27.3 Isodose Planning, Treatment Delivery Methods & Radiobiologic Principles in Radiation Therapy
Key Takeaways
- An isodose curve joins points receiving the same dose, expressed as a percentage of a reference dose, and an isodose plan is the two- or three-dimensional map used to judge target coverage and organ-at-risk sparing.
- Percentage depth dose rises to a maximum at the depth of dose maximum then falls with depth, and the depth of dose maximum increases with beam energy, which produces the skin-sparing effect of megavoltage beams.
- Parallel-opposed, wedged-pair, three-field and multi-field arrangements are chosen to shape the high-dose volume around the target while keeping organs at risk below their tolerance doses.
- The four Rs of radiobiology that justify fractionation are repair of sublethal damage, redistribution within the cell cycle, repopulation, and reoxygenation of hypoxic tumour cells.
- Conventional fractionation delivers about 1.8 to 2 Gy per fraction once daily on weekdays, which exploits the greater repair capacity of late-responding normal tissue relative to tumour.
27.3 Isodose Planning, Treatment Delivery Methods & Radiobiologic Principles in Radiation Therapy
The Enhanced TOS lists Principles of Isodose Planning and Radiobiologic Principles in Radiation Therapy as its own sub-topic worth 2 items, with the competency to distinguish the procedures of isodose planning, methods of treatment, and various dose theories. Section 27.1 covered linear accelerator hardware and the treatment-planning workflow, and Section 27.2 covered teletherapy versus brachytherapy, immobilisation and side effects. This section supplies the dosimetric and radiobiological reasoning that connects them.
1. Isodose Curves and Isodose Plans
An isodose curve is a line joining all points in a phantom or patient that receive the same absorbed dose, normally expressed as a percentage of a stated reference dose — most often the dose at the depth of maximum dose on the central axis, or the prescribed dose at the isocentre.
An isodose chart is the family of such curves for one beam of stated energy, field size and source-to-surface distance. An isodose plan is the composite of all beams in the treatment, displayed on the planning CT.
What a planner reads off a plan:
- Target coverage — the prescription isodose (commonly 95%) should encompass the planning target volume.
- Homogeneity — how uniform the dose is within the target; conventionally the aim is roughly within 95% to 107% of the prescription.
- Conformity — how tightly the high-dose region hugs the target rather than spilling into normal tissue.
- Organ-at-risk doses — read as maximum point dose, or as the volume receiving a stated dose, and compared with published tolerance constraints.
- Hot spots — regions above the prescription, particularly outside the target, which are minimised by wedges, field weighting and beam-angle selection.
Dose-volume histogram. The modern companion to the isodose display, plotting for each structure the fraction of its volume receiving at least a given dose. It compresses a three-dimensional plan into a curve per structure, so target coverage and organ sparing can be compared between competing plans at a glance.
Characteristics of the isodose distribution
| Feature | Behaviour |
|---|---|
| Central axis | Highest dose at a given depth |
| Penumbra | The rapid dose fall-off region at the field edge, widened by a larger source size and by increased distance from the collimator |
| Field size | Larger fields deliver greater dose at depth for the same surface dose, because of increased scatter |
| Energy | Higher energy shifts the curves deeper and sharpens the fall-off beyond the target |
| Tissue inhomogeneity | Lung transmits more, so isodose lines bulge deeper through lung; bone attenuates more |
| Surface obliquity | An angled surface tilts the isodose curves; a wedge or bolus compensates |
2. Percentage Depth Dose and the Depth of Dose Maximum
Percentage depth dose (PDD) is the dose at a given depth expressed as a percentage of the dose at the depth of maximum dose, on the central axis, for a stated field size and distance.
