Transition zone (TZ) adenocarcinoma accounts for 20-30% of all prostate cancers and has distinct morphological and MR features compared to peripheral zone cancers. TZ cancer poses a diagnostic challenge as it develops in the same anatomical region as benign prostatic hyperplasia (BPH) nodules. According to PI-RADS v2.1, primary assessment in TZ lesions is based on T2 morphology — DWI/ADC serves only as an upgrading criterion (unlike PZ). The most important diagnostic finding of TZ cancer is the 'erased charcoal sign': the tumor erases the boundaries and capsules of surrounding BPH nodules, creating homogeneous low T2 signal. TZ cancers are generally diagnosed with lower Gleason scores (ISUP 1-2 more common) and tend to be less biologically aggressive, though advanced-stage disease can also occur.
Age Range
55-80
Peak Age
70
Gender
Male predominant
Prevalence
Uncommon
TZ adenocarcinoma originates from glandular epithelium in the transition zone — since BPH also develops in the same region, they share a common cellular origin but progress through different molecular pathways. TMPRSS2-ERG gene fusion is less frequently found in TZ cancer compared to PZ cancer (15% vs 50%), while SPINK1 overexpression and FOXA1 mutations are more common. The tumor shows infiltrative growth between BPH nodules and destroys the pseudocapsules and boundaries of surrounding BPH nodules — this is reflected as 'erased charcoal sign' on T2-weighted images. The thin hypointense capsules (like charcoal lines) normally seen between BPH nodules in TZ are 'erased' by the tumor and replaced by homogeneous low T2 signal. This homogeneous hypointensity results from the tumor's dense cellular structure — glandular lumens are lost, free water decreases, and T2 relaxation time shortens. TZ cancers are generally diagnosed with higher volume but lower Gleason score — this 'large but indolent' phenotype causes earlier PSA elevation, enabling earlier diagnosis.
On T2-weighted MRI, the thin hypointense capsules and boundaries ('charcoal lines') of BPH nodules in the transition zone have been erased by the tumor and replaced by homogeneous low T2 signal. This finding is pathognomonic for TZ adenocarcinoma and forms the basis of PI-RADS v2.1 TZ assessment. Compared to preserved capsules and boundaries in BPH's 'organized chaos' pattern, the 'erased charcoal sign' indicates infiltrative growth of the lesion and destruction of BPH nodules.
On T2-weighted images, a homogeneous low signal intensity lesion in the transition zone erasing the boundaries and capsules of surrounding BPH nodules is observed. The thin hypointense 'charcoal lines' (pseudocapsules) normally seen between BPH nodules in TZ have been erased by the tumor. The lesion typically shows lenticular (lens-shaped) or round-oval morphology. This finding is the primary criterion for evaluation of TZ lesions in PI-RADS v2.1: homogeneous, non-encapsulated, border-erasing lesion is classified as PI-RADS 4 (<15 mm) or PI-RADS 5 (≥15 mm).
Report Sentence
A ___ mm homogeneous hypointense lesion with lenticular morphology erasing the boundaries of BPH nodules is noted in the transition zone, consistent with 'erased charcoal sign'; transition zone adenocarcinoma should be considered (PI-RADS ___).
The transition zone lesion may show variable degrees of diffusion restriction on DWI. DWI restriction in TZ cancers may be less pronounced compared to PZ cancers because TZ cancers typically have lower Gleason scores. According to PI-RADS v2.1, DWI is only an upgrading criterion in TZ lesions — if T2 morphology is PI-RADS 3 or 4 and DWI is markedly positive, PI-RADS may be upgraded one category. Since stromal BPH nodules can also show mild DWI restriction, evaluation together with T2 morphology is mandatory.
Report Sentence
The transition zone lesion shows focal diffusion restriction on DWI, serving as an upgrading criterion in the PI-RADS assessment.
The TZ lesion shows low ADC values on ADC map. ADC values of TZ cancers are generally higher compared to PZ cancers (due to lower Gleason). The ADC threshold for clinically significant TZ cancer is generally accepted as <900-1000 × 10⁻⁶ mm²/s. Due to risk of overlap with stromal BPH nodules, ADC alone is not diagnostic — must be interpreted together with T2 morphology. TZ lesions with ADC <750 × 10⁻⁶ mm²/s are highly likely to be clinically significant cancer.
Report Sentence
Low ADC values are noted in the transition zone lesion on ADC map (ADC: ___ × 10⁻⁶ mm²/s), when evaluated together with T2 morphology consistent with adenocarcinoma.
