The one idea
A beam is a divergent cone that the patient bends. Every tool in this module either reshapes the cone (jaws, MLC, wedges, compensators, bolus) or accounts for how the patient reshapes it (sloping skin, lung, bone), and every matching problem comes from the cone's divergence.
Module 07 worked on the central axis, in water, with a flat surface. Real plans break all three assumptions. Off axis, the dose depends on the beam profile and on the field edge. In a real patient, the surface slopes, and the beam crosses lung and bone. Where two fields meet, their edges diverge from two different sources and cannot share a plane unless you make them.
Keep two pictures in mind. First, the photon beam sets electrons in motion, and those electrons carry the dose sideways and forward for centimetres. That one fact explains penumbra, the lung problem, and why simple algorithms fail at interfaces. Second, field edges are straight lines from the source. That one fact explains every gap, collimator angle and couch kick.
When an isodose line does something unexpected, ask two questions. Where did the electrons go? Where did the field edge diverge to? One of those answers almost always explains it.
Beam profiles and penumbra
A beam profile is the dose measured across the field at a fixed depth, perpendicular to the central axis. Inside the field it is nearly flat; at the edge it falls steeply from about 100 to a few percent; outside it is a low tail from scatter and leakage. The region where the dose falls is the penumbra. Physicists quote it as the distance between the 80 and 20 percent points (sometimes 90 and 10), measured at a stated depth.
Three effects widen that edge, and each has a different cause.
| Component | Physical cause | What widens it |
|---|---|---|
| Geometric | The source has finite size, so points near the edge see part of the source and not the rest | Larger source, collimator farther from the patient, greater depth or SSD |
| Transmission | Photons pass through the thin corner of the jaw or leaf edge | Edges that do not follow beam divergence, such as rounded MLC leaf ends |
| Scatter | Secondary electrons and scattered photons carry dose across the edge | Higher energy (longer electron range), greater depth, low-density tissue such as lung |
Geometric penumbra
Draw two lines from opposite sides of the source past the collimator edge. They cross at the collimator and fan apart below it. The fan's width at the point of interest is the geometric penumbra. Similar triangles give its size: source width times how far the point is below the collimator, divided by how far the collimator is below the source.
Example: a Co-60 source 2 cm across, collimator at 55 cm, point at 10 cm depth with 80 cm SSD. P = 2 × (80 + 10 − 55)/55 = 1.27 cm. A linac focal spot is a few millimetres or less, so its geometric penumbra is several times smaller. That source size is why Co-60 teletherapy needed penumbra trimmers, and why the linac replaced it for sharp-edged fields.
Profile shape inside the field
A flattening filter is a metal cone, thickest on axis, that turns the forward-peaked bremsstrahlung beam into a flat one. It is designed to be flat at about 10 cm depth. Near the surface the profile therefore shows "horns", shoulders a few percent hot near the field edge. Off-axis photons also pass through less filter, so they are softer (lower mean energy) and attenuate faster with depth. Flatness and symmetry are defined over the central 80 percent of the field and are checked against baseline in routine QA (module 20).
Flattening-filter-free (FFF) beams skip the cone. The profile is peaked on axis, there are no horns, off-axis softening is small, and the head scatters less. For small fields, such as SBRT, the peak hardly matters, and the higher dose rate shortens treatment.
Penumbra widens with depth, source size, SSD and beam energy. Higher energy means longer-range secondary electrons, so 18 MV has a broader physical penumbra in water than 6 MV, and the difference grows in lung. Co-60 has the largest geometric penumbra because of its 1.5 to 2 cm source.
Placing the block or MLC edge exactly on the PTV. The 50 percent line sits at the field edge, and the 95 percent line sits several millimetres inside it. Dosimetrists add a block margin of roughly 5 to 7 mm beyond the PTV for 3D fields so the PTV sits inside the high-dose plateau.
Isodose curves
An isodose curve joins points of equal dose. For a single beam it looks like a set of nested cups: the central axis is the depth dose from module 07, and the sides are the beam profiles from section 02, stacked at every depth. Read them with three habits.
- Know the normalization. A single-field chart is usually normalized to 100 percent at dmax on the central axis (SSD charts) or at isocenter (SAD charts). A plan can be normalized to a point, to a volume, or to the maximum. The same distribution reads very differently depending on which.
- Follow the edges. Low-value lines (20 percent and below) bulge outward with depth because scatter grows; high-value lines (90 percent and above) pull inward near the edge.
