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Y3S1 Planning
| Question | Answer |
|---|---|
| Single vs dual vs partial arcs | Single = smaller volumes Dual = more anatomically complex volumes (may include arc geometry modifications) Partial = unilateral volumes |
| CW and CCW VMAT arcs | 182-178 CW, 178-182 CCW |
| Define physical optimisation | traditional volume-based or max dose objectives/constraints. |
| Describe biological optimisation | gEUD equation (or modified form) (EUDs in Raystation are considered physical optimisation ) |
| Define EUD | the uniform dose distribution that gives a biological effect equivalent to that of a given heterogeneous dose distribution. -> relies on Biological Parameter ‘a’ – tissue-specific parameter a>1 hot spots, a<1 cold spots, a=1 both |
| What is DCAT | Interplay b/w MLCs and mobile lesions (lung) may lead to underosage/overdosage. DCAT avoids leaf interplay - field shape encompasses the complete projection of the treated volume from every direction. Less MUs but could suffer from more OAR dose |
| Isocentre POI placement in VMAT plans | Middle of lowest dose PTV (usually largest volume) |
| Prescription types in h/n vmat plans | Prescribe to highest PTV (or CTV) (primary px) Prescription type = median dose Also prescribe to lower dose EVAL structures |
| Rationale of colly kick | Minimises effects of interleaf leakage and increases optimisation space Opposing collimator angle helps increases optimisation opportunity too (e.g. 350 colly on second arc) |
| H/N: colly considerations | Be cautious about not increasing collimator angle too much -> as you increase, you irradiate higher around the eyes |
| Advantages and pitfalls of dose fall-off function | Dose fall-off function can be used for parallel structures + good for spinal cord dose The more irregular the target volume gets, the less effective - still need standard ring structures |
| Why do we use a contracted external / contracted PTVs | Avoids the system trying to push dose into the build up region -> over-modulation (check with dr if skin dose is desired) |
| What are max DVH goals used for | parallel OARs |
| OARs in h/n | external, acoustics, brain, brainstem, chiasm, glottis, lips, mandible, oral cavity, parotids, spinal cord, TMJ |
| what is the effect of high density artefacts caused by fillings | dose calc accuracy negatively impacted, need to be contoured and density overrides applied - need to consult with physics regarding correct density values for metal objects |
| purpose of obj-ntt contour | used to conform dose to PTV and avoid dose dumping in tissue in the irradiated volume that has not been contoured |
| how does dose fall off work for multiple targets | 'adapt to target dose levels' ticked. the high dose level is reduced around targets with a lower prescribed dose. the system will look at the dose enhancing objectives on lower dose PTVs and use these to inform the high dose level around lower dose levels |
| what is a min dose | used for targets, met when all parts of ROI has a dose greather or ewual to min dose |
| what is a max dose | used for oar and targets, met when ROI has a max dose that is less than or equal to specified dose |
| what is a min DVH | used for targets, met when at the least the specified vol of the ROI receives at least the specified min dse |
| what is max DVH | used for oar and targets, met when only the specified vol of ROI receives more than the specificed max dose |
| what is a uniform dose | used for targets, met when the entire ROI volume receives a dose equal to the specified dose level |
| what is a min EUD | used for targets, a<1 targets cold spots etc. met if EUD value is greater than or equal to dose level |
| what is a max EUD | used for oar and targets, a>1 hot spots, met when EUD value is less than or equal to the dose level |
| what is the dose fall off function | met when all voxel doses within the specified ROI are less than or equal to their respective max dose levels |
| define quality assurance | All procedures that ensure consistency of the medical px, and safe fulfilment of that px, as regards the dose to the TV ++ minimal dose to NTT, minimal exposure of personnel and adequate pt monitoring |
| general workflow of planning/QA + additional checkpoints | sim -> primary and secondary datasets imported into TPS -> RT checks prior to planning -> plan -> check by second RT -> check by physicist ++ day 1 checks, weekly checks, boost/Ph2 checks, final/completion checks |
