Most renal masses today are found incidentally on imaging done for something else — the “incidentaloma” era. The classic triad of flank pain, hematuria, and a palpable mass is now rare and represents advanced disease.
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Most renal masses
today are found incidentally on imaging done for something else —
the “incidentaloma” era. The classic triad of flank pain,
hematuria, and a palpable mass is now rare and represents advanced
disease. The APP's role is to characterize the mass correctly,
distinguish likely benign from malignant, and route patients to the
right pathway: surveillance, biopsy, ablation, histotripsy, or
surgery.
10.1 Characterizing a renal mass
The single most
important question is: does it ENHANCE? Enhancement means blood
supply, and a solid enhancing renal mass is renal cell carcinoma
until proven otherwise. A change of more than ~15–20 Hounsfield
units between the non-contrast and contrast phases is the working
definition. This is why a renal-protocol CT (or MRI) with and without
contrast is the study of choice — a single-phase contrast CT cannot
answer the question.
The Bosniak classification (for cystic lesions)
Bosniak category
Features
Malignancy risk & action
I
Simple cyst: thin wall, no septa, no calcification, no
enhancement
~0% — benign. No follow-up
II
A few hairline septa, fine calcification; < 3 cm hyperdense
~0% — benign. No follow-up
IIF
Multiple hairline septa, minimal thickening, no measurable
enhancement
~5% — surveillance imaging
III
Thickened, irregular walls or septa WITH measurable enhancement
~50% — surgical excision or ablation
IV
Enhancing soft-tissue components
~90% — treat as cancer
Solid masses
Renal
cell carcinoma: the default assumption for a solid enhancing
mass.
Angiomyolipoma
(AML): the important benign exception — macroscopic FAT on CT
(negative Hounsfield units) is diagnostic. Associated with tuberous
sclerosis when multiple/bilateral. Large AMLs (> 4 cm) risk
spontaneous hemorrhage (Wunderlich syndrome) and may warrant
embolization.
Oncocytoma:
benign; may show a central stellate scar, but it cannot be
reliably distinguished from RCC on imaging — this is a common
reason to biopsy.
Also
consider: a renal abscess, an infarct, lymphoma, or a metastasis
to the kidney (in a patient with a known primary).
10.2 The small renal mass (≤ 4 cm, cT1a)
These are
increasingly common and frequently indolent — roughly 20% are
benign, and many malignant ones are low-grade and slow-growing
(typical growth ~3 mm/year). Management is individualized to tumor
features, patient age, renal function, and competing comorbidity.
Overtreating an 82-year-old's 2 cm mass is a real harm.
Option
When it fits
Partial nephrectomy (nephron-sparing)
Preferred for most cT1a masses when technically feasible —
preserves kidney function, which matters for long-term
cardiovascular and renal outcomes
Histotripsy (non-invasive focused ultrasound)
Emerging incisionless option — see below; our practice is first in the
U.S. to offer it for renal masses
Active surveillance
Older/comorbid patients, small masses, competing risks; serial
imaging with intervention if growth is rapid (> 5 mm/yr) or
size exceeds ~4 cm
Thermal ablation (cryoablation / radiofrequency)
Selected small (< 3 cm), peripheral masses; poor surgical
candidates. Slightly higher local recurrence than surgery, but
low morbidity
Renal mass biopsy
When the result would change management — before ablation,
before surveillance in a younger patient, to characterize an
indeterminate mass, or when metastasis/lymphoma is suspected. It
is safe (tract seeding is essentially a non-issue) and underused
Radical nephrectomy
Larger, central, or hilar tumors not amenable to partial — but
avoid it for small masses if a partial is possible
10.3 Renal cell carcinoma (RCC)
Subtypes
Subtype
Share
Notes
Clear cell
~70–75%
Most common and most likely to metastasize. VHL-pathway driven;
the subtype most responsive to immunotherapy and anti-VEGF agents
Papillary
~10–15%
Type 1 (indolent) and type 2 (aggressive); often
multifocal/bilateral
Chromophobe
~5%
Generally the best prognosis
Collecting duct / medullary
Rare
Highly aggressive. Renal medullary carcinoma occurs in patients
with sickle cell TRAIT — a critical association to know
Hereditary
syndromes (~5%): von Hippel-Lindau (clear cell,
bilateral/multifocal, plus hemangioblastomas and pheochromocytoma),
hereditary papillary RCC, Birt-Hogg-Dubé, and hereditary
leiomyomatosis. Consider genetic referral for young patients (<
46), bilateral or multifocal disease, or a suggestive family
history.
Paraneoplastic
syndromes are classic for RCC — hypercalcemia (PTHrP),
polycythemia (erythropoietin), hypertension (renin), and Stauffer
syndrome (reversible non-metastatic hepatic dysfunction). RCC is the
“internist's tumor.”
Staging:
CT chest/abdomen/pelvis. RCC has a particular propensity for
venous extension — a tumor thrombus can grow up the renal vein
into the IVC and even the right atrium, which dramatically changes
the operation. MRI is used to define the thrombus level.
Treatment
Localized:
partial nephrectomy when feasible; radical nephrectomy for
larger or centrally located tumors. Adjuvant pembrolizumab is now an
option for high-risk resected disease.
