Hidradenitis suppurativa (hi-drad-uh-NIE-tis sup-yoo-ruh-TIE-vuh) is a chronic skin condition that features pea-sized to marble-sized lumps under the skin. Also known as acne inversa, these deep-seated lumps typically develop where skin rubs together — such as the armpits, groin, between the buttocks and under the breasts.

The lumps associated with hidradenitis suppurativa are usually painful and may break open and drain foul-smelling pus. In many cases, tunnels connecting the lumps will form under the skin. Hidradenitis suppurativa tends to start after puberty, persist for years and worsen over time. This condition is diffciult to treat and if it doesn't respond to antibiotics or simple surgery, then radiation may be an option. See some of the studies below

Radiotherapy of hidradenitis suppurativa--still valid today? (see review articles about this disease here, and here and here, and here, and here.

Frohlich D,     Strahlenther Onkol. 2000 Jun;176(6):286-9.

PURPOSE: In a retrospective analysis the efficacy of radiotherapy in the treatment of hidradenitis suppurativa was assessed. PATIENTS AND METHODS: Data from 231 patients undergoing radiotherapy for hidradenitis suppurativa in the last years in 2 hospitals were analyzed. The patients were treated with an orthovoltage therapy unit with 175 kV, 0.5 mm copper-filtering. The single doses reached from 0.5 Gy to 1.5 Gy up to total doses between 3.0 Gy and 8.0 Gy in one series. In chronic recurrent disease 2 or more series with a total dose of more than 10.0 Gy were given. RESULTS: Complete relief of symptoms at the end of radiotherapy was achieved in 89 patients (38%). In 92 patients (40%) there was clear improvement of symptoms. Only 2 patients did not react to radiotherapy. No side effects of radiotherapy occurred. CONCLUSION: Radiotherapy should be given more consideration as a treatment option for hidradenitis suppurativa.

Therapeutic effects of low radiation doses.
Strahlenther Onkol. 1994 Jan;170(1):1-12.

Trott KR. Department of Radiation Biology, St. Bartholomew's Medical College, London, Great Britain.

This editorial explores the scientific basis of radiotherapy with doses of < 1 Gy for various non-malignant conditions, in particular dose-effect relationships, risk-benefit considerations and biological mechanisms. METHODS: A review of the literature, particularly clinical and experimental reports published more than 50 years ago was conducted to clarify the following problems. 1. The dose-response relationships for the therapeutic effects on three groups of conditions: non-malignant skin disease, arthrosis and other painful degenerative joint disorders and anti-inflammatory radiotherapy; 2. risks after radiotherapy and after the best alternative treatments; 3. the biological mechanisms of the different therapeutic effects. RESULTS: Radiotherapy is very effective in all three groups of disease. Few dose-finding studies have been performed, all demonstrating that the optimal doses are considerable lower than the generally recommended doses, yet few of these studies meet the required standard. In different conditions, risk-benefit analysis of radiotherapy versus the best alternative treatment yields very different results: whereas radiotherapy for acute postpartum mastitis may not be justified any more, the risk-benefit ratio of radiotherapy of other conditions and particularly so in dermatology and some anti-inflammatory radiotherapy appears to be more favourable than the risk-benefit ratio of the best alternative treatments. CONCLUSIONS: Radiotherapy can be very effective treatment for various non-malignant conditions such as eczema, psoriasis, periarthritis humeroscapularis, epicondylitis, knee arthrosis, hydradenitis, parotitis and panaritium and probably be associated with less acute and long-term side effects than similarly effective other treatments. Randomized clinical studies are required to find the optimal dosage which, at present, may be unnecessarily high. Since no adequate experimental studies have been performed nothing is known about the mechanisms of these therapeutic radiation effects. Good experimental models are now available which should be used to investigate the cellular and molecular mechanisms involved.