The curve rises steeply from the surface to the depth of dose maximum (d-max), then falls approximately exponentially with depth.
| Beam | Approximate depth of dose maximum |
|---|---|
| Superficial / orthovoltage kilovoltage | At or very near the surface |
| Cobalt-60 (1.25 MeV average) | About 0.5 cm |
| 6 MV linear accelerator | About 1.5 cm |
| 10 MV | About 2.5 cm |
| 18 MV | About 3.0-3.5 cm |
Skin sparing is the direct consequence: because the dose builds up over the first centimetres as secondary electrons come into equilibrium, a megavoltage beam deposits far less dose in the skin than at depth. This is the single greatest advantage of megavoltage over orthovoltage therapy, and it is lost when a bolus is deliberately placed on the skin to bring d-max to the surface for a superficial target such as a chest-wall recurrence.
Percentage depth dose increases with higher beam energy, larger field size (more scatter) and greater source-to-surface distance, and decreases with greater depth.
3. Beam Modifiers
| Modifier | Function |
|---|---|
| Wedge (physical, motorised or dynamic) | Progressively attenuates one side of the beam, tilting the isodose curves; used to compensate for an oblique surface and to combine two beams at an angle without a hot spot in the overlap |
| Bolus | Tissue-equivalent material on the skin; removes skin sparing where surface dose is wanted, and compensates for surface irregularity |
| Compensator | A shaped attenuator away from the skin that corrects for missing tissue while preserving skin sparing |
| Blocks / multileaf collimator (MLC) | Shape the field to the target outline and shield organs at risk |
| Flattening filter | Produces a uniform beam profile across the field; flattening-filter-free modes deliver higher dose rates for stereotactic work |
4. Beam Arrangements and Treatment Methods
| Arrangement | Description | Typical use |
|---|---|---|
| Single field | One beam | Superficial targets, some palliative treatments |
| Parallel-opposed pair | Two beams 180 degrees apart | Simple, homogeneous through the midplane; whole brain, some palliative spine and bone treatments |
| Wedged pair | Two beams at an angle with wedges | Superficial and lateralised targets such as some head and neck sites |
| Three-field / four-field box | Anterior, posterior and lateral fields | Pelvic targets; the box confines a high-dose region around the target while sparing anterior and posterior tissue |
| Multi-field conformal (3D-CRT) | Several shaped beams from CT-based planning | The general workhorse |
| Intensity-modulated radiotherapy (IMRT) | Beam intensity varied across each field by moving multileaf collimators | Concave targets wrapping around an organ at risk, such as prostate adjacent to rectum or head and neck adjacent to parotid |
| Volumetric modulated arc therapy (VMAT) | Continuous gantry rotation with simultaneously varying dose rate, leaf position and gantry speed | Fast, highly conformal delivery |
| Stereotactic radiosurgery / body radiotherapy (SRS / SBRT) | Very high dose in one or a few fractions, with steep gradients and rigorous immobilisation and image guidance | Small brain lesions, early lung tumours, oligometastases |
| Brachytherapy | Sealed sources placed within or adjacent to the target — intracavitary, interstitial, intraluminal or surface | Cervix, prostate, oesophagus, skin |
Image-guided radiotherapy (IGRT) — cone-beam CT, planar imaging or surface guidance at the machine before each fraction — verifies position and is what makes tight planning margins safe.
Isocentric (SAD) technique places the isocentre at the target centre so the gantry can rotate around the patient without repositioning, and has largely displaced the older fixed source-to-surface distance (SSD) technique.
5. Radiobiologic Principles and Dose Theories
The therapeutic ratio
The whole enterprise rests on the ratio between tumour control probability and normal tissue complication probability. Everything in planning and fractionation is an attempt to widen the gap between the two curves.
