- Three important areas form the foundation for the evolving use of altered fractionation:
- Tissue response
- Duration of treatment
- Fraction size and number
- Acutely responding tissues:
- Are rather active in ongoing cellular proliferation
- Most tumors (except perhaps prostate cancer, breast cancers, and melanoma) and some normal tissues such as skin, mucous membranes, and gastrointestinal epithelium:
- Share this characteristic:
- These tissues are most affected by the overall treatment duration rather than by the size or number of fractions used
- Share this characteristic:
- Late-responding tissues:
- Have a low proliferative rate and include the spinal cord, brain, bone, and cartilage
- These tissues are most affected by the:
- Size and number of fractions rather than by treatment duration:
- Therefore are spared by decreasing the dose per fraction of radiation delivered
- Size and number of fractions rather than by treatment duration:
- Because most tumors consist of rapidly dividing cells:
- Local tumor control is strongly dependent on the overall treatment duration rather than on the size or number of fractions
- When squamous cell carcinoma of the head and neck is exposed to radiation:
- The less radiosensitive cells within the lesion:
- Can undergo rapid proliferation:
- Approximately 3 to 5 weeks after treatment commences
- This accelerated repopulation can overwhelm the ongoing treatment effects of radiation:
- Which ultimately can lead to local failure
- The clinical significance of this phenomenon is that even with significant regression of the primary tumor mass:
- Local failure still ultimately could result from proliferation of these resistant clones
- Therefore it is essential to complete treatment in as short a time as possible so that accelerated repopulation is minimized:
- Increasing the chance for local control
- For this reason, split-course radiation:
- Which incorporates a treatment break during the course of radiotherapy is not recommended
- Can undergo rapid proliferation:
- The less radiosensitive cells within the lesion:
- Based on the aforementioned principles:
- The goal of altered fractionation schemes:
- Is to improve the therapeutic ratio by maximizing the tumoricidal effect and minimizing acute and late toxicities while using readily available low-LET radiation
- The goal of altered fractionation schemes:
- Two major categories of altered fractionation schemes exist:
- Hyperfractionation
- Accelerated fractionation
- They share basic radiobiological principles yet have their own particular features (Table)
- Accelerated fractionation:
- Is the strategy of choice for rapidly proliferative tumors
- Accelerated fractionation is based on the concept that the shortened overall treatment time:
- Would reduce the opportunity for accelerated repopulation effectively
- Hyperfractionation:
- Is preferred for slowly proliferating tumors
- Hyperfractionation improves the therapeutic ratio primarily through:
- Redistribution of tumor cells into more radiosensitive phases as a result of multiple fractions
- Differential sparing of late-responding normal tissues because of a decrease in the size of the dose per fraction