| plan evaluation checklist items | ◼ Treatment patient, ◼ Correct site ◼ Plan matches prescription ◼ Total dose, fractionation ◼ Daily dose ◼ Treatment machine correct ◼ Beam type and energy |
| plan checking list | ◼ Critical organs not exceeded ◼ iso moves in relationship to landmarks ◼ Individual shielding (MLCs) ◼ Appropriate inhomogeneity correction ◼ Correct bolus ◼ Target volume and field size correlate ◼ DRR generated to the correct Isocentre |
| CBCHOP checklist | contours, beam arrangements, coverage (evaluate on graphic plan and DVH), heterogeneity/hot spots (value and location), organs at risk (constraints, iso lines and DVH), prescription |
| Quantitative Plan Evaluation | • DVH • Clinical goals • ICRU 83 metrics: dose homogeneity (HI) and dose conformity(HI) |
| Define HI | HI = objective tool to analyse the uniformity of dose distribution in the target volume |
| HI formula | D2-D98/D50. HI of 0 = absorbed dose is homogenous |
| CI formula | Vp/PTVp |
| What is the D50 | median dose, should be close to prescribed dose |
| IMRT DVH metrics | near min 98%, near max d2%, median d50% |
| what is the remaining volume at risk | The difference between the volume enclosed by the external contour of the patient and that of the CTVs and OARs on the slices that have been imaged. |
| Importance of RVR | 1. identify unsuspected HDR 2. estimate risk of late effects (carcinogenesis) 3. important for younger patients, long life Assess using DVH and slice-by-slice analysis, to examine the absorbed-dose distribution for all beam paths |
| what is fluence | sum of the contributions from each beamlet modulation factor. ideal fluence converted to mlc segments |
| what two aspects are balanced in clinically acceptable plans | deliverability and complexity |
| list complexity parameters | - Max MUs - Number of control points - Min seg size (area) in IMRT Maximum fluence |
| how does fluence affect complexity | fluence distribution with high tops and deep valleys of values = high complexity. fluctuations more likely to be found in a beam with high max value of the fluence. A beam with a low maximum fluence is therefore believed to be less complex. |
| Define modulation | The process of varying one or more properties of a beam. (MAINLY MLCs) |
| What is a modulation factor | INDEX THAT EXPRESSES COMPLEXICITY OF MLC MOTION a small value = delivery time shortens; however, a small MF value results in poorer dose distribution. necessary to set MF with a balance of delivery time and dose distribution Depends on site etc |
| MF formula | max open time/average open time |
| what is modulation index | ▪ In Helical Tomo Planning, the user sets a value (1.0–5.0) as MF in the design of a plan ▪ MI (Modulation Index): The modulation of the beam fluence, a low MI value is associated with a beam with low complexity. |
| what is modulation complexity score | ▪ Take into account the relative variability on leaf positions, the area of the beam opening and the numbers of MU (1). ▪ A high value of the MCS is associated with a beam with low complexity |
| physics check of 3dcrt | Independent calculation method where planned MU differ from the calculated by x %. Calculation limitations of MU-check programs may contribute to MU differences - may require measurements performed on phantom to validate planned MU calculations |
| physics check of imrt/vmat plans | Commissioning and QA of an IMRT treatment planning system is more complex and time consuming than 3DCRT Physicists can deliver fluence on film and compare it to that in planning system Dose delivery depends on accurate MLC pos |
| What is PSQA and the two ways to perform it | patient-specific verification of IMRT techniques primarily depicts information of variation in planned and measured dose in the PTV 1. Point-dose measurements 2. Verification of dose delivery of separate beams ▪ using 2D (IMRT), 3D/4D detectors (VMAT) |
| Define in vivo dosimetry | directly monitors radiation dose delivered to a patient during radiation therapy, allows comparison of prescribed and delivered doses and thus provides a level of radiotherapy quality assurance that supplement port films and computational double checks |
| Applications of in vivo dosimetry | TBI/TSET - estimate dose to normal structures outside of the treatment fields e.g. lens, pacemakers, foetal and testicular dose. Diode- enabled real-time in vivo dosimetry for immediate investigation/correction of errors during dose delivery. |
| RT role in in-vivo dosimetry | RTS could be involved in Placing diodes, Records the results, Performs simple calculations to compare measured with expected results, and Informs the physicist if a result exceeds |
