The role of stereotactic radiotherapy in the treatment of oligometastases

A new approach to combined proton-photon therapy for metastatic cancer patients

Objective.Proton therapy is a limited resource and is typically not available to metastatic cancer patients. Combined proton-photon therapy (CPPT), where most fractions are delivered with photons and only few with protons, represents an approach to distribute proton resources over a larger patient population. In this study, we consider stereotactic radiotherapy of multiple brain or liver metastases, and develop an approach to optimally take advantage of a single proton fraction by optimizing the proton and photon dose contributions to each individual metastasis.Approach.CPPT treatments must balance two competing goals: (1) deliver a larger dose in the proton fractions to reduce integral dose, and (2) fractionate the dose in the normal tissue between metastases, which requires using the photon fractions. Such CPPT treatments are generated by simultaneously optimizing intensity modulated proton therapy (IMPT) and intensity modulated radiotherapy (IMRT) plans based on their cumulative biologically effective dose (BEDα/β). The dose contributions of the proton and photon fractions to each individual metastasis are handled as additional optimization variables in the optimization problem. The method is demonstrated for two patients with 29 and 30 brain metastases, and two patients with 4 and 3 liver metastases.Main results.Optimized CPPT plans increase the proton dose contribution to most of the metastases, while using photons to fractionate the dose around metastases which are large or located close to critical structures. On average, the optimized CPPT plans reduce the mean brain BED2by 29% and the mean liver BED4by 42% compared to IMRT-only plans. Thereby, the CPPT plans approach the dosimetric quality of IMPT-only plans, for which the mean brain BED2and mean liver BED4are reduced by 28% and 58%, respectively, compared to IMRT-only plans.Significance.CPPT with optimized proton and photon dose contributions to individual metastases may benefit selected metastatic cancer patients without tying up major proton resources.

A multi-national report on stereotactic body radiotherapy for oligometastases: Patient selection and follow-up

Aims Stereotactic body radiotherapy (SBRT) for oligometastases is increasingly used with few evidenced-based guidelines. We conducted a survey to determine patient selection and follow-up practice patterns. Materials and methods Seven institutions from US, Canada, Europe, and Australia that recommend SBRT for oligometastases participated in a 72-item survey. Levels of agreement were categorized as strong (6-7 common responses), moderate (4-5), low (2-3), or no agreement. Results There was strong agreement for recommending SBRT for eradication of all detectable oligometastases with most members limiting the number of metastases to five (range 2-5) and three within a single organ (range 2-5). There was moderate agreement for recommending SBRT as consolidative therapy after systemic therapy. There was strong agreement for requiring adequate performance status and no concurrent chemotherapy. Additional areas of strong agreement included staging evaluations, primary diagnosis, target sites, and follow-up recommendations. Several differences emerged, including the use of SBRT for sarcoma oligometastases, treatment response evaluation, and which imaging should be performed during follow-up. Conclusion Significant commonalities and variations exist for patient selection and follow-up recommendations for SBRT for oligometastases. Information from this survey may serve to help clarify the current landscape.

The radiobiological targets of SBRT: tumor cells or endothelial cells?

The development of stereotactic body radiation therapy (SBRT) and stereotactic radiosurgery (SRS) techniques has revolutionized the practice of radiation oncology. The radiobiological targets that alter the therapeutic response to SBRT remain a subject of debate. The prevailing perspective has been that the radiation-induced damage to endothelial cells and changes in microvasculature facilitate tumor response to SBRT. A provocative study by Moding et al. (PMID: 25761890), challenged this notion by elucidating the role of tumor cells versus endothelial cells in mediating sarcoma eradication following high-dose SBRT. Using dual recombinase technology, they generated primary sarcomas in genetically engineered mouse models (GEMMs). They also modulated the apoptotic pathway and radiosensitization profile using targeted mutations in either tumor cells or endothelial cells. Unlike transplanted tumor models, the findings here suggest that deletion of the pro-apoptotic gene Bax or of the DNA-damage response gene ATM in endothelial cells did not result in tumor eradication to high dose SBRT, despite extensive endothelial cell death. On the other hand, genetic targeting of ATM gene in tumor cells achieved local sarcoma control and tumor eradication. These findings imply that tumor cells rather than endothelial cells act as prime targets affecting a tumor eradication response to SBRT. The translational implications of these findings are of great potential significance. When targeting endothelial cells, delivery of SBRT irradiation can only result in tumor growth delay. The benefit of targeting ATM in this setting will be radiation dose dependent. Curative intent, tumor eradication and local control, on the other hand, are only possible by targeting tumor cells with high dose SBRT (50 Gy in 1 fraction) and with radiosensitization by ATM deletion. In the absence of radiosensitization, only palliation is possible with high dose SBRT. Whether these provocative findings can be extrapolated to other translational tumor models or proved valid in clinical trials remains the subject of future studies. The mechanisms by which tumors compensate to SBRT's endothelial cell damage, such as new vascular recruitment, and/or recruitment of other immune and stromal components, are also critical questions for the field of radiobiology to address. Such mechanistic understanding of the key cellular players mediating SBRT response in a model system that recapitulates human disease will be essential in designing targeted radiosensitizers ultimately aimed at improving the therapeutic ratio.

Image-guided high-dose-rate brachytherapy: preliminary outcomes and toxicity of a joint interventional radiology and radiation oncology technique for achieving local control in challenging cases

PURPOSE

To determine the ability of image-guided high-dose-rate brachytherapy (IG-HDR) to provide local control (LC) of lesions in non-traditional locations for patients with heavily pre-treated malignancies.

MATERIAL AND METHODS

This retrospective series included 18 patients treated between 2012 and 2014 with IG-HDR, either in combination with external beam radiotherapy (EBRT; n = 9) or as monotherapy (n = 9). Lesions were located in the pelvis (n = 5), extremity (n = 2), abdomen/retroperitoneum (n = 9), and head/neck (n = 2). All cases were performed in conjunction between interventional radiology and radiation oncology. Toxicity was graded based on CTCAE v4.0 and local failure was determined by RECIST criteria. Kaplan-Meier analysis was performed for LC and overall survival.

RESULTS

The median follow-up was 11.9 months. Two patients had localized disease at presentation; the remainder had recurrent and/or metastatic disease. Seven patients had prior EBRT, with a median equivalent dose in 2 Gy fractions (EQD2) of 47.0 Gy. The median total EQD2s were 34 Gy and 60.9 Gy for patients treated with monotherapy or combination therapy, respectively. Image-guided high-dose rate brachytherapy was delivered in one to six fractions. Six patients had local failures at a median interval of 5.27 months with a one-year LC rate of 59.3% and a one-year overall survival of 40.7%. Six patients died from their disease at a median interval of 6.85 months from the end of treatment. There were no grade ≥ 3 acute toxicities but two patients had serious long term toxicities.

CONCLUSIONS

We demonstrate a good one year LC rate of nearly 60%, and a favorable toxicity profile when utilizing IG-HDR to deliver high doses of radiation with high precision into targets not readily accessible by other forms of local therapy. These preliminary results suggest that further studies utilizing this approach may be considered for patients with difficult to access lesions that require LC.