- Look at the gradient. Closely spaced lines mean steep dose change: small setup errors there move dose a lot.
| Change | Effect on single-beam isodoses | Why |
|---|---|---|
| Higher energy | Lines move deeper; flatter interior; sharper skin sparing | Less attenuation, longer buildup |
| Larger field | Lines move deeper on axis; low lines bulge more | More phantom scatter |
| Extended SSD | Lines move slightly deeper; field wider at depth | Inverse square costs less over the same depth range |
| Oblique or curved surface | Lines tilt to follow the skin, partly | Missing or extra tissue above each point (section 06) |
| Lung in the path | Lines move deeper behind lung; penumbra widens inside it | Less attenuation; electrons travel farther in low density (section 07) |
Before reading coverage, find the normalization. "Plan normalized so 95 percent of the PTV gets 100 percent" and "normalized to isocenter" can show the same isodose picture with a different hot spot and a different dose to everything else. Module 10 covers prescribing and normalization traps.
Wedges and hinge angle
A wedge makes the beam deliver less dose on one side than the other, so the isodose lines tilt. The thin side (toe) lets more through; the thick side (heel) lets less through. The isodose lines tilt toward the heel: they are shallower under the heel and deeper under the toe.
The wedge angle is the angle through which an isodose line is tilted from perpendicular to the central axis, specified at a reference depth (10 cm in the IEC and ICRU convention; older systems used the 50 percent line). Standard angles are 15, 30, 45 and 60 degrees.
Why two beams need wedges
Two beams at an angle overlap most near the corner between them, on the near side, where both are at shallow depth. Without wedges that corner is hot. Wedges with their heels together (thick ends toward the corner) remove dose there. The hinge angle θ is the angle between the two central axes.
| Wedge type | How it works | Hand-calc factor | Side effects |
|---|---|---|---|
| Physical (hard) wedge | Metal wedge in a tray slot | Wedge factor, about 0.5 to 0.8 depending on angle; varies a little with depth and field size | Hardens the beam, so PDD rises slightly; adds head scatter and out-of-field dose; field size limited in the wedged direction |
| Universal (motorized) wedge | One internal 60° wedge; any angle made by mixing open and wedged dose | Factor of the weighted mix | Effective angle approximately tan θeff ≈ w × tan 60°, where w is the wedged fraction |
| Enhanced or virtual dynamic wedge | One jaw sweeps across the field during delivery | Depends strongly on the field size in the sweep direction | No hardening, no extra scatter, no therapist handling; wide fields possible |
Wedged pair: heels together. Hinge angle 90°, wedge 45°. A physical wedge hardens the beam; a dynamic wedge does not. Wedge factor goes in the denominator of the MU calculation and raises MU (module 07).
Breast tangents use wedges as compensators, not as a wedged pair. The breast thins toward the apex (nipple), so without wedges the apex gets too much dose. Heels toward the apex. Most clinics now use field-in-field segments for the same purpose, which avoids wedge scatter to the contralateral breast.
Combining beams
A single deep field overdoses the entrance region, as module 07 showed. Adding a second field from the opposite side gives a dose curve that is the sum of a falling curve and its mirror image. The sum is flat near midplane and rises toward each surface, peaking just under the skin at dmax.
How much it rises depends on how steeply each single-beam curve falls. A steeper fall (lower energy, thicker patient) leaves a bigger dip in the middle relative to the entrance regions. Physicists call the excess near the surface the tissue lateral effect.
| Increase this | Ratio of peripheral maximum to midplane dose | Why |
|---|---|---|
| Separation | Rises (worse) | Each beam has fallen further by midplane |
| Energy | Falls (better) | Flatter depth dose curves |
| SSD | Falls slightly (better) | Inverse square costs less across the patient |
| Field size | Small effect | Scatter raises both deep and shallow dose |
For a 25 cm pelvis treated AP/PA, 6 MV gives a peripheral maximum around 14 percent above midplane, and 18 MV around 6 percent (worked example 3). That is why pelvic boxes and thick patients get high energy, and why a four-field box spreads the remaining excess over four entrance regions rather than two.
Unequal weighting
Weight a beam more and the high-dose region shifts toward its entrance. Weighting is quoted either at isocenter (each field's dose at the point) or at dmax; know which convention your planning system uses before reading a 2:1 plan.
For parallel opposed fields, keep the peripheral excess below roughly 10 to 15 percent. Above that, go to higher energy, add fields, or accept a hot entrance on purpose.
