general oral questions Flashcards
- What are typical 3D conformal beam arrangements for treating early stage prostate cancer?
o 4-field box (AP, PA, LT, RT)
Anterior beam weight may be increased in order to decrease dose to the rectum (at the expense of potentially increased bladder dose) – in this case, wedges on the lateral beams may be used to compensate for the resulting gradient (thick part toward anterior side).
Typical weighting (without wedges) is AP:PA:RT:LT = 20:20:30:30 with lateral fields given more weight because they don’t go through rectum or bladder.
o 6-field box (LT, RT, LAO, LPO, RAO, RPO)
May provide better OAR sparing (bladder and rectum in particular since no AP/PA fields are used), and a more conformal dose distribution because there are more beams
LT and RT may be given more weight relative to the oblique fields because they avoid the rectum and bladder
- List TG-142 QA tests where constraints for SRS/SBRT are more stringent from conventional.
o Daily:
Laser localization: 2, 1.5, 1 mm
Collimator size indicator: 2, 2, 1 mm
o Monthly:
Couch position indicators: 2 mm/1º, 2 mm/1º, 1 mm/0.5º
Lasers: 2, 1, <1 mm
o Annual:
Coincidence of mechanical and radiation isocenters: 2, 2, 1 mm from baseline
o Daily imaging:
Positioning/repositioning for planar kV, EPID MV: 2, 2, 1 mm
Imaging and Tx coordinate coincidence for all: 2, 2, 1 mm
o Monthly imaging:
Imaging and Tx coordinate coincidence for planar kV, EPID MV: 2, 2, 1 mm
Planar kV scaling accuracy: 2, 2, 1 mm
CBCT geometric distortion: 2, 2, 1 MM
- Water phantom with lung slab. What is CAX depth dose for LBTE solver (e.g., Acuros) vs convolution-superposition technique (e.g., AAA).
Acuros does model secondary electron transport while AAA does not. In low density lung, expect more outscatter in the lung slab.
- Explain trends in CAX dose distribution for slab phantom with water, bone, lung and water slabs
Dose in water enhanced upstream of bone due to increased backscatter from higher density bone. Similarly, dose in lung downstream of bone enhanced due to increased secondary electron fluence from higher density bone. Dose in water after lung initially low due to buildup of secondary electron fluence occurring in the higher density water (lack of TCPE initially).
o How would depth dose change if photon energy increased? Same trends as described above, but more pronounced due to secondary electrons having longer ranges.
o How would depth dose change if field size decreased? Same trends as described above, but more pronounced due to changes in lateral scatter having more of an effect on CAX dose (e.g., less scatter toward CAX in lower density medium; more out scatter; results in dose reduction)
- What is the typical prescription point for each of superficial x-rays, MV x-rays and electrons?
o Electrons: R90
o MV x-rays: near centre of target/on CAX/typically at iso, not in high dose gradient, not near tissue boundary
o Superficial x-rays: patient surface
- Sketch open isodose profiles for 200 kV x-rays, 10 MV x-rays and 9 MeV electrons. What features should be considered in designing a treatment plan?
o Can achieve sharper lateral penumbra with kV photons with cut-out located on patient skin compared to electrons with cut-out on tray on treatment head. However, electron PDD falls off more quickly; better depth sparing.
o For larger fields, electron beam profiles more uniform than KV photons
o Lower energy photon beams have larger penumbra, more bulging out of low value isodose curves (low energy photons result in more lateral scatter). Bulging is due to electrons scattering out of the beam.
- A patient presents with basal cell carcinoma on the tip of his nose. What are treatment options? Pros and cons of these processes?
o Options: orthovoltage and electrons. See pros and cons above.
o If you only have electrons, how would you setup and treat this patient?
Lateral POPs to avoid having beams pointed toward patient’s face. Consider shielding eyes from scatter.
- What can we do to mitigate issue of backscatter from lead shield placed in contact with patient?
o Examples: in nostril for nose treatment, behind ear for ear treatment.
o Cover with wax to absorb electron scatter (photoelectric dominant).
- How to measure HVL (TG-61 beam quality specifier)?
o Use diaphragm to create narrow beam geometry with beam diameter no larger than 4 cm.
