Stereotactic body radiation therapy for

Advanced cancer frequently leads to bone metastases, with autopsy reports indicating that 70-85% of patients had bone metastases at the time of diagnosis.

While conventional palliative radiation therapy has been shown to reduce pain and enhance the quality of life, there is no evidence of an increase in overall survival. Recent advancements in radiation therapy may have the potential to enhance both overall survival and local control rates.

The treatment of oligometastatic disease has seen a recent technological breakthrough in the form of Stereotactic Body Radiation Therapy (SBRT) for bone metastases. Compared to conventional radiation therapy, SBRT can administer substantially higher biologically equivalent doses (BED).

The Canadian Association of Radiation Oncologists (CARO) describes SBRT as “the precise delivery of highly conformal and image guided hypofractionated external beam radiotherapy, delivered in a single or few fraction(s), to an extracranial body target with doses at least biologically equivalent to a radical course when given over a protracted conventionally fractionated (1.8− 3.0 Gy/fraction) schedule” SBRT enables a shift in the objective of therapy from simply providing relief from pain and symptoms to achieving maximum local tumor control while reducing pain.

As SBRT is still a relatively new field, information on the toxicities, and outcomes associated with such treatment are still to be learned. There are three primary reasons for considering SBRT as a treatment option in the case of bone metastases:

1. Treating a location that has already been subjected to conventional external beam radiation therapy in the past is referred to as retreatment.
2. In cases of oligometastases where there are five or fewer metastatic sites.
3. Oligometastatic progression in patients who have metastases that are spread extensively throughout their body but one or two areas may be disproportionately affected.
   SBRT can focus on these specific areas that are either causing discomfort or showing progression on radiography.

For patients who have bone metastases outside of the spine, SBRT seems to be a viable and secure treatment alternative. Pooling and analyzing data can be difficult due to the inconsistent endpoints across clinical trials. Establishing a consensus on SBRT endpoints is crucial to standardize outcome evaluation reporting and enable comparisons across various trials.

An international consensus on clinical endpoints has been developed for bone metastases in the context of conventional radiation therapy. Patient-reported pain scores and the quantity of analgesics used are used to determine the response categories.

A pain score of 0 out of 10 at the treated site with no concurrent increase in analgesic consumption is the definition of a complete response to treatment, whereas a pain reduction of 2 or more at the treated site without any increase in analgesic consumption or a 25% reduction in analgesics with no increase in pain score or 1 point above the baseline is the definition of a partial response.

An increase in pain score of 2 or more above the baseline with a constant analgesic intake or a 25% increase in analgesic consumption with a stable pain score is the definition of pain progression.

Responses that do not fall within the definitions mentioned above are considered indeterminate. The definitions mentioned above cannot be directly applied to patients undergoing SBRT treatment because the indications for SBRT treatment differ from those of conventional radiation therapy.

As mentioned before, the potential indications for SBRT differ significantly from those of conventional treatment, with three major indications being considered: need for retreatment, oligometastatic disease, and oligometastatic progression.

If the objective of treatment is to alleviate pain, the evaluation can be performed using the endpoints established by the international consensus. For patients with oligometastatic disease, it is appropriate to use the Response Evaluation Criteria in Solid Tumors (RECIST) criteria to evaluate asymptomatic lesions.

Radiological imaging becomes more significant in asymptomatic lesions. According to the RECIST criteria, in bone metastases with soft tissue involvement, response rates should be defined as follows: a complete response is the disappearance of all lesions, a partial response is a decrease in size of ≥30% from baseline, progressive disease is defined as an increase in size of ≥20%, and stable disease is neither a partial response nor progressive disease.

However, the RECIST criteria are not applicable to assess the response of bone metastases without any involvement of soft tissue. Studies in the literature suggest that a more fractionated treatment schedule may be more effective in promoting the recalcification of bone, possibly due to its greater biological efficacy.

Likewise, osteolytic lesions can undergo remineralization following palliative radiotherapy, with a gradual increase in median percent density change as the dose and fractionation are increased. The higher biologically effective dose (BED) of SBRT may lead to increased recalcification in patients undergoing this treatment.

For patients with oligometastatic disease, the assessment of local control and prevention of distant metastases could serve as relevant endpoints to evaluate the efficacy of SBRT. Secondary endpoints such as tumor markers like prostate-specific antigen (PSA) in prostate cancer, progression-free survival, and overall survival are also crucial in assessing the outcomes of SBRT.

For patients with oligometastatic progression, distinct endpoints need to be defined. Patients with widespread metastases with oligometastatic progression may experience symptoms or increased size in one to two new areas. If these lesions cause symptoms, the endpoint definitions outlined in the international consensus should be applied.

In case the lesions are asymptomatic and have a soft tissue component involved, the RECIST criteria can be used to assess the response. In this patient population, the occurrence of distant metastases becomes a relatively less significant concern as they already have metastases in other parts of the body.

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