Sloping skin, compensators and bolus
Isodose charts assume a flat surface perpendicular to the beam. When the skin slopes away, points under the low side have less tissue above them, so they get more dose than the chart predicts. The isodose lines move deeper there. They do not follow the skin all the way, because the point under missing tissue is still at the same distance from the source; only the attenuation changed.
Hand corrections for missing tissue
Three classical methods give the same direction and similar magnitude. The effective SSD method slides the chart to the actual skin and corrects inverse square. The TAR (or TMR) ratio method multiplies by TMR at the actual tissue depth over TMR at the chart depth. The isodose shift method moves each isodose line by a fraction k of the missing or excess tissue h.
| Beam energy | k (approx.) |
|---|---|
| Up to 1 MV | 0.8 |
| Co-60 to 5 MV | 0.7 |
| 5 to 15 MV | 0.6 |
| 15 to 30 MV | 0.5 |
Values from Khan and Gibbons. Lower energy, larger k, because the beam attenuates faster and missing tissue matters more.
Compensators and bolus
Two ways to fix a sloping surface. Bolus fills the missing tissue with tissue-equivalent material on the skin. It flattens the distribution and also removes skin sparing, because buildup now happens in the bolus. A compensator does the same job from inside the beam: a shaped attenuator in the tray, far from the skin, which preserves skin sparing. It need not be as thick as the missing tissue; with a dense material and a scatter-reducing thickness ratio (roughly 0.7) it is much thinner.
Choose by intent. If you want skin dose (mastectomy scar, superficial nodes, skin involvement), use bolus. If you want uniformity without skin dose, use a compensator, wedge, field-in-field or IMRT.
Obliquity raises surface dose and moves dmax shallower, because electrons from the oblique path reach the skin sooner. Tangential breast fields deliver substantial skin dose without bolus for this reason. Bolus placed on the skin removes skin sparing; a compensator in the tray keeps it.
Heterogeneity corrections
Planning uses electron density from CT (module 12), because Compton scattering dominates at megavoltage and its probability scales with electron density. Lung has a density around 0.2 to 0.3 g/cm³; cortical bone around 1.8. Two separate effects follow.
Primary attenuation. Beyond lung, the beam has crossed less mass, so dose is higher than a water calculation predicts. Beyond bone, the reverse. This effect is largest at low energy, where attenuation per centimetre is greatest.
Electron transport. In lung, secondary electrons travel three to five times farther. Inside a small field, more electrons leave the beam than return, so dose on axis falls (lateral disequilibrium). At the edge, penumbra widens. Where the beam enters a tumor from lung, the dose builds up again over the first few millimetres, a re-buildup region that can underdose the tumor periphery. These effects are largest at high energy and in small fields.
| Beam | Dose change beyond 1 cm of lung | Dose change beyond 1 cm of bone |
|---|---|---|
| Co-60 | ≈ +4% | ≈ −3.5% |
| 4 MV | ≈ +3% | ≈ −3% |
| 10 MV | ≈ +2% | ≈ −2% |
| 20 MV | ≈ +1% | ≈ −1% |
Approximate rules of thumb for points beyond the inhomogeneity, after Khan and Gibbons. They describe attenuation only, not the electron transport effects inside lung.
The effective depth correction
The simplest correction scales depth by density. A point under 2 cm of tissue, 8 cm of lung at density 0.25 and 2 cm of tissue sits at 12 cm physical depth but only 6 cm water-equivalent depth. The correction factor is the ratio of tissue ratios at the two depths.
Behind lung, dose rises; the correction is larger at lower energy. Inside lung, small high-energy fields lose dose on axis and gain penumbra. For lung tumors, 6 MV is generally preferred over 15 to 18 MV for this reason.
Dose algorithms
Every algorithm splits the problem the same way. First, the energy released by photon interactions at each point (TERMA). Second, how that energy is carried away and deposited by electrons and scattered photons (the kernel). The algorithms differ in how honestly they bend the kernel when density changes.
| Algorithm | How it handles heterogeneity | Accuracy in lung |
|---|---|---|
| Correction-based (effective depth, Batho, ETAR) | Scales depth along the ray; water distribution otherwise | Poor near interfaces and in small fields |
| Pencil beam convolution (type A) | Density scaling only along the beam direction; lateral spread computed as in water | Overestimates dose in and around lung tumors, especially small fields and high energy |
| Convolution/superposition, collapsed cone, AAA (type B) | Kernel scaled by density in all directions | Good; models lateral disequilibrium and penumbra broadening approximately |
| Grid-based Boltzmann solver (Acuros XB) | Solves the transport equation on a grid for each tissue's composition | Close to Monte Carlo; reports dose to medium or to water |
| Monte Carlo | Simulates individual photon and electron histories | Reference standard; result carries statistical noise that falls with more histories |
The type A and type B labels come from a comparison by Knöös and colleagues (2006): type A algorithms do not change lateral electron transport with density, type B algorithms do. AAPM TG-65 reviews inhomogeneity corrections; TG-105 covers Monte Carlo planning.