Purpose of narrow beam geometry is to minimize scattering from the attenuator.
o Monitor chamber upstream from diaphragm and out of path of photons that will go through the diaphragm
o Diaphram 50 cm from source; detector 100 cm from source. Don’t want scatter photons from attenuator to reach detector. Ion chamber should have air equivalent walls
- What happens to the dose profile across the radiation field at the level of the prescription point when the field size increases from 2x2 cm2 to 10x10 cm2 for 100 kV vs 6 MV x-rays. Explain.
o MV photons: the depth dmax increases with increasing FS below 5x5 due to establishing lateral equilibrium; dmax decreases with increasing FS above 5x5 due to increasing scatter resulting in buildup happening sooner
o Larger FS: beams don’t fall off as quickly due to more scatter contributions reaching CAX
o As field size decreases, the penumbra represents a larger proportion of the field.
- Why might you be reluctant to use electron beam therapy near the eye?
o Electron beams have larger lateral penumbra
- Compare the two lung plans shown below.
o In both cases the same prescription, beam geometry and weighting were used. What do you conclude regarding plans A and B? Inhomogeneity correction is on in A and is off for B. Can tell because in A, the dose is higher near mid-separation, especially where the beam traverses more lung tissue, as expected since there is less attenuation in lung. Also, the isodose lines near the field edge bulge out a bit more in A, as you would expect in low density lung.
o What would be the appropriate energy to treat this lung case? 6 or 10 MV photons
o Why would you not use a higher energy like 15MV? Because with higher photon energies, secondary electron ranges are longer so interface effects (e.g., buildup in higher density tumour after travelling through lung tissue) are more pronounced – can end up with unwanted lower dose on periphery of tumour as a result.
- What is the typical prescription for a prostate cancer treatment which involves permanent implant? Which isotope is commonly used? LDR monotherapy for low risk/early stage prostate cancer: 144 Gy to the prostate with I-125 or 125 Gy with Pd-103.
o Give an example of a typical prescription for an external-beam RT prostate cancer treatment.
71.4/28 to prostate + 50.4/28 to nodes or 60,44/20 hypofractionation
* 76/38 and 78/39 are more historically traditional
66/33 to prostate bed + 52/33 nodes
Also, can use HDR to reduce number of EBRT fractions required: 37.5/15 EBRT + 15 Gy HDR or 46/23 + 15 Gy HDR.
o Draw and compare the Prostate DVH for the external beam treatment and the Permanent Implant. Showing cumulative DVHs:
- Describe the construction of an internal lip shield for electron therapy.
o Energy of beam determines thickness required. Range [cm] of electron in water ~ energy [MeV] / 2. Pb density is 11.34 g/cc. Can divide by this (or divide by 10 as rough approx.) to get Pb thickness required.
o Cover with wax to absorb low energy scatter off lead.
o Also want to cover with something because lead is toxic and this will go in someone’s mouth.
- What contributes to the potential overdose or underdose of tissue in the following
o Photon/electron match - electron lower value isodose curve bulging out, which is especially pronounced at deeper depths (also larger SSD, higher energies, smaller FS) resulting in hotspot deeper and cold spot shallower. Consider oblique incidence of electron beam to reduce over/underdosing
o Photon/photon match - must pay attention for beam divergence. Based on target depth, locations of OARs, must decide on surface gap/where junction will be.
- Whole brain RT is a common CNS irradiation technique. When is WBRT used? Describe a technique for WBRT (field borders, isocentre placement). Which primary sites tend to metastasize to the brain?
o Common Rx: 30/10
o Use PCI = prophylactic cranial irradiation when there is high probability of metastases in the brain. Also use WBRT when there are too many to target individually (palliative).
PCI standard for small cell lung cancer primaries. Breast cancer can also spread to brain
o Entails treatment of brain and brainstem with uniform dose of radiation administered with opposed lateral fields with blocks or MLC shaping to shield eyes. Flash ant, post, sup
PCI: inf border below C1 or C2 vertebra including temporal lobe posteriorly and 0.5 cm below cribiform plate anteriorly (helmet). Want to include all places where CSF circulates
For WBRT non-PCI, inf border is cranial base/foramen magnum.
Most likely to fail at cribiform plate, which is often underdosed to spare eyes. Cribiform plate is bone layer between anterior part of brain and nasal cavity
Choose colli angle to optimize MLC shielding
Isocentre at field border near eye to minimize divergence into eye.
- Evaluate the following Rt Breast treatment plan and determine if it is appropriate for treatment. How would you modify this plan to achieve your clinical goals for this patient?
