Introduction

Sarcomas are a rare type of cancer, accounting for less than 5% of adult and 20% of child cancer cases. They develop in and affect the body’s connective and supportive tissue, with soft tissue sarcomas forming in muscles and fat, and osteosarcomas forming in bones and cartilage. Sarcomas are usually treated with surgery or radiotherapy, but these treatments can cause significant short- and long-term complications that impact a patient’s quality of life.

Proton therapy is a different type of radiotherapy which has been shown to be effective since some types of sarcoma, without the severe side effects associated with traditional cancer treatments.¹ Read on to learn more about this novel therapeutic. 

Sarcomas

What are sarcomas?

Sarcomas are an extremely rare form of cancer, with around 5000 new cases diagnosed in the UK every year. About 44% of sarcoma cases are found in the thigh, and some types are more common in children than in adults.² Sarcomas account for approximately 2% of cancer-related deaths overall. 

Common types of sarcoma

Leiomyosarcoma

Leiomyosarcoma is the most commonly diagnosed sarcoma in UK adults. It affects smooth muscle, particularly in the uterus and abdomen. It is often aggressive, with 90% of patients being diagnosed with stage 2 or 3 cancer. The prognosis for leiomyosarcoma depends on the stage, location, and size of the tumour, with uterine tumours often having the worst outlook.³

Osteosarcoma

Osteosarcoma affects the bones. It is far more common in children than adults, and is the eighth most common childhood cancer. It mostly affects children aged 10-14, aligning with the growth spurt associated with puberty. The most common locations for osteosarcomas to develop are the femur (thigh bone), the tibia (lower leg bone) and the humerus (upper arm bone). Osteosarcoma has a much worse prognosis in patients over 40, with patients over 60 having the worst outlook.⁴

Ewing sarcoma

Ewing sarcoma is an aggressive bone cancer which is highly metastatic (very likely to spread). In the past 40 years, treatments such as chemotherapy have improved the five-year survival rate from 20% to over 70%. Still, approximately 25% of patients experience a relapse of their cancer. Ewing sarcoma is more likely to be found in long bones, such as those in the arms and legs, but can develop in other locations such as the pelvis. The prognosis depends on the location and size of the tumour, and whether the cancer has metastasised before being detecrted. The prognosis is much worse in patients with metastatic cancer.⁵

What is proton therapy?

Radiotherapy is an effective treatment for many localised cancers, but can be toxic and damaging to surrounding tissues - including major organs. Proton beam therapy (PBT) is an emerging form of radiotherapy which causes less damage to organs than photon radiotherapy. In fact, studies have shown that proton therapy can reduce irradiation of surrounding tissues by as much as 50% compared to photon radiotherapy.⁶ 

Proton therapy uses a beam of photons - very small positively charged particles which make up part of the nucleus of atoms. The proton beam penetrates the skin, and the photons deposit their energy at the end of the beam. What sets proton therapy apart from photon radiotherapy is that the energy from the proton beam is more focused at the end of the beam (directly on the tumour) while the energy from photon radiotherapy spreads more. This is how proton therapy is able to spare more healthy tissue.⁶

Proton therapy and sarcomas

The targeted nature of proton therapy makes it a suitable treatment for sarcomas, as they are often located near critical structures, such as blood vessels, nerves, and bones. As such, it is essential that the therapies used to treat sarcomas are precise enough to not damage the delicate tissues surrounding them. Proton therapy also has the added benefit of being able to deliver high doses of radiation to deep-located tissues. 

Proton therapy has been found to be effective against many different types of sarcoma. Below, we will discuss how proton therapy has been used to treat a range of sarcomas.

Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common childhood soft tissue sarcoma. It forms in skeletal muscle. A study that took place between 2000 and 2014 on 83 childhood RMS patients who were treated with proton therapy found that after 10.5 years, tumour recurrence or progression had occurred in just 16 patients and the survival rate five years after treatment was 80.6% of patients.⁷ 

Osteosarcoma

Radiotherapy is used to treat osteosarcoma in cases where surgery is not possible. It requires a very high dose of radiation, so proton therapy is much more suitable (and much less harmful). However, very few studies have assessed the effect of proton therapy on osteosarcoma. One study reporting on the treatment of osteosarcomas with radiotherapy and/or proton beam therapy. 56.1% of the patients recieved proton beam therapy (either alone or in combination with traditional radiotherapy). The study found that 10 out of 41 patients who received radiotherapy experienced complications requiring hospitalisation or surgery later in life. 

56.1% whose treatment involved proton therapy, ten patients had radiotherapy-related complications later in life. Out of these ten, five had received proton therapy. Despite this, five years following treatment, 68% of the patients were in remission, and no resurgence was observed.^8,9

Ewing sarcoma

A study on 30 Ewing sarcoma patients treated with proton therapy alone found complete local control rates of 92% after three years. Where radiotherapy was combined with surgery, local control rates were 100%. For patients whose cancer was in the pelvic and mid-to-lower spinal areas, no radiotherapy-related complications were observed in the bowel, bladder, heart or lungs. However, some patients whose cancer was located at the base of the skull experienced bilateral hearing loss and vasculopathy (blood vessel disease) later in life.⁵

Advantages and disadvantages of proton therapy

Advantages of proton therapy include:¹⁰

• It poses a much lower risk of damaging vital organs compared to traditional radiotherapy
• Can be used at higher doses than traditional radiotherapy
• Lower risk of cancer resurgence

On the other hand, challenges facing proton therapy include:

• Very limited availability - there are only two NHS centres in the UK which offer proton radiotherapy
• It may not be suitable for every type of sarcoma

Summary

Sarcomas are a group of rare cancers that can occur in the body’s soft, connective tissues such as blood vessels, cartilage and fat. Examples include leiomyosarcoma, osteosarcoma and Ewing sarcoma. Sarcomas are more common in children than adults, and are most commonly treated with surgery or photon radiotherapy.

Proton therapy is an emerging form of radiotherapy which aims a beam of very small positively charged particles (protons) at a tumour to kill cancer cells and shrink the tumour. The major advantage of proton therapy is that it is less damaging to surrounding vital organs and tissues than traditional radiotherapy. This is because the majority of its energy is focused at the end of the proton beam, meaning it is focused on the target tumour.

This precision makes it an ideal treatment for sarcomas, which are often situated near vital organs and tissues. Some sarcomas that proton therapy has been found to be effective against are rhabdomyosarcoma, osteosarcoma and Ewing sarcoma. One major challenge of proton therapy is that it is not currently widely available, with only two NHS centres offering it in the UK.