Dose to medium versus dose to water matters mostly in bone. Monte Carlo and Acuros naturally compute dose to the tissue itself; historical clinical experience and most protocols are in dose to water. In soft tissue the two differ by about a percent; in cortical bone, by roughly 10 percent or more. Know which one your plan reports.
RTOG 0236 (lung SBRT) prescribed 60 Gy in 3 fractions without heterogeneity correction. Recalculated with a type B algorithm, that dose corresponds to roughly 54 Gy in 3. Later trials such as RTOG 0813 and 0915 required heterogeneity correction. When you compare a plan with a historical regimen, ask how the trial calculated dose.
Trusting a pencil beam plan for a small lung lesion. It shows good PTV coverage that a type B or Monte Carlo recalculation reveals as underdosed at the tumor edge, because the lateral electron loss in lung was never modeled.
Field matching: the gap
Two fields placed edge to edge on the skin overlap at depth, because each edge diverges away from its own central axis. Below the skin, the overlap grows with depth and makes a hot triangle. Over the spinal cord that hot spot can mean myelopathy. The cure is to separate the fields on the skin by a gap so the edges (the 50 percent lines) meet at a chosen depth.
Each field edge moves outward by half the field length times depth over SSD, by similar triangles. The gap is the sum of the two movements.
Choose the match depth for the structure you care about. For a spine field matched to another posterior spine field, match at the depth of the cord or the anterior vertebral body, depending on whether you worry more about the cord or about underdosing the target. Gaps for typical spine fields come out around 1 to 2 cm.
Other ways to remove divergence
- Half-beam block. Close one jaw to the central axis. The edge at the axis does not diverge, so two half-blocked fields sharing an isocenter match on a plane at every depth.
- Angle the fields. Rotate the gantry, collimator or couch so one field's edge lies along the other's divergent edge (section 11).
- Feather the junction. Move the junction 0.5 to 1 cm every few fractions so any residual hot or cold spot is spread over several centimetres.
Gap formula uses half the field length of each field at the surface, the match depth and each SSD. If the fields abut on the skin, there is overlap at every depth below the surface. Orthogonal fields (lateral and posterior) are matched with a half-beam block or with a gantry angle equal to the divergence of the other field.
Junction bench
Two fields side by side, seen from the side. Set their lengths and SSDs and the depth where you want the edges to meet. The bench computes the skin gap. Then move the evaluation depth and watch the profile across the junction: cold above the match depth, hot below it. Try applying no gap at all to see the overlap you get from abutting fields on the skin.
Junction bench
Teaching model, not for clinical useLengths are measured along the junction direction at the skin. Each field is normalized to 100 percent inside its own field at the evaluation depth, with a model penumbra that widens with depth.
The side view stretches the horizontal axis so the divergence is visible; real edges diverge at only a few degrees.
Craniospinal and breast matching
Craniospinal irradiation
The classic 3D technique uses opposed lateral cranial fields and one or two posterior spine fields, with the patient prone or supine. Two divergences have to be removed at the cranial and spinal junction.
The spine field's superior edge diverges upward into the cranial fields. Rotate the collimator of each lateral cranial field so its inferior border runs parallel to that divergent edge. The angle is the spine field's half-length divergence.
The cranial fields' inferior edge diverges toward the spine field, in the lateral direction. Rotate the couch (a couch kick) so that edge becomes perpendicular to the spine, or use a half-beam block on the cranial fields so their inferior edge does not diverge at all.
The junction is then feathered, typically by moving it 0.5 to 1 cm every few fractions. Modern practice increasingly uses VMAT or helical tomotherapy with gradient-matched junctions, or proton therapy, which makes the junction less sensitive to small setup errors. The board still tests the 3D geometry.