- Not valid for treatment
o Hot spot of >120% - consider FiF technique or wedges to achieve intensity modulation.
o Normalization point appears to be at lung tissue interface. Move it closer to the center of the breast to reduce lung dose (also having norm point near tissue interface where dose uncertainty is high is undesirable since this results in uncertainty across the entire dose distribution.
o Consider different gantry angles or isocentre location to reduce dose to contralateral breast.
- Typical beam arrangements (breast, lung, prostate, H&N IMRT)
o Breast: POP oblique tangents (medial and lateral). Plus AP/PA POP 4-field block if SCN treated as well. Plus anterior oblique electron field to treat IMN.
o Lung: 5-field: Ant, post, ant oblique, post oblique, lateral
o Prostate: 4-field box, 6-field (LT, RT and 4 obliques)
- A patient is being treated for 5000 cGy in 25 fractions with a single wedged field. After 2 fractions, it is discovered that the physical wedge was forgotten. What do you advise?
o Typical hard wedge factors: for 6 MV, 10x10 cm2 field range from 0.7 to 0.4 for 15-60˚ wedges. So MUs would be 1.7-3.3 too high. These wedge factors are closer to unity (they increase) with increasing FS (e.g., from 0.53 to 0.60 for ESQ from 5 cm to 50 cm). They also increase with increasing energy.
- What do you do if MLC positions are not downloaded for 1 IMRT field out of 7 for 1 fraction?
o Similar to above cases, recalculate plan with this open field instead of MLC defined field. Use plan sum feature in TPS to determine overall effect assuming rest of the fractions are delivered properly. Discuss dosimetric effect with RO. Consider changing field weight on one or more of the remaining fractions to compensate.
o Magnitude of effect depends on how much weight is given to this field (how many MU).
- What organs at risk would you be concerned about for a radical Nasopharynx treatment?
o Nasopharynx is the most superior part of the pharynx.
o Most relevant OARs:
Larynx (located anterior to pharynx; Dmean < 44 Gy),
Esophagus (located inferior to pharynx; Dmean < 34 Gy)
Oral cavity
parotids (Dmean < 25 Gy for combined glands or Dmean < 20 Gy is only one gland spared)
Cochlea (Dmean < 45 Gy)
Optic nerves/chiasm: Dmax < 55Gy
brainstem (Dmax < 54 Gy for whole organ, Dmax < 64 Gy for 3D-CRT),
spinal cord (Dmax < 50 Gy),
brain (Dmax < 60 Gy)
- Your clinic has been treating prostate cancer with 7600 cGy in 38 fractions. They have decided to treat with only 19 fractions. What is this approach called? What dose should they use to get equivalent probability of tumour control? Should anything else be discussed?
o Hypofractionation: a smaller number of larger dose fractions
Adjust (i.e., decrease) total dose to produce equal acute/early/tumour effects
Will result in more severe late effects
o Hyperfractionation: larger number of smaller fractions, same or longer overall treatment time (e.g., two fractions per day)
Greater sparing of late-responding normal tissues
Adjust (i.e., increase) total dose to get equal or slightly improved tumour control (use BED = nd (1 + d/(alpha/beta))). Also get same or slightly increased acute/early effects
However, do want to keep overall treatment time short to minimize tumour proliferation.
- CHART (continuous hyperfractionated accelerated radiation therapy): 36 fractions delivered over 12 consecutive days; 3 fractions per days at 6 hour intervals; 1.4-1.5 Gy per fx, 50-54 Gy total. Conventional: 70 Gy in 35 fx over 7 weeks. Discuss expected outcomes from CHART compared to conventional.
o More fractionated so expect lower incidence of late complications
However, myelopathies were recorded probably because interfraction interval of 6 hours was not sufficient for the full repair of sublethal damage in this tissue
o Tumour control similar to conventional – treatment time is short – this is good for minimizing tumour cell proliferation
o Compliance high because acute reactions did not peak until after end of Tx
o Using alpha/beta = 3 Gy for late effects, 10 Gy for early effects:
CHART: BED = 62 Gy10, 81 Gy3
Conventional: BED = 84 Gy10, 117 Gy3
HOWEVER, it doesn’t make much sense to compare these because the overall treatment times differ so drastically (12 days vs 7 weeks) so cell proliferation plays a very important role and BED doesn’t take this into account.