| Role of board_vacbag_rest | categorised as a 'fixation device' to ensure density is included in calculations |
| how to use uniform dose in head and neck | use on smallest ptv (ptv 70), not so good for surrounding ptvs (can affect dose dropping gradient) or irregular shaped rings |
| Jaw setting and colly for H/N | 10x10, 10 degrees colly |
| in head and neck, what is max eud good for | combined/whole parotids (a=1), PAROTID_OBJ (a>1) |
| Typical dose drop-off (%/mm) | 3% per mm |
| Considerations of uniform dose | can make vols cold, make uniform dose slightly higher than reference dose |
| Qualitative plan evaluation | IDL, coverage, volume and location of max dose, visual homogeneity and conformity |
| IGRT considerations when plan checking | Arc 1 direction relative to CBCT acquisition Tolerances and priorities for CBCT matching e.g. turn on 45Gy / 54Gy line and assess spinal cord. We may be meeting (green) on the score card but the patient moves |
| why is IMRT good for breast | to minimise entry and exit dose through contralateral organs |
| what is chestwall contour in RBWH breast planning | expansion of ipsilateral lung (2cm expansion axially) to encompass ribs and sternum for some, a smaller expansion of 1-1.5 is used |
| purpose of chest wall and opt robust in RBWH breast planning | to create robust datasets and evaluate robust plans |
| purpose of flash in RBWH breast | for treatment team to assess during image matching |
| what is opt robust in RBWH breast planning | PTV Eval w two most SUP/INF slices & 'chestwall' retracted • Used in optimisation + simulate organ motion • Retracting CW avoids rib deformation in simulated organ motion datasets • Retracting SUP/INF minimises PTV ‘dragging’ inf along abdo in SOMD |
| what is flash robust in RBWH breast planning | structure that accounts for change in breast shape whilst on treatment using simulated organ motion datasets that mimic this change and are surrogates for potential shape change during treatment by ensuring that the MLCs give sufficient overshoot |
| IMRT beam placement and colly for breast | • Beams should NOT cross midline • Beams should NOT enter through contralateral breast • Colly usually 0° – 10 degrees. 5 good • Depends on breast/PTV shape • Depends on arm → steep collimator angle may treat more of arm • Visualise in BEV |
| jaw assingment for IMRT breast | • **Essential** - Use Limits as Max • Visualise the fields in BEV to ensure adequate overshoot: • Approx. 1cm SUP/INF • Approx. 1.5-2cm ANT • Approx. 1cm POST (close post jaw as much as possible without restrict coverage to PTV) |
| what is the PURPOSE of robust optimisation | • To force the MLCs to open in the region of the FLASH Robust – across the two biggest (0.97cm) SOMD • Ensures adequate breast coverage and reduces excess dose on skin surface when there are changes in breast shape within the specified range of SOMD |
| ideal sup/inf ptv coverage | 3-4 slices sup/inf of PTV with 95% isodose |
| adding additional beams for imrt | usually 7-15 deg off tangents, consider entry and exit: do not enter through CL breast, do not exit too far into heart/lung, do not rotate too steep on post oblique due to collision risk with the treatment couch |
| how to review organ motion datasets in imrt breast | reasonably quick check to ensure there is no excessively high skin dose. if >120-130% hotspots consider going back to optimisation and increasing weight on external max dose objective VISUAL ASSESS ONLY, NO CLINICAL GOALS |
| limitations of 3dcrt for whole pelvis+ what technique | 4 field box. small bowel may fall into empty space in the true pelvis -> increased risk of acute and late GI complications we also need to deliver dose to paracervical/nodal tissues which are at highest risk of recurrence - unfeasible with 3DCRT |
| define SIB | delivery of different doses per fraction in different target regions imrt/vmat simultaneously deliver 2 or more volumes concurrently |
| prostate + nodes sib | 74-78Gy to prostate, 45-50.4 to LNs |
| prostate + SVs sib | Prostate 78Gy, SVs 54Gy |
| Benefit of IMRT/VMAT for whole pelvis (general statement) | can reduce GI, GU and haematological toxicities |
| cervix/endo px | 50.4/28 plus boost 5.4/3 OR 50.4 + 45Gy to nodes |
| rectum px dose levels | 54Gy, 51Gy, 47Gy |
| importance of ext genitalia contour in whole pelvis planning | volumes often extend to skin and near the external genitals -> hotspots often cling here |
| imrt techniques | 7-9 beams with gantry of 45, 90, 115, 180, 245, 280,. 320 |
