Radiation oncologists at the Roberts Proton Therapy Center have developed protocols, techniques and approaches expressly for patients who have complications that might alter the efficacy of, or preclude, radiotherapy. These complex presentations typically involve the presence of cancer in proximity to radiosensitive organs and comorbidities that necessitate dose reduction.
Standard cancer radiotherapy involves an intricate balance to determine the minimum exposure required to treat malignancy without toxicity to nearby normal tissues. On one hand, overexposure to standard radiotherapy may increase the risk of adverse events. Efforts to minimize radiation exposure, on the other hand, may result in inadequate dose delivery, increasing the risk of cancer recurrence.
The benefits of radiotherapy are diminished when patients have comorbidities that may be exacerbated by significant radiation exposure. Although not ideal candidates for standard radiotherapy, these complex patients may benefit from proton therapy, which can preserve the therapeutic ratio and deliver safe treatment.
Among the important properties of proton therapy is a rapid dose fall off at the distal edge of the target (Bragg-Peak effect), a characteristic that allows for significant reductions in integral dose to normal organs by comparison to standard photon therapy (Figure 1). [1] In addition, proton therapy permits conformal, precision treatment delivery, improved dose homogeneity and the opportunity for dose escalation to achieve optimal radiation dosing to the target lesion.
Case Study
Mr. G, a 68-year-old man, was referred to the Roberts Proton Therapy Center for post-operative radiotherapy after he was found to have mediastinal lymph node involvement at the time of his lobectomy, where the initial PET/CT showed no evidence of abnormal lymph nodes. Typically, it is recommended that patients diagnosed with stage III lung cancer undergo sequential chemotherapy followed by radiation.
Mr. G’s case was made more challenging, however, by a diagnosis of symptomatic severe aortic stenosis prior to surgery. As a temporizing measure, Mr. G then had a valvuloplasty with a planned valve replacement to follow the completion of his treatment for cancer. In addition to aortic stenosis, Mr. G’s condition was complicated by the presence of atrial fibrillation and rapid ventricular rate (RVR) during his original lobectomy. Post-operative radiotherapy in this situation has been a challenge causing real morbidity and mortality in the past.
Improvements in techniques over the last decade have improved the safety of this treatment, but in Mr. G’s case, there were several incentives for a reduction in radiation dose. Because the disease to be targeted was located in the mediastinum, a substantial decrease in dose to the heart would be appropriate. A further diminishment in dose to the aortic valve was required to decrease complications arising from the anticipated valve replacement post-radiotherapy. The presence of atrial fibrillation and RVR at his prior surgery also mandated dose reduction.
A multidisciplinary discussion with Mr. G’s cardiac surgeon then took place to determine which areas of the vasculature should be avoided to minimize this risk. Mr. G received radiotherapy at the Roberts Proton Center for five and a half weeks, with minimal dosing to his heart and nearly no dose to the area of the aorta valve. Three weeks after the conclusion of radiotherapy, he had a transaortic valve replacement (TAVR) procedure to correct his aortic stenosis.
At his one-year follow-up, he noted no deterioration in his capacity for normal daily activity. No evidence of cancer recurrence was noted at this time.
References
1. Chang JY, Jabbour SK, De Ruysscher D, et al. Consensus Statement on Proton Therapy in Early-Stage and Locally Advanced Non–Small Cell Lung Cancer. Rad Oncology 2016;95:505–516.
