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Oncology-Chemotherapy Combination Therapy
This guide summarizes the rationale behind using combination chemotherapy to treat cancer.
I. Limitations of Single Cytotoxic Agents:
Effective combination therapy relies on several key principles:
This guide summarizes the rationale behind using combination chemotherapy to treat cancer.
I. Limitations of Single Cytotoxic Agents:
- Mechanism: Cytotoxic chemotherapy kills cancer cells by targeting:
- Nucleic acid chemistry
- DNA/RNA production
- Cell division mechanics (e.g., spindle poisons)
- Lack of Selectivity: A major limitation is the lack of selectivity. Existing drugs damage both cancer and normal dividing cells.
- Classification of Cytotoxic Agents: They are classified by:
- Chemical properties/mechanisms of action
- Source (e.g., natural products)
- Cell cycle specificity (phase-specific or not)
Effective combination therapy relies on several key principles:
- Single-Agent Activity: Each drug must effectively kill cancer cells on its own within the specific tumor type.
- Different Mechanisms of Action: Drugs should target different cellular processes to avoid overlapping effects and resistance.
- Non-Overlapping Toxicity: Drugs should have different side effect profiles to minimize overall toxicity to the patient, allowing for higher doses.
- Cell Cycle Diversity: Selecting drugs that target different phases of the cell cycle maximizes the number of cells killed, as tumor cells divide asynchronously.
- Avoiding Shared Resistance Mechanisms: Drugs should not all be susceptible to the same resistance mechanisms, such as multidrug resistance (MDR).
- Increased Fractional Cell Kill: The primary goal is to increase the proportion of cancer cells killed, leading to improved tumor response. Higher doses generally lead to greater cell kill (within limits).
- Addressing Asynchronous Cell Division: Tumors contain cells in different stages of the cell cycle; combination therapy with drugs acting at different stages increases efficacy.
- Overcoming Multidrug Resistance (MDR): Some tumors exhibit MDR, often due to efflux pumps that expel drugs. Combination therapy can help circumvent this resistance.
- Targeted Therapies: These newer therapies specifically inhibit processes crucial to cancer cell survival (e.g., angiogenesis inhibition, EGFR blockade). They often target oncogene function.
- Integration with Existing Therapies: Much research is ongoing to determine the optimal way to combine targeted therapies with cytotoxic chemotherapy to maximize patient benefit.
- Cytotoxic Chemotherapy: Treatment that kills cancer cells directly.
- Selectivity: The ability of a drug to target cancer cells without harming normal cells.
- Cell Cycle: The series of events leading to cell division.
- Phase-Specific Drugs: Drugs that are most effective during specific phases of the cell cycle.
- Multidrug Resistance (MDR): The ability of cancer cells to resist multiple chemotherapy drugs.
- Targeted Therapies: Drugs that specifically target cancer-related molecules or pathways.
- Angiogenesis Inhibition: Blocking the formation of new blood vessels that supply the tumor.
- EGFR Blockade: Inhibition of the Epidermal Growth Factor Receptor, a protein involved in cell growth and division.
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Oncology-Genetic Predisposition to Cancer
I. Increased Cancer Risk in Families:
Familial cancers may be indicated by several factors:
I. Increased Cancer Risk in Families:
- Moderate Increase: Epidemiological studies show a 2-3 times higher cancer risk among first-degree relatives of cancer patients. This isn't always due to inherited gene alterations; shared environmental factors or increased susceptibility to carcinogens also play a role.
Familial cancers may be indicated by several factors:
- Rare, Genetic Cancers: Examples include bilateral retinoblastoma.
- Associated Phenotypic Features: These are observable physical characteristics associated with specific genetic syndromes, such as:
- FAP (Familial Adenomatous Polyposis): Multiple polyps.
- Peutz-Jeghers Syndrome: Mucosal pigmentation.
- DNA Repair Disorders: Chromosome breakage.
- Family History Clues: These suggest a higher likelihood of inherited predisposition:
- Early Onset: Cancer diagnosed at an unusually young age.
- Multiple or Bilateral Tumors: More than one tumor, or tumors on both sides of the body (e.g., both breasts).
- Familial Clustering: Multiple family members with the same or related cancers (e.g., breast and ovarian cancer; colon and uterine cancer).
- Three or more close relatives (same side of family) with the same common cancer (or related cancers).