| bladder cons of gynae planning | Bladder filling in gynae is variable - some empty, some full, some neutral Post-surgical conditions and considerations - can they hold? does the bladder fill the same? |
| whole pelvis oars | bowel bag, bladder, rectum solid, femoral heads/necks, kidneys, bone marrow - pelvic bone contoured as a surrogate |
| varian max leaf movement and jaw length | 15cm max leaf travel Y jaw = 19, X=14 |
| whole pelvis colly | 10-35, can use sup-inf leaf travel to shield bowel and exploit sup-inf modulation space - however be weary of increased irradiation of the bowel |
| elekta vs varian MLCs | or varian, MLCs are below jaws, in ELEKTA MLCs are above |
| varian halcyon (tomo) for whole pelvis | utilise a dual iso technique (max 8cm difference), can extend FS up to 36cm. need to evaluate junction dose |
| pixels vs voxels | pixel = picture element voxel = volume element |
| stage 1 vs 2 in monaco optimisation | stage 1 (PB algorithm) produces ideal fluence distribution for evaluation of how accurately cost functions achieve goals stage 2: segmentation begins which converts ideal fluence into deliverable distribution, shapes and weights are optimised |
| what type of densities does monaco work with | relative electron densities (not physical densities) |
| what is a sector in monaco | before stage 1 opt, the system divides the sequence into sectors to simulate the arc during stage 1 sector = arc length / increment More sectors = better plan BUT increases planning time |
| describe arc increment in monaco | if you use a large increment, monaco creates fewer sectors - poor quality plan with smaller incremenets, we have more sectors this increases planning time/delivery head and neck = smaller increment (20) simple case = larger increment (30) |
| what is sweep sequencer in monaco | leaves move from their start position to end position in a continuous, undirectional manner (length is determined by the sector). leaves move to the left side of BEV in the first sector, then change and move to right side of BEV Min width = 5mm |
| What is segment shape optimisation in monaco | Improves plan quality + deliverability. Allows for areas of high and low modulation to better meet constraints. includes smoothing, sequencing and optimisation of beam weights and shapes. |
| what is fluence smoothing in monaco | a parameter that controls the smoothing of the fluence in stage one of the optimisation (off, low, medium, high). medium is a recommended starting point. smoother plans have less plan quality and control points |
| describe off, low, medium and high fluence smoothing | off - creates many segments low - creates more segments (use for complex plans) medium - creates an average number of segments (less complex plans) high - creates few segments |
| statistical uncertainty for MC algorithm | use of MC can create hot spots - cost functions used to control these this is a more accurate representation of actual patient dose statistical uncertainty per control point should not go above 10 (poor isodose visualisation). 1.5% per calc or 5% per CP |
| constraint vs pareto *** | constrained = sets constraints on health tissue while it administers dose to target volumes - normal tissue priority pareto = prioritises target underdoses on tumour volumes and relax constraints on healthy tissue - target volume priority |
| layering in monaco | targets at top and patient skin surface at bottom - determines how optimiser treats the voxels in the volume where the structures overlap |
| dose constraints in monaco | target penalty, target eud, quadratic overdose, max dose, parallel or serial, traditional dvh (underdose or overdose), conformality function |
| physical vs biological cost functions | physical - logical, easier to use bio - intuitive way to control dose distribution compared to a DVH point method, accounts for response of tissues to dose as well as the volume effect of organs |
| define EUD | the dose that causes the same effect if applied homogenously to the entire organ volume. the eud represents any two or more dose distributions that yield the same radiobiological effect |
| isoconstraint vs isoeffect | isoconstraint = eud we are aiming for isoeffect = calculated eud of what we are receiving |
| target eud in monaco | biological cost function OBJECTIVE for targets cell sensitivity default 0.5 higher cell sensitivity = inc penalty paid for cold spots, inc pressure to deliver dose to cold spots |
| describe quadratic overdose and RMS | physical cost function CONSTRAINT. isoconstraint is RMS and we add a dose excess |
| quadratic overdose vs max dose | QO has RMS, max dose does not |
