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*****All information below contained in more inclusive Benign Renal Tumours Chapter Notes

If a complex cyst is first identified on US, contrast-enhanced axial imaging should be performed to better characterize the cyst
See Table 1 from Original Guideline
Risk of malignancy

Bosniak II: 5%, likely gross overestimation of the true risk, as most of the malignant category II lesions had features that made them too complex to be considered a true category II cyst
Bosniak IIF: 8-27%
Bosniak III: 54%
Bosniak IV: 88%, malignant until proven otherwise

Majority will grow over time; growth should not necessarily be considered a sign a malignancy
Transformation into a more complex cyst is rare and has been reported in only a handful of cases

Intervention is only warranted if the cyst becomes symptomatic (i.e., bleeding, recurrent infection or pain)

Treatment options include: percutaneous management (aspiration +/- sclerotherapy) or surgery
Percutaneous cyst decompression may also be considered prior to offering definitive treatment as a means to confirm that the source of symptoms are cyst-related.

≈15% of these category IIF cysts will progress in complexity (to Bosniak category III or IV) over time
Should be followed with a contrast-enhanced CT scan or MRI every 6 months for the first year. Cases without progression should be followed annually for at least 5 years.

Given the low metastatic potential of cystic RCC, the panel feels that reduced surgical margins and controlled cyst decompression (if required) can be performed with low risk of tumour recurrence.
Likewise, due to the same reason, active surveillance and thermal-ablation therapies may also be considered as appropriate treatment alternatives in select cases

Most of these malignant cysts are thought to have low metastatic potential and thus, more conservative management may be safely considered in select cases

The vast majority of cystic RCCs are multilocular cystic RCCs (mcRCC,) but all RCC subtypes may present in a predominantly cystic form. Cystic RCCs need to be distinguished from solid renal masses with necrotic components, which behave more aggressively.
mcRCCs have consistently better cancer-specific and overall survival compared with solid RCCs.

There is yet to be a report demonstrating metastases or recurrence of mcRCCs. To reflect this indolent behaviour, the International Society of Urological Pathology (ISUP) has recently modified its terminology and now recommends calling these lesions multilocular cystic renal neoplasm with low malignant potential.
One potential explanation for this better prognosis is that the majority of mcRCCs tumour volume is fluid and thus, the actual tumour burden is much lower when compared to similar sized solid tumours. As the outcomes of these tumours do not seem to be influenced by the overall lesion size, some experts have even suggested to abandon the current pathological T staging for mcRCC and to reassigned them a new stage called pT1c (c for cystic).

Given their relatively indolent nature, there is emerging evidence suggesting that these lesions (especially Bosniak classification III) can be safely managed by active surveillance. If active surveillance is considered, it seems reasonable to follow these lesions with abdominal imaging every 6 months for the first 2 years, followed by yearly imaging thereafter, if the lesion is stable.
Triggers for interventions are yet to be clearly defined and validated, but may include:
1. Progression from Bosniak III to IV
2. Growth of solid nodule > 3 cm
3. Fast-growing nodule

Given the limited data, RFA should be limited to patients with small Bosniak category III and IV cysts who are poor operative candidates and in whom active surveillance is not being considered
The role of cryotherapy in the management of Bosniak III or IV cysts is not well-defined, with only a handful of cases reported to have been treated by the approach in the literature.

There is evidence that RTBs are significantly less informative for the diagnosis of cystic lesions than for solid ones.
It is generally felt that RTB is not diagnostic for most Bosniak III cysts, as there is minimal targetable solid component.
For Bosniak IV cysts, a biopsy of the solid component may be considered to confirm the presence of a malignant tumour and to help with decision-making in select cases (elderly, multiple comorbidities, unfit for treatment, etc).