Breast tangents and supraclavicular field
The same problem in a different place. The supraclavicular field diverges downward into the tangents; the tangents diverge upward into the supraclavicular field. Common solutions:
- Half-beam block the supraclavicular field inferiorly, so its lower edge does not diverge.
- Kick the couch for each tangent by arctan(Ltangent/2 ÷ SAD), so the tangents' superior edge becomes vertical.
- Single-isocenter technique: isocenter on the match plane, the supraclavicular field blocks its lower half and the tangents block their upper half. No divergence at the plane, no couch kick.
Mixing up which field gets which rotation in craniospinal. The collimator rotation goes on the cranial fields, and its size comes from the spine field's length. The couch kick also goes with the cranial fields, and its size comes from the cranial field's own length.
Worked examples
Work each one on paper first. The numbers are given in the problem, as they are on the exam.
Geometric penumbra: cobalt versus linac
(a) Co-60, source diameter 2 cm, collimator 55 cm from the source, 80 cm SSD. (b) Linac, focal spot 0.2 cm, lower jaw 40 cm from the source, 100 cm SSD. Find the geometric penumbra at 10 cm depth for each.
Show solution
Co-60 ≈ 1.3 cm; linac ≈ 0.35 cm
Sanity check: the cobalt answer should be several times the linac answer because its source is ten times larger. The common error is dividing by SSD instead of SDD, which gives about 0.9 cm for cobalt.
Choosing wedges
(a) A maxillary sinus is treated with an anterior and a lateral field at 90° to each other. (b) A parotid bed is treated with anterior and posterior oblique fields 120° apart. Which wedge angle for each pair, and which way do the heels point?
Show solution
45° wedges and 30° wedges; heels together, toward the hinge corner
Sanity check: a wider hinge angle needs a smaller wedge, and two opposed beams (180°) need none: 90 − 90 = 0. If your formula gives a larger wedge for a wider hinge, it is inverted.
Opposed fields: is 6 MV good enough?
A pelvis 25 cm thick is treated AP/PA at 100 cm SSD on each side, 100 cGy from each field at midplane (12.5 cm). 6 MV: PDD(12.5) = 61%, PDD(23.5) = 39%, dmax 1.5 cm. 18 MV: PDD(12.5) = 73%, PDD(22) = 55%, dmax 3 cm. Find the dose at the anterior dmax point for each energy, relative to midplane.
Show solution
The anterior dmax point gets the AP field's dmax dose plus the PA field's dose at depth 25 − dmax. Each field's dmax dose is 100 ÷ PDD(12.5).
6 MV ≈ 1.14 × midplane; 18 MV ≈ 1.06 × midplane
Sanity check: the ratio must exceed 1 and must be smaller at higher energy. Forgetting the opposing field's contribution gives 164 cGy (0.82), less than midplane, which is impossible for opposed fields at this separation.
Correcting for lung
6 MV, 10 x 10 field, isocentric. The calculation point is 12 cm deep: 2 cm chest wall, 8 cm lung of relative density 0.25, then 2 cm of tumor and tissue. TMR(6 cm) = 0.884, TMR(12 cm) = 0.749. Find the heterogeneity correction factor by the effective depth method.
Show solution
CF ≈ 1.18: the point receives about 18% more than a water calculation predicts
Sanity check: the rule of thumb of 2 to 3 percent per centimetre of lung at 6 MV gives 16 to 24 percent for 8 cm. The common error is inverting the ratio (0.85), which would mean lung shields the tumor. Remember this simple method ignores the lateral electron loss inside lung.
The gap between two spine fields
Two adjacent posterior spine fields, both at 100 cm SSD. The upper field is 20 cm long and the lower field 30 cm long at the skin. Match the edges at 5 cm depth, the depth of the cord. What skin gap?
Show solution
≈ 1.25 cm
Sanity check: spine gaps are usually 1 to 2 cm. Using full field lengths instead of half gives 2.5 cm and a cold cord; ignoring the gap gives a hot cord. Above 5 cm depth the junction is cold, below it hot.
Craniospinal angles
Craniospinal irradiation. The spine field is 36 cm long at the skin at 100 cm SSD. The lateral cranial fields are 20 cm long along the patient's axis, treated isocentrically at 100 cm SAD. Find the collimator rotation and the couch kick for the cranial fields.
Show solution
Collimator ≈ 10°; couch ≈ 6°
Sanity check: both angles are small, under about 15°. Swapping the lengths (using the cranial length for the collimator angle) gives the wrong pair, 5.7° and 10.2°, and is the most common error.