| constraints vs objectives | objective = a desired txt goal constraint = boundaries (limits) that the system must satisfy, regardless of how objectives are changed constraints decrease the speed of optimisation because they restrict the solutions available to the optimiser |
| how to use quadratic overdose for conformality | use QO cost functions with a series of stepped shrink margins (shrink margins used instead of ring optimisation structures). works well to create transition doses between multiple target vols and control hotspots in an overlap region |
| parallel cost function in monaco | does not have to be used only on parallel structures. e,g, rectum has dose volume response. reference dose with isoconstraint (mean organ damage % to the structure - aka how much can be sacrificed) and power law exponent (k) 1-4 |
| describe k values 1-4 for parallel cost function | 1 = greatest penalty applied to region of low dose and applies pressure over most o the curve 4 = applies penalty in the region of the reference dose 2 is a good midpoint |
| describe serial cost function in monaco | preferred constraint for serial OARs. applies large penalties for hotspots, even if small in volume. isoconstraint is EUD. k 1-20 |
| formula for serial cost function k value | k = 0.15 x d50 |
| what is an overdose or underdose dvh | equivalent to max or min dvh. underdose dvh to be handled with caution |
| max dose in monaco | can be used to control global max when applied to external contour. penalty kicks in immediately when voxels cross max dose threshold so it can be troublesome. preferable to use QO with RMS dose excess |
| conformality cost function in monaco | for use with OARs. physical function to shape high dose volume around targets. works well for single target volumes where there are large NTT areas but not h/n w overlapping vols. if dose is squeezed too much (low value) can end up with high dose in PTVs |
| what is the conformality function in monaco best used with | still use quadratic overdose in outer NTT. conformity function works 4cm - 8cm from edge of target |
| what is a shrink margin in monaco | variable shrink margins can be assigned when you have competing targets e.g, gtv in ctv - can create shrink margin of ctv away from gtv to work voxels in ctv to create a dose gradient |
| surface margin in monaco | used for tumours on surface, 0.5cm margin -> akin to eval |
| relative impact in monaco | + 0-0.25 ++ 0.25 -0.5 +++0.5-0.75 ++++0.75 -1.0 |
| what is multicriterial optimisation | optimiser tries to achieve even lower dose (tighten the constraint) to OAR as long as still meeting target objective. typically means OAR and target are separated and that target coverage is not significantly affected by a tighter constraint on the OAR |
| in what type of SABR plans are R50 and d2 concepts used | lung cases - not so much in spine and liver |
| SBRT vs SABR vs SRS | SRS for intracranial legions. Single fractions >12Gy SABR and SBRT for extracranial lesions. 1-8 fxs (SABR very high) |
| planning features of SABR | small margins, very steep gradient, inhomogenous target dose, many MUs and longer beam on time |
| what coverage do we expect in SABR | highly dependent on site and context. 85-95% accepted, max 125-140% brain - 100% spine - 85% (nearby cord) lung - 95-98% |
| Define D2 | a mechanism for evaluating dose fall-off geometrically (2cm expansion from PTV in which we want the 50% IDL to fall) |
| define R50 | the ratio of the 50% prescription isodose vol to the PTV vol (gradient index) |
| why is GI better than CI in SABR | R50 is a function of the size of the PTVs - differentiates plans with similar conformity but different gradients |
| max dose loc for SABR | must be within GTV. higher max translates to faster dose fall-off outside the target, so if max is higher R50 and D2 should be betteer |
| prescription isodose / max dose for SABR | If PD = 100%, max dose must be at least 111.11% but no more than 166.6% |
| coverage (isodose) for SABR | 95% target vol should be conformally covered by the prescription isodose (100%) 99% of PTV receives a minimum of 90% prescription isodose |
| dose reporting for SABR | PTV d50 and d near min/max |
| impact of increasing segment shape optimisation and high precision leaf positions | inc optimisation time, inc plan quality, decrease delivery time, decrease segment number, may increase MU |
| what is a voxel based tps | the entire volume is split into tiny voxels - planner control voxels (not structures) that extend out from the isocentre and are based on the grid size |