Advanced/metastatic:
systemic therapy has shifted decisively to immune checkpoint
inhibitor combinations (e.g., ipilimumab + nivolumab) and IO plus
VEGF-targeted tyrosine kinase inhibitor doublets. Cytoreductive
nephrectomy is now selective, not automatic. Metastasis-directed
therapy (surgery or SBRT) is used for oligometastatic disease. our practice
infuses immunotherapy in-house (Chapter 20).
Important:
RCC is essentially unresponsive to conventional cytotoxic
chemotherapy and to radiation as a primary treatment. Do not be
surprised when oncology does not offer chemo.
Practical points
Protect renal function:
track eGFR before and after any nephron loss, especially in
solitary kidneys, CKD, diabetics, and hypertensives. Chronic
kidney disease from nephron loss is a cardiovascular risk factor,
not just a lab value.
An enhancing solid renal
mass is malignant until proven otherwise — never dismiss it as
a cyst based on a non-contrast scan.
Renal medullary carcinoma in a young Black patient with
sickle cell trait is a specific, aggressive entity — do not
mistake it for something indolent.
Clinical Pathway
Click any node to expand
Most renal masses today are found incidentally — the "incidentaloma" era. The APP's role is to characterize the mass correctly, distinguish likely benign from malignant, and route the patient to surveillance, biopsy, ablation, histotripsy, or surgery.
The single most important question is: does it ENHANCE?
Select a box to open its teaching details.
Your job
Characterize the mass correctly, distinguish likely benign from malignant, and route the patient to the right pathway: surveillance, biopsy, ablation, histotripsy, or surgery.
Ch 10.1
The working definition
A change of more than ~15–20 Hounsfield units between the non-contrast and contrast phases
Pitfall
A single-phase contrast CT cannot answer the question — you need with AND without
Never dismiss a mass as a cyst based on a non-contrast scan
What a positive answer means
A solid enhancing renal mass is renal cell carcinoma until proven otherwise.
Ch 10.1
Cystic or solid?
Bosniak categories
I — simple cyst: thin wall, no septa, no calcification, no enhancement. ~0%, benign, no follow-up
II — a few hairline septa, fine calcification; < 3 cm hyperdense. ~0%, benign, no follow-up
III — thickened, irregular walls or septa WITH measurable enhancement. ~50% — surgical excision or ablation
IV — enhancing soft-tissue components. ~90% — treat as cancer
Ch 10.1 — Bosniak classification
The rule
An enhancing solid renal mass is malignant until proven otherwise.
Ch 10.1 — solid masses
What to know
Macroscopic FAT on CT — negative Hounsfield units — is diagnostic
Associated with tuberous sclerosis when multiple/bilateral
Large AMLs > 4 cm risk spontaneous hemorrhage (Wunderlich syndrome) and may warrant embolization
Ch 10.1 — solid masses
Consequence
This is a common reason to biopsy.
Ch 10.1 — solid masses
Also consider
Renal abscess
Infarct
Lymphoma
Metastasis to the kidney — in a patient with a known primary
Ch 10.1 — solid masses
≤ 4 cm, cT1a. Roughly 20% are benign, and many malignant ones are low-grade and slow-growing (~3 mm/year). Overtreating an 82-year-old's 2 cm mass is a real harm.
Select a box to open its teaching details.
The numbers that set expectations
Roughly 20% are benign
Many malignant ones are low-grade and slow-growing — typical growth ~3 mm/year
Pitfall
Overtreating an 82-year-old's 2 cm mass is a real harm.
Ch 10.2
Biopsy when
Before ablation
Before surveillance in a younger patient
To characterize an indeterminate mass
When metastasis or lymphoma is suspected
Reassurance
It is safe — tract seeding is essentially a non-issue — and underused.
Ch 10.2
Management options for cT1a
Why
Preserves kidney function, which matters for long-term cardiovascular and renal outcomes.
Ch 10.2
Intervene if
Growth is rapid — > 5 mm/yr
Size exceeds ~4 cm
Ch 10.2
Trade-off
Slightly higher local recurrence than surgery, but low morbidity.
Ch 10.2
Pitfall
Avoid radical nephrectomy for small masses if a partial is possible
Ch 10.2
Watch especially
Solitary kidneys
CKD
Diabetics
Hypertensives
Ch 10.3 — practical points
Subtype, hereditary risk, paraneoplastic clues, staging for venous extension, and treatment that is nothing like other solid tumors.
Select a box to open its teaching details.
The subtypes
Clear cell (~70–75%) — most common and most likely to metastasize; VHL-pathway driven; the subtype most responsive to immunotherapy and anti-VEGF agents
Papillary (~10–15%) — type 1 indolent, type 2 aggressive; often multifocal/bilateral
Renal medullary carcinoma occurs in patients with sickle cell TRAIT — in a young Black patient this is a specific, aggressive entity; do not mistake it for something indolent
Ch 10.3 — subtypes
Refer for
Young patients — under 46
Bilateral or multifocal disease
A suggestive family history
The syndromes
von Hippel-Lindau — clear cell, bilateral/multifocal, plus hemangioblastomas and pheochromocytoma