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Myint et al. Mini-invasive Surg 2018;2:34 I http://dx.doi.org/10.20517/2574-1225.2018.52 Page 3 of 8

can be as high as 20%-30%. CXB alone is not suitable as the low energy X-rays have limited penetration
that will not reach the lymph nodes in the meso-rectum. The usual standard of care is to offer these
[1,2]
patients surgery . However, if the patient is not suitable for surgery or refuses it, they can be offered an
alternative treatment using external beam radiotherapy with or without chemotherapy. If the patient is fit,
external beam chemo-radiotherapy, with a total dose of 45 Gy in 25 fractions over 5 weeks, or a biologically
[6]
equivalent dose, can down-size the tumor. There is published evidence that in radio-responsive tumors ,
the malignant tissue is not just down-sized, but usually down-staged to either ypT0 or ypT1 as well. If the
patient is not fit enough for this treatment or has a poor renal function, a short course of radiation (25 Gy
in 5 fractions over 5 days) can be offered, with consideration of performing CXB boost after 4-6 weeks to
[4,7]
improve local control .

TECHNIQUES
CXB uses a high dose (90 Gy) of low energy (50 KVp) X-rays which are targeted directly on the tumor
under visual guidance. There are two machines currently available for CXB cancer therapy. First, the
Papillon + X-ray brachytherapy unit is currently marketed by the British company Ariane Medical Systems,
Ltd (Alfreton, UK). Additionally, the Xoft® Axxent® Electronic Brachytherapy System® (iCAD, Inc., San
Jose, CA, USA) is currently only approved for breast, skin and gynecological cancers, but it is undergoing
development for treatment of rectal neoplasms. The radiation dose applied at each treatment is quite high (30
Gy) but because the radiation energy is low (50 kV) and applied directly to the tumor in a small volume ( <
5 cc), the collateral damage to the normal surrounding tissues is limited. The treatment is given three times
(30 Gy X 3) every two weeks. This regimen allows the normal tissues to recover during the 2-week break.
There are three applicator sizes available: 30, 25 and 20 mm. The choice of applicator size depends on the
tumor size which should be less than 30 mm (if the tumor size is > 30 mm, then EBCRT or EBRT is offered
initially to down-size the tumor before CXB). The tumor is treated with a margin of 5 mm. In a responsive
tumor, the lesion usually regresses centripetally [Figure 1], beginning immediately after the first fraction
[4,7]
but mostly after the second fraction as illustrated in our case study .

The treatment can be given as a day patient as the whole procedure usually takes less than 30 min. This
includes the initial assessment with endoscopy and the treatment time is less than 150 s. The patient can
be treated supine or prone, in a knee-chest position [Figure 2]. A rigid sigmoidoscope is inserted to assess
the tumor size, position, and to select the size of the rectal applicator. Then the radiation is applied using
a suitable rectal treatment applicator. The radiation dose of 30 Gy is delivered to the surface of the normal
surrounding rectal mucosa. Therefore, exophytic lesions which protrude into the treatment applicator
receive a much higher dose of radiation than 30 Gy at the surface of the tumor. In a radio-responsive
tumor, the treated layer is shaved off after each radiation treatment until the tumor regresses completely to
the base of the bowel wall, and finally is flush with the surface of the surrounding normal rectal mucosa.
The deeper layers then get treated with subsequent fractions. At a depth of 5 mm below the surface of the
rectal mucosa, where the muscularis propria (deep muscle) of the rectal wall is situated, the dose of CXB
is reduced to 50% of the surface (applied) dose, and at a 10 mm depth, the dose is attenuated to 30% of
the surface dose [Figure 3]. There is published evidence that 98% of the residual tumor is usually confined
within the muscularis propria (5 mm deep from the rectal mucosa) for early stages (cT1, cT2) of rectal
[6]
tumors . We normally offer CXB boost treatments 4-6 weeks following EBCRT. However, if the residual
tumor following EBCRT is still bulky and infiltrates more than 5 mm below the rectal mucosa (beyond the
rectal wall full thickness) we can delay the treatment by few more weeks to see if there is further regression
of the residual tumor before proceeding with the CXB boost.

Follow-up
[4,7]
The risk of local neoplastic regrowth is usually highest within the first 2 years and close follow-up is