| signs and symptoms of brain mets | headaches, seizures, dizziness, unsteady gait, memory issues, verbal issues, blurred vision, diplopia, confusion, irritability |
| scan parameters for SBRT brain | scan entire skull, top of shellboard, including top of shoulders for NCP angles 1mm slices |
| Image fusion for SBRT brain | Standardly non contrast CT with MRI contrast |
| Brain image fusion matchpoints | Clivus, internal carotid arteries, craniotomies, temporal lobes, sinuses, sulci |
| Dose fractionations for SABR brain | 18-24Gy/1, 24/3, 27/3, 25/5, 27.5/5, 30/5 |
| considerations of dose/fractionation in SABR brain | tumour location/surrounding OARs, tumour size, previous XRT |
| fractionations for brain lesions <2 (non hypo and hypo) | 20-24/1, 27-30/3 OR 30-35/5 |
| fractionations for brain lesions >2-3 (non hypo and hypo) | 18/1, 27/3 OR 30/5 |
| fractionations for brain lesions >3-4 (non hypo and hypo) | 27/3, 30/5 |
| OC/ONs tol for 3fx vs 5fx stereo | 3=15-18Gy 5=22-25Gy |
| brainstem tol for 3fx vs 5fx stereo | 3=21-24 5=25-30 |
| hippocampus tol for 3fx vs 5fx stereo | <3 mean <4 mean (ALARA) |
| lens tol for 3fx vs 5fx stereo | 6Gy both |
| brain tol for 3fx vs 5fx stereo | V20<20cc V24<20cc |
| How do margins differ between intact and post-op brain SABR? | Intact lesions - CTV is not as important -> less risk of microscopic disease remaining and spread of microscopic disease GTV -> PTV margin = 1.5mm For post-op. HRTV + GTV + 2mm = CTV CTV + 1.5mm = PTV |
| Beam arrangement for SABR brain | Always use a CP full arc - allows control of low dose NCP arcs are useful but do not add if not necessary - may lead to unnecessary low dose wash and OAR irradiation, inc txt time Avoid 'mohawk' arc for central sites - irradiates central structures |
| Explain process for irradiating PTV overlapping with OARs e.g. brainstem | ROI algebra Whole PTV to 25Gy Overlap PTV to max of 25.5 -> push hard Non overlap area to plan max (111%) -> max dose allowed to be in this area |
| Ring approach for brain SABR | 3 ring approach to help tailor dose gradient/drop-off and control low dose wash The smaller an optimisation structure, the better/easier it is to work -> preference for rings over working NTT hard |
| Intact vs post op brain planning | Intact generally small and spherical, deeper seated lesions (close to OARs), dose escelate to 130-135%, larger dose per fc Post op bulky/irregular shape, on peripheries of skull, 110-115% dose and smaller dose per fx |
| Why do we escelate dose to intact brain tumours | Max doses differ BECAUSE with a post-op site there will be more normal tissue around / regenerating |
| Explain WBRT | previously used for brain mets but no longer rec for most patients due to radionecrosis and severe cognitive decline. HS WBRT options |
| Describe SRS (including contraindications) | precise delivery of single, high dose radiation using linac, gamma knife or cyber knife. 18-24 Gy /1. not suitable for large lesions and nearby oars |
| describe brain SRT (including indications) | 24-30/2-5. steep dose gradients. indications: improve therapuetic ratio, larger lesion, OAR/eloquent location |
| Plan eval for SRT brain | px isodose covering ptv by >99% px IDL covered evenly, not splaying into rings/NTT check min dose of GTV, PTV and CTV hotspot in GTV or CTV if post-op check OAR doses - drive parallel ALARA check low dose wash - uniform unless otherwise specified |
| Describe R50 in relation to brain SRT | using NCP will reduce R50, smaller PTVs will have a larger R50 |
| pitfalls of conformity index | can have same vol of PTV but not in target (diagram) |
| describe cyber knife | robotic arm with several hundred treatment beams and mroe than 1000 poss beam directions. non-isocentric inverse txt planning. small circular fields of varying size. weighted with different MUs |
| describe gamma knife | 192 collimators, rapid dose drop off with sub mm accuracy. no margins involved (treat to GTV). frame (single fraction with no IGRT) or frameless (single or multi, daily CBCT) |
| for lung SABR with multiple lesions what are our planning options | 1 iso 2 isos if >5cm apart. -> two px and a summed plan or a bias plan on monaco |
| arc arrangement for SABR lung | aim to minimise amt of lung in entry and exit paths, avoid full arcs to reduce combined lung mean and low dose wash 200 length usually required to meet dose fall off to meet R50/D2 - longer arcs allow a more isotropic dose fall off |
| in SABR, which OARs would we compromise coverage for | brainstem, optic nerves/chiasm, spinal cord, bowel |
Created by:
jrichardss