TZ adenocarcinoma may show focal enhancement on DCE MRI, but since BPH nodules also show enhancement, DCE has limited diagnostic value in TZ. According to PI-RADS v2.1, DCE is NOT USED as a criterion for evaluation of TZ lesions — it only plays an upgrading role in PZ lesions. Enhancement assessment in TZ is unreliable because stromal BPH nodules can also show early enhancement. However, enhancement kinetics of TZ cancer may differ from BPH: cancer tends to show more homogeneous and more intense enhancement.
Report Sentence
The transition zone lesion shows enhancement on DCE, however per PI-RADS v2.1 DCE is not used as a primary criterion in TZ assessment; T2 morphology is the basis of evaluation.
On TRUS, TZ adenocarcinoma is generally indistinguishable from BPH nodules — mostly appearing isoechoic or hypoechoic. TZ cancers are more difficult to detect by TRUS due to their anterior location. MR-TRUS fusion biopsy is recommended to target TZ lesions detected on mpMRI. The miss rate for TZ cancers with systematic biopsy is 20-30% because standard biopsy needles target the peripheral zone. TRUS sensitivity for TZ cancer detection is below 30%.
Report Sentence
A lesion in the transition zone that cannot be reliably distinguished from BPH nodules is observed on TRUS; mpMRI-TRUS fusion-guided biopsy is recommended.
TZ adenocarcinoma shows intense PSMA uptake in the anterior prostatic region on PSMA PET-CT. PSMA PET-CT is used especially for staging and biochemical recurrence evaluation. Since BPH nodules can also show mild PSMA uptake, mpMRI remains the preferred modality for primary tumor evaluation. However, in high Gleason TZ cancers, PSMA PET-CT can provide additional information beyond mpMRI.
Report Sentence
Intense PSMA uptake is observed in the anterior prostatic region (transition zone) on PSMA PET-CT, consistent with TZ adenocarcinoma.
Criteria
Lesion developing adjacent to anterior fibromuscular stroma (AFMS)
Distinct Features
High risk of AFMS invasion, EPE through anterior capsule possible. Highest miss rate with standard biopsies, MR-guided biopsy mandatory. Most difficult subtype to detect with TRUS.
Criteria
Lesion developing around urethra, originating from periurethral zone
Distinct Features
Early obstructive symptoms, may be incidentally found in TURP specimen. Can be symptomatic even at small sizes. Early clinical presentation due to urethral invasion.
Criteria
Extensive infiltrative lesion occupying large portion of TZ
Distinct Features
Homogeneous hypointensity occupying entire TZ, BPH nodules completely erased. PI-RADS 5. Frequently higher Gleason and EPE/SVI positive. Generally >15 mm.
Distinguishing Feature
BPH nodules are encapsulated, well-circumscribed with heterogeneous T2 signal ('organized chaos'). TZ cancer is homogeneously hypointense and erases capsules ('erased charcoal sign'). Normal TZ tissue is preserved between nodules in BPH. Stromal BPH nodules may be T2 hypointense but are round and encapsulated — cancer is lenticular and border-erasing.
Distinguishing Feature
Prostatitis can show diffuse or focal T2 hypointensity in TZ but usually shows wedge-shaped or striated pattern, not lenticular morphology. Boundaries of BPH nodules are generally preserved in prostatitis. Clinical findings (fever, dysuria, transient PSA elevation) are distinguishing.
Distinguishing Feature
AFMS is a normal anatomical structure appearing as an anteriorly located homogeneous T2 hypointense area. Unlike TZ cancer, AFMS shows no mass effect, does not erase BPH nodule boundaries, and shows no DWI restriction. AFMS is a normal structure present in every patient.
Urgency
urgentManagement
surgicalBiopsy
NeededFollow-up
specialist-referralMR-guided or MR-TRUS fusion biopsy is recommended for PI-RADS 4-5 TZ lesions — systematic biopsy misses TZ cancers at a rate of 20-30%. TZ cancers are generally diagnosed with lower Gleason scores and tend to be less biologically aggressive than PZ cancers. However, advanced-stage TZ cancer (anterior EPE, AFMS invasion) is more difficult to detect and surgically more challenging. Treatment options are similar to PZ cancer: low risk → active surveillance, intermediate-high risk → radical prostatectomy or radiotherapy.
TZ adenocarcinomas generally have lower Gleason scores and better prognosis than PZ cancers. However, there is risk of delayed diagnosis due to being hidden within BPH. mpMRI + MRI-TRUS fusion biopsy improves diagnostic accuracy.