- Two or more close relatives (same side of family) with the same common cancer (or related cancers), with at least one diagnosis before age 50.
- One close relative with early-onset cancer (e.g., breast cancer <40, bowel cancer <45).< />pan>
- One close relative with multiple primary cancers.
- Two or more relatives with the same uncommon cancer (e.g., sarcoma, glioma, pancreatic cancer).
- Clinical Examination: May reveal specific features of genetic syndromes like Cowden syndrome, Gorlin syndrome, or neurofibromatosis. Know the characteristic features of each.
- Genetic Testing: Possible only when a causative germline mutation can be identified in a blood sample from an affected relative. Often, testing isn't possible due to:
- Lack of a suspected mutation in known predisposition genes.
- Unavailability of samples from affected relatives.
- Failure to identify the causative mutation in relevant genes.
- Cancer Surveillance: May be arranged for at-risk relatives, but effectiveness is often unproven. Participation in relevant clinical trials is encouraged.
- Germline mutation: A mutation present in an organism's germ cells (sperm or eggs), which can be passed on to offspring.
- Phenotypic features: Observable physical characteristics.
- Familial clustering: The occurrence of the same or related cancers in multiple members of a family.
- Early-onset cancer: Cancer diagnosed at a younger age than typically expected.
- Bilateral tumors: Tumors occurring on both sides of the body.
- Clinical genetics services: Specialized medical services that evaluate and manage inherited conditions, including cancer predisposition.
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Oncology -Bowel Cancer Family History
I. Recognizable Syndromes & Polyposis:
I. Recognizable Syndromes & Polyposis:
- Increased risk can stem from identifiable syndromes characterized by polyp presence.
- Genetic testing is effective in identifying causative mutations in most cases.
- Key Syndromes & Polyposis:
- FAP (Familial Adenomatous Polyposis): Classically >100 bowel polyps. Autosomal dominant; mutations in the same gene as attenuated FAP, differing in mutation site and phenotype.
- Attenuated FAP: <100 polyps, later onset, high bowel cancer risk, and risk for upper gi malignancy. autosomal dominant, same gene as fap.< />pan>
- MYH-associated polyposis: Autosomal recessive; predisposition to adenomatous bowel polyps; often presents with sibling history of bowel cancer. Testing focuses on two common MYH gene mutations.
- More common than recognizable syndromes.
- Characterized by: Familial aggregation of bowel cancer with fewer polyps.
- Genetic basis: Primarily due to mutations in mismatch repair genes (MLH1, MSH2, MSH6, PMS2). Defective DNA repair leads to increased mutation rates and faster cancer progression.
- Microsatellite instability (MSI): A laboratory finding in tumor tissue indicating a likely mismatch repair gene mutation. Used in risk assessment.
- Family History Assessment: Crucial for determining risk level. Factors considered:
- Low Risk: One affected relative >45 years; two relatives >55 years or on different sides of the family. Reassurance may be appropriate.
- Moderate Risk: One relative <45 years; two relatives (one <55 years); three affected at any age. tumor tissue analysis is warranted.< />pan>
- High Risk (Amsterdam Criteria): At least three individuals with bowel cancer across two generations; at least one affected individual <50 years; familial polyposis excluded.< />pan>
- Tumor Tissue Analysis (for moderate risk):
- MSI testing: Identifies potential mismatch repair gene mutations.
- Loss of expression of mismatch repair proteins: Helps determine which gene to analyze first.
- BRAF V600E mutation testing: Its presence suggests sporadic cancer (not germline mutation) and hypermethylation.
- Genetic Testing: Offered if mutation suspected (moderate or high risk). Guides surveillance strategies.
- Surveillance Strategies:
- Moderate risk: Colonoscopy at presentation and at age 55.
- High risk (known gene carriers): 2-yearly colonoscopies from age 25 or 35 (depending on family history).
- Additional Considerations: Families with known mismatch repair gene mutations should also consider surveillance for endometrial cancer (high risk) and other cancers (e.g., gastric cancer in those >50).
- Autosomal Dominant: One affected allele leads to the condition.
- Autosomal Recessive: Two affected alleles are required.
- Mismatch Repair Genes: Genes involved in DNA repair; mutations increase cancer risk.
- Microsatellite Instability (MSI): A marker of mismatch repair deficiency.
- Germline Mutation: A mutation present in all cells of the body, inherited from parents.
