. 2017 Jan;71(1):81–92. doi: 10.1016/j.eururo.2016.05.033

Table 3.

METastasis Reporting and Data System for Prostate Cancer regional response assessment categories

RAC	Classification	Region	Descriptions^a
1	Highly likely to be responding	Local, nodal, and visceral	• Consistent with RECIST v1.1/PCWG criteria for unequivocal response (partial/complete; see below)
1	Highly likely to be responding	Bone	Return of normal marrow in areas previously infiltrated by focal/diffuse metastatic infiltration • Decrease in number/size of focal lesions sufficient to indicate high likelihood response • Evolution of diffuse neoplastic pattern to focal lesions Decreasing soft tissue associated with bone disease • Dense lesion sclerosis (edge to edge), sharply defined, very thin/disappearance of hyperintense rim on T2W-FS images • The emergence of intra/peritumoural fat within/around lesions (fat dot/halo signs) • Previously evident lesion shows increase in ADC from ≤1400 μm²/s to >1400 μm²/s^b • ≥40% increase in ADC from baseline with corresponding decrease in high b-value SI; and morphological findings consistent with stable or responding disease^c
2	Likely to be responding	Local, nodal, and visceral	• Changes depicting tumour response that do not meet RECIST v1.1/PCWG criteria for partial or complete response (see below)
2	Likely to be responding	Bone	Evidence of improvement, but not enough to fulfil criteria for RAC 1. For example: • Previously evident lesions showing increases in ADC from ≤1000 μm²/s to <1400 μm²/s^b • >25% but <40% increase in ADC from baseline with corresponding decrease in high b-value SI; and morphological findings consistent with stable or responding disease^c
3	No change	All	• No observable change
4	Likely to be progressing	Local, nodal, and visceral	• Changes depicting tumour progression that do not meet RECIST v1.1/PCWG criteria for progression (see below)
4	Likely to be progressing	Bone	• Evidence of worsening disease, but not enough to fulfil criteria for RAC 5 • Equivocal appearance of new lesion(s) • No change in size but increasing SI on high b-value images (with ADC values < 1400 μm²/s) consistent with possible disease progression^b • Relapse disease: re-emergence of lesion(s) that previously disappeared or enlargement of lesion(s) lesions that had partially regressed/stabilized with prior treatments Imaging depicted bone lesions that might be clinically significant (therefore excludes asymptomatic fractures in noncritical bones) • Soft tissue in spinal canal causing narrowing not associated with neurological findings and not requiring radiotherapy
5	Highly likely to be progressing	Local, nodal, and visceral	• Changes depicting tumour progression that meet RECIST v1.1/PCWG criteria for unequivocal progression (see below)
5	Highly likely to be progressing	Bone	• New critical fracture(s)/cord compression requiring radiotherapy/surgical intervention → only if confirmed as malignant by MRI signal characteristics • Unequivocal new focal/diffuse area(s) of metastatic infiltration in regions of prior normal marrow • Unequivocal increase in number/size of focal lesions Evolution of focal lesions to diffuse neoplastic pattern • Appearance/increasing soft tissue associated with bone disease • New lesions/regions of high signal intensity on high b-value images with ADC value between 600 μm²/s and 1000 μm²/s
RECIST v1.1 categories [24] • Complete response: disappearance of all target lesions • Partial response: at least a 30% decrease in the sum of the longest diameter (LD) of target lesions, taking as reference the baseline sum LD • Stable disease: neither sufficient shrinkage to qualify for partial response nor sufficient increase to qualify for progressive disease, taking as reference the smallest sum LD since the treatment started • Progressive disease: at least a 20% increase in the sum of the LD of target lesions, taking as reference the smallest sum LD recorded since the treatment started or the appearance of one or more new lesions Progression of local prostate disease • Use RECIST v1.1 for progression criteria above applied to local disease Progression of nodes • <1.0 cm nodes have to have grown by at least 5 mm in short axis from baseline or treatment nadir and be ≥1 cm to be considered to have progressed • For nodes that are 1.0–1.5 cm that have grown by at least 5 mm in short axis from baseline or treatment nadir and are ≥1.5 cm in short axis can be considered to have progressed • For nodes ≥1.5 cm short axis use RECIST v1.1 progression criteria Progression of visceral disease • Use RECIST v1.1 progression criteria above applied to visceral disease

ADC = apparent diffusion coefficient; FS = fast spin; MRI = magnetic resonance imaging; PCWG = Prostate Cancer Clinical Trials Working Group; RAC = response assessment category; RECIST = Response Evaluation Criteria in Solid tumours; SI = signal intensity; W = weighted.

Multiple criteria need to be met to category response.

Apparent diffusion coefficient cut-off values determined by measurements of untreated lesions [18], [41], [42].

Based on the reproducibility of apparent diffusion coefficient values of <20% [43], [44].