.e. prevents tumor recurrence) and improves overall survival.
   Section::::Side effects.
  
  Common side effects include fatigue, abdominal pain, nausea and anorexia, usually self-limiting. Post-radioembolization syndrome occurs in 20-70% of patients that undergo traditional radioembolization, presenting with shakes, chills, fatigue, nausea/vomiting, abdominal pain/discomfort, and/or cachexia and possibly hemodynamic changes, rarely requiring admission. Unfortunately, most data, if not all, is derived from traditional radioembolization outcomes studies and more will be needed to assess the actual incidence and risk of post-radioembolization syndrome in RL.
   BULLET::::- nausea, vomiting and fatigue
  
  Gastrointestinal symptoms are often found together with those in other systems including genitourinary disorders and sexual dysfunction. The burden of symptoms substantially impairs the patients' quality of life.
  
  Nausea, vomiting, fatigue and diarrhea may happen early during the course of radiotherapy. Radiation enteropathy represents the longer-term, chronic effects which may be found after a latent period most commonly of 6 months to 3 years after the end of treatment. In some cases, it does not become a problem for 20–30 years after successful curative therapy.
  
  Section::::Signs and symptoms.:Associated conditions.
  
  BULLET::::- Small intestinal bacterial overgrowth
   Radiotherapy has been used to treat recurrent or persistent lesions. However, considering the benign nature of this disease, radiation should be considered only in cases of recurrent, disfiguring lesions.
  
  Surgery has been considered the mainstay of therapy. However, recurrence after surgery is common.
   On completion of delivery of the radioactive sources, the applicators are carefully removed from the body. Patients typically recover quickly from the brachytherapy procedure, enabling it to often be performed on an outpatient basis.
  
  Between 2003 and 2012 in United States community hospitals, the rate of hospital stays with brachytherapy (internal radiation therapy) had a 24.4 percent average annual decrease among adults aged 45–64 years and a 27.3 percent average annual decrease among adults aged 65–84 years. Brachytherapy was the OR procedure with the greatest change in occurrence among hospital stays paid by Medicare and private insurance.
  
  Section::::Radiation sources.
   Radiation therapy is in itself painless. Many low-dose palliative treatments (for example, radiation therapy to bony metastases) cause minimal or no side effects, although short-term pain flare-up can be experienced in the days following treatment due to oedema compressing nerves in the treated area. Higher doses can cause varying side effects during treatment (acute side effects), in the months or years following treatment (long-term side effects), or after re-treatment (cumulative side effects). The nature, severity, and longevity of side effects depends on the organs that receive the radiation, the treatment itself (type of radiation, dose, fractionation, concurrent chemotherapy), and the patient.
   Radiation therapy, which kill cancer cells with focused doses of radiation, is often indicated in the treatment of rhabdomyosarcoma, and the exclusion of this treatment from disease management has been shown to increase recurrence rates. Radiation therapy is used when resecting the entirety of the tumor would involve disfigurement or loss of important organs (eye, bladder, etc.). Generally, in any case where a lack of complete resection is suspected, radiation therapy is indicated. Administration is usually following 6–12 weeks of chemotherapy if tumor cells are still present. The exception to this schedule is the presence of parameningeal tumors that have invaded the brain, spinal cord, or skull. In these cases radiation treatment is started immediately. In some cases, special radiation treatment may be required. Brachytherapy, or the placement of small, radioactive “seeds” directly inside the tumor or cancer site, is often indicated in children with tumors of sensitive areas such as the testicles, bladder, or vagina. This reduces scattering and the degree of late toxicity following dosing. Radiation therapy is more often indicated in higher stage classifications.
   Most people, including those treated with ASCT, relapse after initial treatment. Maintenance therapy using a prolonged course of low-toxicity medications is often used to prevent relapse. A 2017 meta-analysis showed that post-ASCT maintenance therapy with lenalidomide improved progression-free survival and overall survival in people at standard risk. A 2012 clinical trial showed that people with intermediate- and high-risk disease benefit from a bortezomib-based maintenance regimen.
  
  Section::::Treatment.:Chemotherapy.:Relapse.
   In addition, malignancies (including primary and many metastatic liver cancers) are often hypervascular; tumor blood supplies are increased compared to those of normal tissue, further leading to preferential deposition of particles in the tumors.
   Radiation therapy may be added, especially if chemotherapy does not seem sufficient on its own. Radiation may cause other health problems later, such as breast cancer, and there is some debate about the best approach to it.
  
  FDG-PET scanning is not as useful for predicting treatment success in PMBL as it is in other lymphomas.
  
  Section::::Prognosis.
  
  Most people with PMBL are successfully treated and survive for many years. However, if the initial treatment is unsuccessful, or if it returns, the long-term prognosis is worse. Relapses generally appear within 12 to 18 months after the completion of treatment.
  
  Section::::Epidemiology.
   Section::::Principles.
   In 2013, Alexander V. Akleyev described the chronology of the clinical course of CRS while presenting at ConRad in Munich, Germany. In his presentation, he defined the latent period as being 1-5 years, and the formation coinciding with the period of maximum radiation dose. The recovery period was described as being 3-12 months after exposure ceased. He concluded that "CRS represents a systemic response of the body as a whole to the chronic total body exposure in man." In 2014, Akleyev's book "Comprehensive analysis of chronic radiation syndrome, covering epidemiology, pathogenesis, pathoanatomy, diagnosis and treatment" was published by Springer.
   Treatment consists of surgical excision (the extent of which ranges from tumor excision to limb amputation, depending on the tumor) and in almost all cases radiation. Radiation eliminates the need for limb amputation and there is level I evidence to show that it leads to equivalent rates of survival (Rosenberg et al. NCI). Radiation may be delivered either pre-op or post-op depending on surgeon and multidisciplinary tumor board's recommendations. Radiation can be omitted for low grade, Stage I excised tumors with 1 cm margin (NCCN). Chemotherapy remains controversial in MFH.
   DIPG has a 5-year survival rate of <1%. The median overall survival of children diagnosed with DIPG is approximately 9 months. The 1- and 2-year survival rates are approximately 30% and less than 10%, respectively. These statistics make DIPG one of the most devastating pediatric cancers. Although 75–85% of patients show some improvement in their symptoms after radiation therapy, DIPGs almost always begin to grow again (called recurrence, relapse, or progression). Clinical trials have reported that the median time between radiation therapy and progression is 5–8.8 months. Patients whose tumors begin to grow again may be eligible for experimental treatment through clinical trials to try to slow or stop the growth of the tumor. However, clinical trials have not shown any significant benefit from experimental DIPG therapies so far.
   Section::::Medical uses.
  
  Different cancers respond to radiation therapy in different ways.
  
  The response of a cancer to radiation is described by its radiosensitivity.
  
  Highly radiosensitive cancer cells are rapidly killed by modest doses of radiation. These include leukemias, most lymphomas and germ cell tumors.
  
  The majority of epithelial cancers are only moderately radiosensitive, and require a significantly higher dose of radiation (60-70 Gy) to achieve a radical cure.
  