- Somatic Mutation: A mutation acquired during an individual's lifetime, not heritable.
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Oncology-Predictive Genetic Testing
I. Predictive Genetic Testing: The Basics
The focus on predictive testing often overlooks the implications for the affected individual whose genetic material initiates the testing process. These implications include:
I. Predictive Genetic Testing: The Basics
- Purpose: To determine if a family member has inherited a specific gene mutation known to increase cancer risk. This is offered after a mutation is identified in a family member and they consent to share their results.
- Process: Typically conducted through a clinical genetics service, preceded by genetic counseling to fully explain the implications.
- Result Interpretation:
- Positive Result: Does not guarantee cancer development, but informs surveillance needs and risk-reducing surgery decisions. It also clarifies risks for future generations.
- Negative Result: Does not guarantee against cancer development. It simply means the specific familial mutation wasn't found; a genetic predisposition may still exist.
- Considerations: Psychological distress for the individual being tested and their family should be anticipated and addressed. Potential insurance implications are also a concern, although guidelines exist to mitigate these in many regions.
The focus on predictive testing often overlooks the implications for the affected individual whose genetic material initiates the testing process. These implications include:
- Negative Test Results: A negative result (no mutation identified) does not rule out a genetic predisposition in the family. It simply means the specific mutation causing cancer in the family hasn't been found yet. More research may be needed to discover the underlying genetic cause.
- Uncertain Test Results: Sometimes a "sequence change" is found but its significance (benign or pathogenic) is unclear. Further testing is often required for clarification.
- Positive Test Results: A positive result can lead to:
- Psychological Distress: Including guilt over potentially passing the mutation to family members, especially children.
- Increased Cancer Risks: Identifying a specific mutation might reveal a heightened risk for other cancers (e.g., ovarian cancer with a BRCA1 breast cancer mutation) not initially apparent from the family history (metachronous cancers or cancers at associated sites).
- No guarantees: Neither positive nor negative predictive genetic testing results guarantee the development or absence of cancer.
- Psychological impact: The emotional consequences for the tested individual and their family must be carefully considered.
- Further investigation: Negative or unclear results may require additional testing or research.
- Broader implications: Testing results can significantly impact medical decisions, family planning, and insurance
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Oncology- Multistep Carcinogenesis
I. The Driver Mutation Concept:
II. Progression from Benign to Malignant:
I. The Driver Mutation Concept:
- Origin: The idea of "driver mutations" emerged from sequencing studies, but the observation of frequent specific mutations in certain cancers predates this by over 20 years. These mutations are not random; they play a crucial role in initiating and driving cancer development.
- Initiating Events: Certain mutations are highly prevalent in specific cancers, suggesting they are initiating events. Examples include:
- APC: Frequently mutated in colorectal cancer (78-80%). Leads to benign polyps.
- KRAS: Frequently mutated in pancreatic ductal adenocarcinomas (up to 95%). Leads to pre-malignant PanINs.
- BRAF: Frequently mutated in benign nevi (skin moles), precursors to melanoma.
II. Progression from Benign to Malignant:
- Second Hits and Beyond: Progression from benign lesions to malignant tumors requires the acquisition of additional mutations ("second hits" and subsequent hits). These are not random; they often act in response to the effects of the initiating oncogene.
- Example: Overcoming Senescence: KRAS and BRAF mutations often induce cellular senescence (growth arrest), preventing further progression. However, mutations in genes that counteract senescence, such as p16, PTEN, and p53, allow the tumor to bypass this growth arrest and progress to malignancy.
- Multistep Progression: The multistep model is best exemplified in colon cancer. Following the initiating APC mutation, further mutations frequently occur in:
- KRAS
- TP53
- Genes in the TGFβ pathway
- Genes in the PI3 kinase pathway
- Pathway Involvement: Most colon cancers show mutations in at least three of these five pathways. While other mutations exist outside these pathways, their roles in tumor progression are less well understood and require further research.
- Driver vs. Passenger Mutations: Driver mutations are functionally important for tumor initiation and progression. Passenger mutations have no significant effect.
- Multistep Nature of Carcinogenesis: Cancer development is a multi-step process requiring accumulation of multiple mutations over time.
- Context-Specific Mutations: The importance of a specific mutation depends on the context of other mutations present within the cell.
- Therapeutic Implications: Understanding the specific driver mutations in a cancer helps to select targeted therapies.