In the planning room
You will sign plans where the physics in this module is already done by software. Your job is to know where software can mislead you and what to ask.
Ballpark numbers to carry in your head
| Situation | Rough number | What it tells you |
|---|---|---|
| 95% line inside a 3D field edge | several mm | Why block margins exist beyond the PTV |
| Wedge for a 90° hinge | 45° | 90 − θ/2 |
| Dose behind 1 cm of lung, 6 MV | ≈ +2 to 3% | Large in a mediastinal or lung plan with lots of lung in the path |
| Spine field gap at cord depth | ≈ 1 to 2 cm | Larger means longer fields or deeper match |
| CSI collimator and couch angles | ≈ 10° and 5 to 6° | Larger angles suggest long fields or an error |
| Junction feathering | 0.5 to 1 cm shifts | Spreads residual hot and cold spots |
| Peripheral excess, 6 MV opposed, 25 cm | ≈ 14% | Time to consider higher energy or more fields |
| Say this to physics or dosimetry | Why it lands |
|---|---|
| "Which algorithm is this calculated with, and is it dose to water or dose to medium?" | Shows you know a lung or bone plan can differ by several percent between algorithms and reporting modes |
| "This is a small lung lesion. Can we keep 6 MV rather than 10 or 15?" | Higher energy widens penumbra and deepens the re-buildup in lung |
| "What depth did we match the junction at, and are we feathering?" | Ties the gap to the cord depth and the plan for residual hot spots |
| "Is the bolus meant to bring dose to the skin here, or was it added for uniformity?" | Separates the two reasons for bolus and the skin reaction you should expect |
| "The hot spot is at the apex of the breast. Can we add field-in-field segments or change the wedge?" | Names the missing-tissue cause and the two standard fixes |
Tumor regression and weight loss change the matching geometry too. A craniospinal junction set at the cord depth on simulation is still set there if the patient loses weight, but the cord is now shallower relative to the skin. Repeat imaging and a check of the junction belong in the weekly review.
Board pearls and traps
Geometric = s(SSD + d − SDD)/SDD.
And in lung, most at high energy.
Horns at shallow depth; softer beam off axis. FFF: no horns.
Heels together for a wedged pair.
Dynamic wedge does not.
Missing tissue moves lines deeper.
About +4%/cm at Co-60, falling to +1%/cm at 20 MV.
Pencil beam does not bend lateral transport; collapsed cone and AAA do.
Differs from dose to water mostly in bone.
Cold above the match depth, hot below.
Applied to the cranial fields.
Or half-beam block the cranial fields.
Both lower the peripheral excess.
Obliquity alone raises surface dose.
- Using full field lengths in the gap formula. It is half of each.
- Shifting isodose lines toward the source under missing tissue. They move away.
- Expecting lung correction to matter more at high energy. Attenuation correction is larger at low energy; electron transport problems are larger at high energy.
- Reading a wedged plan as if the heels point away from the hot corner. Heels go toward it.
- Comparing a modern plan's dose with an uncorrected historical prescription without asking how it was calculated.
- Thinking a gap at the skin means no cold spot. The skin and superficial tissue above the match depth are underdosed by design.
Question bank
Exam-style single best answer. Pick one to see the explanation.
Formula card and sources
Sources
- American Board of Radiology. Study Guide for Medical Physics for Radiation Oncology, Qualifying Exam (2023 domain ranges, verified 2022, still the posted version).
- Khan FM, Gibbons JP. The Physics of Radiation Therapy, 5th ed. Chapters on dose distribution and scatter analysis, treatment planning (isodose distributions, patient data and corrections, field separation).
- Ford E. Primer on Radiation Oncology Physics. CRC Press, 2020.
- Papanikolaou N et al. Tissue inhomogeneity corrections for megavoltage photon beams: AAPM TG-65. AAPM Report 85, 2004.
- Chetty IJ et al. Report of the AAPM TG-105: issues associated with clinical implementation of Monte Carlo-based photon and electron external beam treatment planning. Med Phys 2007;34:4818.
- Knöös T et al. Comparison of dose calculation algorithms for treatment planning in external photon beam therapy for clinical situations. Phys Med Biol 2006;51:5785.
- Timmerman R et al. Stereotactic body radiation therapy for inoperable early stage lung cancer (RTOG 0236). JAMA 2010;303:1070.
The junction bench uses an idealized edge model with a depth-dependent penumbra. It teaches geometry and direction. Clinical junctions are planned and verified by physics on your machine.