The four Rs of radiobiology — why treatment is fractionated
| R | Mechanism | Effect |
|---|---|---|
| Repair of sublethal damage | Normal tissue, especially late-responding tissue, repairs sublethal damage between fractions more effectively than tumour | Favours normal tissue — the principal justification for fractionation |
| Redistribution (reassortment) | Cells surviving a fraction redistribute through the cell cycle into more radiosensitive phases | Favours tumour kill; cells are most sensitive in G2 and M, most resistant in late S |
| Repopulation | Surviving clonogens proliferate between fractions | Favours tumour if treatment is prolonged, which is why gaps and unplanned interruptions are avoided |
| Reoxygenation | Hypoxic tumour cells become oxygenated as the tumour shrinks | Favours tumour kill; the oxygen enhancement ratio for x-rays is roughly 2.5-3, so oxygenated cells are far more radiosensitive |
A fifth R, intrinsic radiosensitivity, is often added to explain why tumours of the same histology respond differently.
The law of Bergonie and Tribondeau
Cells are most radiosensitive when they are highly mitotic, undifferentiated, and have a long mitotic future. Applied to therapy, this is why rapidly dividing tumours and rapidly renewing normal tissues — bone marrow, intestinal mucosa, skin basal layer, gonads — respond and react earliest.
Early versus late responding tissue
| Early (acute) responding | Late responding | |
|---|---|---|
| Tissues | Skin, mucosa, bone marrow, intestinal epithelium | Spinal cord, lung, kidney, subcutaneous fibrosis, blood vessels |
| Timing of reaction | During and shortly after treatment | Months to years later |
| Sensitivity to fraction size | Relatively insensitive | Highly sensitive — large fractions disproportionately damage late-responding tissue |
| Reversibility | Usually heals | Often permanent |
This distinction is the reason conventional fractionation uses about 1.8 to 2 Gy per fraction, once daily, five days a week: small fractions spare late-responding normal tissue, while the cumulative dose still controls the tumour.
Altered fractionation schedules
| Schedule | Pattern | Rationale |
|---|---|---|
| Conventional | About 1.8-2 Gy per fraction, once daily, 5 days a week | Standard balance |
| Hyperfractionation | Smaller fractions, more than once daily, similar overall time, higher total dose | Further spares late-responding tissue while escalating tumour dose |
| Accelerated fractionation | Standard or slightly reduced fraction size, shorter overall time | Counters repopulation in rapidly proliferating tumours |
| Hypofractionation | Larger fractions, fewer of them | Shorter courses; used in palliation and, with modern conformality and image guidance, in breast, prostate and stereotactic treatments |
| Split course | A planned gap mid-treatment | Improves tolerance, but the gap permits repopulation and is generally avoided |
Dose concepts
- Absorbed dose is measured in gray (Gy), one joule per kilogram. Radiation therapy is prescribed in gray, not sievert.
- Fraction size, number of fractions and overall treatment time together determine biological effect; total dose alone does not.
- The linear-quadratic model describes cell survival with an alpha component (single-hit, non-repairable) and a beta component (multi-hit, repairable), and the alpha/beta ratio characterises tissue response: roughly 10 Gy for early-responding tissue and most tumours, roughly 3 Gy for late-responding tissue. The lower the alpha/beta ratio, the more sensitive that tissue is to increased fraction size — the quantitative statement of the early-versus-late distinction above.
- Tolerance doses for organs at risk are the planning constraints, conventionally expressed as the dose associated with a stated complication probability at a stated interval.
Radiosensitisers such as concurrent chemotherapy and oxygen increase tumour cell kill; radioprotectors aim to shield normal tissue. Hypoxia is the classic cause of radioresistance, which is why reoxygenation across a fractionated course matters so much.
What does an isodose curve represent on a radiotherapy plan?
A 6 MV photon beam has its depth of dose maximum at approximately 1.5 cm rather than at the skin surface. What is the clinical consequence, and how is it deliberately removed when required?
Which of the four Rs of radiobiology most directly explains why prolonging a course of radiotherapy through unplanned treatment gaps reduces tumour control?
Why does conventional external-beam radiotherapy deliver about 1.8 to 2 Gy per fraction rather than fewer, larger fractions?