DOSE VOLUME HISTOGRAM: 

Graphical representation of dose to volumes of tissue. DVH statistics considers each small voxel (a voxel is essentially like a pixel but in 3D. It’s a small volume based division of tissue in the radiation field and derived from a planning CT scan) independently. The treatment planning system mathematically calculates the dose delivered to that volume. All voxels which receive the same dose are in the same “isodose”. Graphically when representing with a histogram, each bar is an “isodose bin”

First create a Frequency DVH histogram (% volume vs dose) 

Can create a cumulative DVH after: How much volume is receiving ATLEAST set dose. (i.e. x Gy or above). Axes remain the same (% volume vs dose) 

V20Gy ( VOLUME that received at least 20 Gy, i.e. 20Gy or more) IS X % volume 

D1cc ( DOSE received to 1cc volume) IS X Gy dose 

Excellent online resources that explain the concept of a DVH, frequency vs cumulative DVH and its pitfalls can be found here: https://www.slideshare.net/sasikumars/dose-volume-histogram

Understanding how to read DVH metrics found here: https://www.carlosjanderson.com/understanding-the-meaning-of-dvh-metrics/#comments

ICRU 50 (1993) and 62 supplement (1999)

ICRU 50 1993

Define clearly and concisely : Intent, volume to the treated, dose to be treated to 

GTV: palpable or visible or demonstrable extent and location of malignant growth 

CTV: margin that encompasses microscopic malignant disease 

PTV; accounts of variation in CTV due to organ motion, step up error. Ensures CTV received prescription dose within statistical limits. 

Treatment volume TV = volume receiving dose appropriate to treatment purpose (generally 95%)

Irradiated volume (IR) volume receiving significant dose relative to OAR tolerances (usually about 20% dose)

OAR: tissues whose radiation sensitivity may influence treatment plan 

DOSE Prescribed and reported to a point. ICRU prescription point and ICRU dose reference point. Often same point but different purposes. 

Criteria of both ICRU prescription point and dose reference point

Easy to define 

Clear 

Within PTV 

Clinically relevant 

Avoid high dose gradients 

Representative of dose throughout PTV 

Often both points are centre of PTV or isocentre – but need not be. Sometimes necessary to move if centre of PTV is outside PTV (in chest wall for geometrically unusual lung tumours, or in rectal tumours – do not want it in lumen (air) but in tissue) 

Dose reporting: 

PTV : max and min dose. dose to reference point. Average dose, DVH 

OAR : max point dose and volume receiving that dose. Ideally DVH data 

Hotspot size and POSITION to be recorded. Definition of significant hotspot: >100% dose outside PTV and >15mm diameter significant.

3 levels of dose reporting

Level 1 – max min and ref point 

Level 2 – some volume dose data 

Level 3 – full 3D volume dose data and DVH stats 

PLANNING EVALUATION 

Dose delivered should be Homogenous as possible to PTV – 95-107%  

Spatial representation of doses in according with ICRU 42 (isodose lines) 

ICRU 62 1999

CTV to PTV divided into 2 parameters: 

IM and SM : (internal margin and set up margin) 

IM can be defined as ITV based on 4DCT 

PRV; planning organ at risk volume: analogous to PTV to OAR (but doesn’t really include a CTV margin – mainly accounts for IM + SM of OAR) 

Conformity Index (CI) – defines how conformal a treatment plan is. CI = TV volume/ PTV volume 

Doesn’t actually look at spatial overlap though – just numerical volumes!!

Further defines OARs are series or parallel  

serial – spinal cord 

parallel – lung 

serial-parallel – heart (serial coronary arteries and parallel myocardium) 

DOSE reporting – new stuff in ICRU 62 : DVH in all volumes and OAR 

ICRU 50 and 62 problems:

Recommends prescribing to a point dose 

Not practical to find a point inIMRT plans with modulation that meets this criteria 

Very steep dose gradients and hence not practical. Max and min doses to a point may also not be as relevant 

dose to a volume is more practicable for IMRT

ICRU 83 new IMRT concepts

Greater emphasis on DVH 

Clarified importance of consistent outlining of OARs 

Reporting near max and near min doses 

ICRU reference point replaced by median dose to PTV reporting

Use DVH statistics 

D50% PTV (median absorbed dose or 50% of volume of PTV is receiving at least this much dose)

Near min absorbed dose D2% (2% of volume of pTV is receiving at least this much dose) 

D98% (98% of pTV volume is receiving at least this dose or more) 

Reports based on PTV with uncompromised margin 

Subvolumes PTV – (SC+1cm) may be used to add detail to PTV dosing

PLANNING EVALUATION TOOLS

Check isodose lines 

DVH data 

Max and min doses within PTV (to relevant volume, rather than point. Volume is more representative as the patient is not going to be in the exact same position during treatment. Volumes used are small e.g. 1cc or 0.5cc) 

Check OARs – isodose and DVH 

Remember series or parallel OAR 

Check for HOTSPOTS