- EORTC trial: accelerated treatment consisting of 72 Gy in 45 fraction (3 fractions of 1.6 Gy per day) over a total of 5 weeks, with 2 week rest period in the middle. Conventional: 70 Gy in 35 fx over 7 weeks.
o Results: 15% increase in locoregional control. This did not translate into a survival advantage. Increased acute effects (expected; see below). Unexpected increase in late effects (including lethal complications). Conclusions: Pure accelerated treatment must be used with extreme caution.
o What do you expect for acute effects? Total dose is similar (actually slightly higher) and delivery time is shorter (therefore less time for repair) so expect worse acute effects. In reality, acute effects were increased significantly.
o What do you expect for late effects? On one hand, more fractions and lower dose per fraction should correspond to reduced late effects. However, there were serious, sometime lethal, late effects. There are likely 2 reasons for this: these late effect are “consequential” late damage developing out of very severe acute effects. Second, there is incomplete repair between dose fractions if several fraction are given per day
- Your clinic has been treating esophageal cancer with 3D conformal radiation therapy. They set up to marks on the skin. They have decided to implement daily setup using kV-CBCT. What aspects of the treatment will change? Should the added dose from daily kV-CBCT be incorporated in the plan?
o From AAPM TG-180 (Image guidance doses to RT patients): “It is recommended that imaging dose be considered part of the total dose at the treatment planning stage if the dose from repeated imaging procedures is expected to exceed 5% of the prescribed target dose.”
- The radiation therapists report that the output of the linac is off by 4%. What do you do?
o Check past QA results to determine if change was sudden or gradual drift.
o If gradual drift, then can adjust output, do second check on adjustment and release for clinical service. If sudden, then measure profiles using simplest equipment (e.g., 2D detector array).
o ALSO consider current weather: outdoor pressure variation due to storm can cause output to be off.
o TG-142 tolerances: daily 3%, monthly 2%, annual 1%.
CPQR tolerance/cation: daily output constancy 2%/3%, monthly relative dosimetry 2%/3%, annual reference dosimetry 1%/2%
o If > action level, then must cease treatments, no question. If > tolerance but < action then can continue but must investigate after treatments are done for the day.
o Ask them to repeat measurement (with same equipment). Or you can repeat the measurement. If still getting same value, then repeat measurement with another measurement system (e.g., monthly QA setup). If this is also consistent, then can ask a second person to do setup and measurement.
* Consider question above but with other parameters off e.g., energy: PDD at 10 cm dropped from 65% to 62% What do you do?
-Check profiles, energy
-if off, requires more work and dosimetric measurements
- Situations where loss of coverage of PTV okay?
o OAR nearby or possibly overlapping with PTV.
o PTV up to edge of body (air adjacent); buildup must occur.
- Refer to image: identify organs 1, 2, 3, and 4
- Right kidney, 2. Spinal cord, 3. Liver, 4. heart
- Refer to image: in this 3-field rectum plan, the 95% isodose is not covering the PTV. What would you do to obtain coverage?
Assuming you want to stay with this particular beam arrangement, you could improve coverage (and dose uniformity across the target) by adding wedges to the lateral beams with the thicker parts of the wedges oriented posteriorly (where the PA beam is incident from). Adding an AP beam would also improve coverage, but this beam would go through the bladder, which is not desirable.
- Refer to image: what is wrong with this 4-field prostate plan? If answered correctly, then follow- up with: what will this do?
The normalization point (ICRU reference point) is in a steep dose gradient region, is not at or near the centre of the PTV, and the dose at this point is not representative of the dose to the entire target. Because the norm point is in the periphery of the treated volume (in the dose fall of region), the dose throughout the target is ~110% of the Rx dose, resulting in a large hotspot.
- Refer to image: in this breast plan, with just open fields, where would the hot spot be expected? How would you increase dose homogeneity in this plan?
- Expect hot spots where separation is smallest (near nipple). Hot spots are expected to be located near mid-separation, where contributions from overlapping beams are largest due to increasing buildup. Posteriorly, the separation is larger, and the hotspots that do exist closer to chest wall will be located closer to dmax (near ~1.5 cm for 6x). In general, want dose within the target to be 95%-107%.
- Can increase homogeneity in the following ways:
o Adding in subfields
o Add wedges.
o Or consider higher energy beam which is more penetrating
typical cutout factors
typical applicator factors
typical SSDeffective
